Introdução

No geral, os contratos factory são uma ferramenta útil para os desenvolvedores criarem e gerenciarem várias instâncias de contratos inteligentes de maneira mais eficiente e simplificada, fornecendo um bloco de construção para a criação de aplicativos descentralizados complexos no ecossistema Web3.

Ele permite a implantação de muitos contratos semelhantes com uma única transação, reduzindo o custo e a complexidade da implantação de vários contratos individualmente. A palavra-chave "new" é um elemento-chave no padrão de fábrica no Solidity e desempenha um papel significativo na implantação e gerenciamento de contratos Factory.

Neste video mostraremos como você pode realizar um deploy re um smart contract utilizando Factory na rede Alfajores (rede de teste) da Celo.

Pré-requisitos

Para este tutorial não é necessário conhecimento prévio. As ferramentas utilizadas serão:

• Remix: Remix é uma Integrated Development Environment (IDE) para desenvolvimento de smart contracts na blockchain Ethereum. Ele fornece uma plataforma fácil de usar para escrever, testar e depurar smart contracts escritos na linguagem Solidity. Além disso, a IDE Remix também fornece ferramentas para simular e executar smart contracts em diferentes ambientes de rede, incluindo a rede principal Ethereum e redes de teste. Ele é uma ferramenta popular entre desenvolvedores de smart contracts, pois oferece recursos avançados para facilitar o desenvolvimento e teste de contratos inteligentes.
• Faucet: Faucet é utilizada para adicionar fundos a sua conta de teste na rede Alfajores.
• Alfajores: É a rede de teste da Celo que utilizaremos para demonstração a implantação de um contrato inteligente e também realizar transações dos ativos.

Requisitos​

• Criar duas contas na MetaMask usando a rede Alfajores da Celo

Tutorial​

Confira no video como construir e realizar deploy de contratos Factory no blockchain Celo.

Conclusão​

Parabéns! Você concluiu o tutorial e agora sabe como criar um contracto Factory no blockchain da Celo 🎉 .

Próximos passos​

Como próximos passos sugiro a você consultar outros videos.

Além disso, convido você a ver nossos próximos conteúdos em português.

Sobre o Autor​

Eu sou um empreendedor serial, founder da Guizo Studios e sempre disponível para ajudar o ecossistema Celo.

LinkedIn

Introduction​

Non-fungible tokens (NFTs) have gained significant popularity in recent years due to their ability to represent unique digital assets such as artworks, collectibles, and even virtual real estate. One specific type of NFT, called ERC1155, allows for the creation and management of both fungible tokens within a single smart contract.

Minting your own NFTs on Celo is a relatively straightforward process. this tutorial will provide the necessary steps to successfully mint your ERC115 NFT on the Celo Blockchain network, with tips and best practices for successful minting, using the Remix IDE.

Additionally, this tutorial will provide a summary of the advantages of using the Celo blockchain to mint ERC1155 NFTs. And helps you demystify the process and provides the tools and resources necessary for successful NFT minting.

Prerequisites​

Remix: You should be familiar with working on the Remix IDE. This tutorial will make use of the Remix IDE to create, deploy and mint your ERC token on the Celo Blockchain.

OpenZeppelin: For creating your ERC1155 Token standard contract, you’ll make use of an already created and secured contract on the openzeppelin library available remotely on the internet.

This tutorial requires you to have a solid foundation knowledge of basic web3 concepts like NFTs, openzeppelin, smart contracts, solidity code, etc.

Note: Although this tutorial will cover creating and deployment of your ERC contract, it will not include an in-depth explanation of how to create and deploy your ERC115 contract.

Requirements​​

To follow this tutorial you should have the following:

• Celo Wallet Extension: This tutorial will require you to have an account on an installed celo wallet extension, or if you’re using another wallet like metamask, you should have the celo alfajores network added. Here is a link to guide you on how to add the alfajores testnet to your custom wallet.

• Faucets: You should also have your wallet funded with Celo test funds. Here is a link to request celo faucets.

• Node & node package management npm or yarn: This tutorial will require you to use a preinstalled node package manager, yarn. You should also know about working with any package manager: npm or yarn.

• IPFS: Although this tutorial will not cover uploading your NFT images on IPFS, you should also be familiar with the concept of uploading on IPFS, and how to upload files on IPFS.

Note: IPFS (InterPlanetary File System) is a distributed file system that allows users to store and access content from anywhere in the world. It is built on the same principles as the Ethereum blockchain, making it the ideal choice for hosting and delivering content on the blockchain.

The ERC115 Token Standard​

Before getting started with writing code and minting your ERC115 token, here is a quick reminder on what the ERC1155 token is and how it is usually minted.

The ERC115 Token Standard is one of the most popular Ethereum-based token standards, that is used for creating, minting, storing, and transferring Non-Fungible Tokens (NFTs).

It is an extension of the ERC20 Token Standard and allows users to create unique tokens with different attributes, such as name, symbol, and metadata. And a powerful tool for developers to easily create and manage digital assets on a blockchain network.

Tokens created with the ERC1155 Token Standard are also interoperable with other ERC20 tokens, for the creation of multiple asset types, such as collectibles, game items, digital artwork, and more. To mint a new token, developers need to call the ERC1155 contract and pass in the token's contract address, an array of token IDs, and an array of tokenURI strings. The tokenURI string is used to identify each token and can contain information such as a token’s title, description, or image.

The following code example shows the functions to mint a new ERC1155 token from a smart contract on a blockchain network. From openzeppelin’s standard ERC1155 contract:

function _mint(address to, uint256 id, uint256 amount, bytes memory data) internal virtual {
        require(to != address(0), "ERC1155: mint to the zero address");
 
        address operator = _msgSender();
        uint256[] memory ids = _asSingletonArray(id);
        uint256[] memory amounts = _asSingletonArray(amount);
 
        _beforeTokenTransfer(operator, address(0), to, ids, amounts, data);
 
        _balances[id][to] += amount;
        emit TransferSingle(operator, address(0), to, id, amount);
 
        _afterTokenTransfer(operator, address(0), to, ids, amounts, data);
 
        _doSafeTransferAcceptanceCheck(operator, address(0), to, id, amount, data);
    }

Uploading the NFT image on IPFS​

To get started with minting your NFT on the Celo blockchain. First, you need to have your image uploaded on IPFS, although this tutorial will not cover how to upload your pictures, here is a video reference to learn how to upload your NFT images to IPFS using NFT UP, Since you will need an already uploaded NFT image for this tutorial, you can use the link to an NFT folder with the images PHENZIC000 and PHENZIC001, uploaded on IPFS.

Minting your ERC1155 Token Standard​

Remix is an open-source web IDE that simplifies the process of creating and deploying smart contracts to a blockchain network. It offers a simple graphical user interface that enables you to write and deploy Ethereum contracts quickly and easily. To get started with minting and interacting with your tokens, you’ll need to create a basic template for your smart contract on Remix, hence the following steps:

1. First, head over to the Remix IDE using this link,
2. You will need to download the Celo plugin on Remix, for interacting, compiling, and deploying on the Celo blockchain. Following the images and steps below.

• a. Click on the plug-like icon on the left bottom part of the screen.

• b. Enter Celo into the search bar to search for the Celo plugin.

• c. Click Activate to add the Celo plugin to the left plain, you will notice the Celo icon has been added to the plugins on the left, click on the Celo Icon.

3. Next, create a new file under the contracts directory, and name the file MyToken, where you will have your smart contract written.

4. Copy and paste the following code below into your MyToken contract:

• a. The code below simply initializes the contract by importing the standard ERC1155 token contract from openzeppelin into your contract, including all the functionalities of a standard ERC1155 token:

// SPDX-License-Identifier: MIT
 
pragma solidity ^0.8.0;
 
import "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
import "@openzeppelin/contracts/utils/Strings.sol";

Note: OpenZeppelin is an open-source framework for developing secure, reliable, and upgradable smart contracts on the blockchain. It provides a large library of reusable, secure, and well-tested components that can be used to quickly develop and deploy applications on the Ethereum blockchain.

Copy and paste the code above inside your MyToken.sol contract file.

• b. Next, the function below creates a new contract as a sub-contract of the standard openzeppelin ERC1155 contract and also initializes two constants for the NFTs you will be creating.

contract MyToken is ERC1155 {
    uint256 public constant PHENZIC000 = 0;
    uint256 public constant PHENZIC001 = 1;
}

Copy and add the code above to your MyToken contract file.

• c. The Next function is the constructor function that takes in the link to your NFT images uploaded on IPFS and calls the _mints function to mint your NFTs.

    constructor() ERC1155("https://bafybeiaqqz4unoubpu2oz2rsgowh3irdqnpcqjoyspzwrepnrwql7rgvy4.ipfs.nftstorage.link/"){
        _mint(msg.sender, PHENZIC000, 200, "");
        _mint(msg.sender, PHENZIC001, 100, "");
    }

Copy and add the code above to your token contract.

• d. Finally the last function uri take in unit256 digit 0 or 1. And returns the direct link to any of the NFT's locations on IPFS, either the first no or the second.

    function uri(uint256 _tokenId) override public pure returns (string memory) {
        return string(abi.encodePacked("https://bafybeiaqqz4unoubpu2oz2rsgowh3irdqnpcqjoyspzwrepnrwql7rgvy4.ipfs.nftstorage.link/PHENZIC00",Strings.toString(_tokenId),".jpeg")
        );
    }

Copy and add the code below inside your token contract.

• e. Finally, your complete contract should look exactly like the one code below, you can copy and replace your entire code with this for uniformity's sake.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
 
import "@openzeppelin/contracts/token/ERC1155/ERC1155.sol";
import "@openzeppelin/contracts/utils/Strings.sol";
 
contract MyToken is ERC1155 {
    uint256 public constant PHENZIC000 = 0;
    uint256 public constant PHENZIC001 = 1;
 
    constructor() public ERC1155("https://bafybeiaqqz4unoubpu2oz2rsgowh3irdqnpcqjoyspzwrepnrwql7rgvy4.ipfs.nftstorage.link/"){
        _mint(msg.sender, PHENZIC000, 200, "");
        _mint(msg.sender, PHENZIC001, 100, "");
    }
 
    function uri(uint256 _tokenId) override public pure returns (string memory) {
        return string(abi.encodePacked("https://bafybeiaqqz4unoubpu2oz2rsgowh3irdqnpcqjoyspzwrepnrwql7rgvy4.ipfs.nftstorage.link/PHENZIC00",Strings.toString(_tokenId),".jpeg")
        );
    }
}

5. Next click on the Celo plug-in icon you activated earlier, to compile and deploy your contract on celo.

6. Now, click on the connect button attached to the first input bar on the side plain, to connect to your wallet, you will notice it automatically returns the amount of native token on your alfajores wallet and also tells you the network you are on currently.

Note: Also ensure you are current on your celo alfajores testnet account on any wallet of your choice, and the account should already be funded with Celo testnet native token.

7. Next, Click on the compile button, and Remix should compile the contract without any errors.

8. Now click on the deploy button below, and your wallet will pop up on the right side of your screen for you to sign the transaction.

9. Click on confirm button, and you should have your contract already deployed to the celo alfajores testnet with the address of your contract on the display tab attached to the deploy button.

Note: The NFTs minted will be signed to the address of the person that deployed the contract and called the function.

10. Now, that you have deployed your token contract with the minting function being called inside the contract’s constructor function. Your NFTs have automatically been minted to your address.

Interacting with the Deployed ERC1155 Token​

Now that you have your newly minted ERC1155 token deployed and minted on Celo alfajores signed with your address. You can run some checks to interact with your token on the blockchain.

• To check the number (balance) of NFT currently available on your wallet address:

a. Click on the function call balanceof at the bottom left side of your screen.

b. You will notice two drop-down input tabs to add your connected wallet address and the token Id of the NFT you want to view its balance, Copy your connected wallet address and past it in the first field, and input 0 in the second tab to view the balance of the NFT.

c. Click Call to view the amount of the PHENZIC000 token available in your address.

• Assuming you have a Gaming application where you welcome each new player by gifting them with the PHENZIC000 token and gifting the old-timer player with certain levels attained in the game with the PHENZIC001 token. You can have this done automatically on your Dapp but in this case, you can equally send these tokens by calling the safeTransferFrom function.

a. First, click on the safeTransferFrom function call like in the image below to add the required parameters to call the functions.

b. Copy and add your connected wallet address as the first input for the variable from.

c. Next, add the wallet address you want to send the NFT to, any remote alfajores address will work fine.

d. Enter the id of the token you want to send 0 or 1, and the amount you want to send for this example you can use 0 for id, 50 for the amount, and use 0x00 for the data input.

e. Click transact, and to check the balance of the remaining token.

f. Head back to the balanceof add your connected wallet address and 0 or the id of the token you sent from, and click call. You will notice the reduction in the token amount.

• The uri function call overrides simple taken in a token_id, and returns the link to either the first minted NFT or the second minter NFT on IPFS, depending on the token_id you input.

Conclusion​

Minting your ERC1155 NFT on the Celo blockchain is a relatively straightforward process, but it does require some technical knowledge and setup. By following the steps outlined in this tutorial, you can successfully mint your ERC115 NFT and begin it in the Celo ecosystem.

You would have used the Celo plugin on the Remix IDE to interact with the Celo blockchain and, understand other concepts like how the IPSF works and how to mint your token contract using Remix.

About the Author​

Mayowa Julius Ogungbola

is a Software Engineer and Technical writer always open to working on new ideas. I enjoy working on GitHub and you can also connect with me on LinkedIn.

Next Steps​

Here are some other NFT-related tutorial articles you might be interested in:

• How to Build an NFT Collection on Celo
• How to Deploy an ERC721 Smart Contract using Tatum API
• How to Create, Deploy and Mint your ERC1155 token on Celo

References​​

Here are links to some video tutorials you might be interested in following along with while reading this tutorial:

• Uploading your NFTs to IPFS
• Publishing your Minted NFTs on Opensea
• Minting your ERC1155 contract on Remix

Introduction​​

In this tutorial, we will be creating an escrow NFT platform on the Celo network using Eth-Brownie Python. Escrow is a financial arrangement where a third party holds and regulates the transfer of funds or assets between two other parties. In this case, the third party is the smart contract. The two parties are the buyer and the seller. The escrow NFT platform will work like this, the seller will lock NFT in smart contract and the buyer will be able to buy the NFT by paying the price set by the seller. The smart contract will hold the NFT until the buyer pays the price. If the buyer pays the price, the NFT will be transferred to the buyer. If the buyer fails to pay the price, the NFT will be returned to the seller. In this tutorial, we will be using the Celo network, but the same concept can be applied to other evm-compatible blockchain networks.

Prerequisites​

These tutorials assume that you have some basic knowledge of solidity and python.

Requirements​

• Python3 or greater
• NodeJs >= v14.0.0 and npm >= 6.12.0 (For Ganache)
• Ganache (For local blockchain)
• eth-brownie (For interacting with the blockchain)
• python-dotenv (For loading environment variables from .env file)

This is a list of what we’ll cover 🗒

• ✅ Step 1: Project setup
• ✅ Step 2: Write project code
• ✅ Step 3: Configure deployment settings
• ✅ Step 4: Deploy your Contract
• ✅ Step 5: Integration with frontend

Step 1: Project setup​

First, we will create a new directory for our project. Open your terminal and run the following command to create a new directory called escrow-nft and change directory to it.

mkdir escrow-nft && cd escrow-nft

next, we will install eth-brownie, python-dotenv, and ganache-cli. Run the following command to install them.

# Install eth-brownie and python-dotenv
pip3 install eth-brownie python-dotenv

# Install ganache
npm install -g ganache

Now, after installing all the required packages, we need to initialize our project. Run the following command to initialize our project.

brownie init

Here's what a successful initialization looks like:

Next, after initializing our project, we will create two files, brownie-config.yaml and .env in root directory. brownie-config.yaml is a configuration file for brownie. It contains the default settings for our project. We will use this file to configure our project settings. .env is a file that contains environment variables. We will use this file to store our mnemonic phrase. We will use this mnemonic phrase to deploy our contract to the Celo network.

brownie-config.yaml file

reports:
    exclude_contracts:
        - SafeMath
depedencies:
    - OpenZeppelin/openzeppelin-contracts@4.8.0
compiler:
    solc:
        version: 0.8.15
        optimizer:
            enabled: true
            runs: 200
        remappings:
            - "@openzeppelin=OpenZeppelin/openzeppelin-contracts@4.8.0"
networks:
    default: celo-alfajores
console:
    show_colors: true
    color_style: monokai
    auto_suggest: true
    completions: true
    editing_mode: emacs
dotenv: .env
wallets:
    from_mnemonic: ${MNEMONIC}

.env file

MNEMONIC="your mnemonic phrase"

if you want to read more about brownie-config.yaml file, you can read it here.

lastly for this step, we will add Celo network to our brownie project. Run the following command to add Celo network to our brownie project.

brownie networks add Celo celo-mainnet host=https://forno.celo.org chainid=42220 explorer=https://explorer.celo.org

brownie networks add Celo celo-alfajores host=https://alfajores-forno.celo-testnet.org chainid=44787 explorer=https://alfajores-blockscout.celo-testnet.org

You can check if the network has been added by running the following command.

brownie networks list

result if the network has been added successfully.

You can see that we have added two networks, celo-mainnet and celo-alfajores to our brownie network list.

Step 2: Write project code​

In this step, we will write the code for our smart contract. Create a new file called escrowNFT.sol in the contracts directory. This is where we will write our smart contract code. Here is an look at what our smart contract code will look like.

// SPDX-License-Identifier: MIT
pragma solidity 0.8.15;

import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

contract escrowNFT is Ownable {
    using SafeMath for uint256;

    // Declare state variables
    uint256 public fee;
    uint256 private escrowDigit = 16;
    uint256 private modulus = 10**escrowDigit;

    enum Status {
        Pending,
        Accepted,
        Rejected,
        Cancelled
    }

    struct Escrow {
        uint256 tokenId;
        uint256 paymentAmount;
        uint256 deadline;
        address tokenAddress;
        address buyerAddress;
        address sellerAddress;
        Status status;
    }

    mapping(uint256 => Escrow) public escrow;

    event NewEscrow(
        uint256 txId,
        uint256 tokenId,
        uint256 paymentAmount,
        address tokenAddress
    );

    event CancleEscrow(
        uint256 txId,
        uint256 tokenId,
        uint256 paymentAmount,
        address tokenAddress
    );

    event PayEscrow(
        uint256 txId,
        uint256 timestamp,
        uint256 tokenId,
        uint256 paymentAmount
    );

    event RejectEscrow(
        uint256 txId,
        uint256 tokenId,
        uint256 paymentAmount,
        address tokenAddress
    );



    modifier onlySeller(uint256 _txId) {
        require(
            msg.sender == escrow[_txId].sellerAddress,
            "Only seller can call this function"
        );
        _;
    }

    modifier onlyBuyer(uint256 _txId) {
        require(msg.sender == escrow[_txId].buyerAddress, "Only buyer can call this function");
        _;
    }

    constructor(uint256 _fee) {
        fee = _fee;
    }

    function updateFee(uint256 _fee) external onlyOwner {
        fee = _fee;
    }

    function claimFee() external onlyOwner {
        (bool status, ) = payable(msg.sender).call{value: address(this).balance}("");
        require(status, "Transfer failed");
    }

    function generateTxId(
        address _sellerAddress,
        address _buyerAddress,
        address _nftAddress,
        bytes memory _secret
    ) external view returns (uint256 txId) {
        txId =
            uint256(
                keccak256(
                    abi.encodePacked(
                        _sellerAddress,
                        _buyerAddress,
                        _nftAddress,
                        _secret
                    )
                )
            ) %
            modulus;
    }

    function createEscrow(
        uint256 _txId,
        uint256 _tokenId,
        uint256 _paymentAmount,
        address _tokenAddress,
        address _buyerAddress
    ) external {
        require(_paymentAmount > 0, "Payment amount must be greater than 0");
        require(_tokenAddress != address(0), "Token address cannot be 0x0");
        require(_buyerAddress != address(0), "Buyer address cannot be 0x0");
        IERC721 nft = IERC721(_tokenAddress);
        nft.transferFrom(msg.sender, address(this), _tokenId);
        escrow[_txId] = Escrow(
            _tokenId,
            _paymentAmount,
            block.timestamp + 1 days,
            _tokenAddress,
            _buyerAddress,
            msg.sender,
            Status.Pending
        );
        emit NewEscrow(_txId, _tokenId, _paymentAmount, _tokenAddress);
    }

    function cancleEscrow(uint256 _txId) external onlySeller(_txId) {
        require(
            block.timestamp > escrow[_txId].deadline,
            "Deadline not reached"
        );
        require(
            escrow[_txId].status == Status.Pending,
            "Escrow is not in pending status"
        );
        IERC721 nft = IERC721(escrow[_txId].tokenAddress);
        escrow[_txId].status = Status.Cancelled;
        nft.transferFrom(address(this), msg.sender, escrow[_txId].tokenId);
        emit CancleEscrow(
            _txId,
            escrow[_txId].tokenId,
            escrow[_txId].paymentAmount,
            escrow[_txId].tokenAddress
        );
    }

    function payEscrow(uint256 _txId) external payable onlyBuyer(_txId) {
        require(block.timestamp < escrow[_txId].deadline, "Deadline reached");
        require(
            escrow[_txId].status == Status.Pending,
            "Escrow is not in pending status"
        );
        IERC721 nft = IERC721(escrow[_txId].tokenAddress);
        uint256 amountAfterFee = _calculateFee(msg.value);
        escrow[_txId].status = Status.Accepted;
        (bool status, ) = payable(escrow[_txId].sellerAddress).call{value: amountAfterFee}("");
        require(status, "Transfer failed");
        nft.transferFrom(address(this), msg.sender, escrow[_txId].tokenId);
        emit PayEscrow(
            _txId,
            block.timestamp,
            escrow[_txId].tokenId,
            escrow[_txId].paymentAmount
        );
    }

    function rejectEscrow(uint256 _txId) external onlyBuyer(_txId) {
        require(block.timestamp < escrow[_txId].deadline, "Deadline reached");
        require(
            escrow[_txId].status == Status.Pending,
            "Escrow is not in pending status"
        );
        IERC721 nft = IERC721(escrow[_txId].tokenAddress);
        escrow[_txId].status = Status.Rejected;
        nft.transferFrom(address(this), escrow[_txId].sellerAddress, escrow[_txId].tokenId);
        emit RejectEscrow(
            _txId,
            escrow[_txId].tokenId,
            escrow[_txId].paymentAmount,
            escrow[_txId].tokenAddress);
    }
    
    function _calculateFee(uint256 _paymentAmount) private view returns(uint256 amountAfterFee) {
        uint256 feeAmount = _paymentAmount.mul(fee).div(100);
        amountAfterFee = _paymentAmount.sub(feeAmount);
    }
}

Let us analyze the code line by line.

// SPDX-License-Identifier: MIT
pragma solidity 0.8.15;

First line, we declare the SPDX license identifier of the relevant license for the contract. The second line, we declare the solidity version we are using.

import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

We'll be using OpenZeppelin contracts for our project. Ownable contract will be used to make sure only the owner of the contract can call certain functions. IERC721 is the interface for ERC721 tokens. SafeMath is a library that provides mathematical functions and prevents integer overflow.

uint256 public fee;
uint256 private escrowDigit = 16;
uint256 private modulus = 10**escrowDigit;

The fee variable will be used to store the fee percentage. The escrowDigit variable will be used to generate a unique transaction ID. The modulus variable will be used to generate a unique transaction ID.

enum Status {
        Pending,
        Accepted,
        Rejected,
        Cancelled
    }

Enum Status will be used to store the status of the escrow.

struct Escrow {
        uint256 tokenId;
        uint256 paymentAmount;
        uint256 deadline;
        address tokenAddress;
        address buyerAddress;
        address sellerAddress;
        Status status;
    }

Escrow struct will be used to store the details of the escrow.

mapping(uint256 => Escrow) public escrow;

The escrow mapping will be used to map the transaction ID to the Escrow struct.

event NewEscrow(
    uint256 txId,
    uint256 tokenId,
    uint256 paymentAmount,
    address tokenAddress
);

event CancleEscrow(
    uint256 txId,
    uint256 tokenId,
    uint256 paymentAmount,
    address tokenAddress
);

event PayEscrow(
    uint256 txId,
    uint256 timestamp,
    uint256 tokenId,
    uint256 paymentAmount
);

event RejectEscrow(
    uint256 txId,
    uint256 tokenId,
    uint256 paymentAmount,
    address tokenAddress
);

These events are emitted when a new escrow is created, an escrow is cancelled, an escrow is paid, and an escrow is rejected.

modifier onlySeller(uint256 _txId) {
    require(
        msg.sender == escrow[_txId].sellerAddress,
        "Only seller can call this function"
    );
    _;
}

modifier onlyBuyer(uint256 _txId) {
    require(msg.sender == escrow[_txId].buyerAddress, "Only buyer can call this function");
    _;
}

These modifiers are used to make sure only the seller or the buyer can call certain functions.

function updateFee(uint256 _fee) external onlyOwner {
    fee = _fee;
}

function claimFee() external onlyOwner {
    (bool status, ) = payable(msg.sender).call{value: address(this).balance}("");
    require(status, "Transfer failed");
}

The updateFee function is used to update the fee percentage. The claimFee function is used to claim the fee collected by the contract.

function generateTxId(
    address _sellerAddress,
    address _buyerAddress,
    address _nftAddress,
    bytes memory _secret
) external view returns (uint256 txId) {
    txId =
        uint256(
            keccak256(
                abi.encodePacked(
                    _sellerAddress,
                    _buyerAddress,
                    _nftAddress,
                    _secret
                )
            )
        ) %
        modulus;
}

generateTxId function is used to generate a unique transaction ID to be used for the escrow. The transaction ID is generated using the seller address, buyer address, NFT address, and the secret. secret parameter is used to make sure the transaction ID is unique for each escrow.

function createEscrow(
    uint256 _txId,
    uint256 _tokenId,
    uint256 _paymentAmount,
    address _tokenAddress,
    address _buyerAddress
) external {
    require(_paymentAmount > 0, "Payment amount must be greater than 0");
    require(_tokenAddress != address(0), "Token address cannot be 0x0");
    require(_buyerAddress != address(0), "Buyer address cannot be 0x0");
    IERC721 nft = IERC721(_tokenAddress);
    nft.transferFrom(msg.sender, address(this), _tokenId);
    escrow[_txId] = Escrow(
        _tokenId,
        _paymentAmount,
        block.timestamp + 1 days,
        _tokenAddress,
        _buyerAddress,
        msg.sender,
        Status.Pending
    );
    emit NewEscrow(_txId, _tokenId, _paymentAmount, _tokenAddress);
}

createEscrow is used to create a new escrow. The function takes the transaction ID, token ID, payment amount, token address, and buyer address as parameters. The function first checks if the payment amount is greater than 0, and check token, buyer address are not 0x0. Then it transfers the NFT from the seller to the contract. Then it creates a new escrow in the escrow mapping and emits the NewEscrow event.

function cancleEscrow(uint256 _txId) external onlySeller(_txId) {
    require(
        block.timestamp > escrow[_txId].deadline,
        "Deadline not reached"
    );
    require(
        escrow[_txId].status == Status.Pending,
        "Escrow is not in pending status"
    );
    IERC721 nft = IERC721(escrow[_txId].tokenAddress);
    escrow[_txId].status = Status.Cancelled;
    nft.transferFrom(address(this), msg.sender, escrow[_txId].tokenId);
    emit CancleEscrow(
        _txId,
        escrow[_txId].tokenId,
        escrow[_txId].paymentAmount,
        escrow[_txId].tokenAddress
    );
}

cancleEscrow function is used to cancel an escrow. The function takes the transaction ID as a parameter. First, function will check if the deadline is reached and the escrow status is pending. If both conditions are true, the function will transfer the NFT back to the seller and emit the CancleEscrow event.

function payEscrow(uint256 _txId) external payable onlyBuyer(_txId) {
    require(block.timestamp < escrow[_txId].deadline, "Deadline reached");
    require(
        escrow[_txId].status == Status.Pending,
        "Escrow is not in pending status"
    );
    IERC721 nft = IERC721(escrow[_txId].tokenAddress);
    uint256 amountAfterFee = _calculateFee(msg.value);
    escrow[_txId].status = Status.Accepted;
    (bool status, ) = payable(escrow[_txId].sellerAddress).call{value: amountAfterFee}("");
    require(status, "Transfer failed");
    nft.transferFrom(address(this), msg.sender, escrow[_txId].tokenId);
    emit PayEscrow(
        _txId,
        block.timestamp,
        escrow[_txId].tokenId,
        escrow[_txId].paymentAmount
    );
}

payEscrow function is used to pay an escrow. The function takes the transaction ID as a parameter. First, function will check if the deadline is not reached and the escrow status is pending and buyer only sends the payment amount equal to the payment amount in the escrow. If all conditions are true, the function will transfer the NFT to the buyer and pay the seller the payment amount minus the fee. Then it emits the PayEscrow event.

function rejectEscrow(uint256 _txId) external onlyBuyer(_txId) {
    require(block.timestamp < escrow[_txId].deadline, "Deadline reached");
    require(
        escrow[_txId].status == Status.Pending,
        "Escrow is not in pending status"
    );
    IERC721 nft = IERC721(escrow[_txId].tokenAddress);
    escrow[_txId].status = Status.Rejected;
    nft.transferFrom(address(this), escrow[_txId].sellerAddress, escrow[_txId].tokenId);
    emit RejectEscrow(
        _txId,
        escrow[_txId].tokenId,
        escrow[_txId].paymentAmount,
        escrow[_txId].tokenAddress);
}

rejectEscrow function is used to reject an escrow. The function takes the transaction ID as a parameter. First, function will check if the deadline is not reached and the escrow status is pending. If both conditions are true, the function will transfer the NFT back to the seller and emit the RejectEscrow event.

function _calculateFee(uint256 _paymentAmount) private view returns(uint256 amountAfterFee) {
    uint256 feeAmount = _paymentAmount.mul(fee).div(100);
    amountAfterFee = _paymentAmount.sub(feeAmount);
}

_calculateFee private function is used to calculate the fee amount. The function takes the payment amount as a parameter and returns the amount after fee.

Step 4: Deploy your Contract​

Now that we have written our smart contract, we need to deploy it to the blockchain. First, we need to compile our smart contract, and then we will deploy it to the blockchain. Use the following command to compile the smart contract.

brownie compile

result if the compilation is successful

Now, we need to deploy our smart contract to the blockchain. First create a new file scripts/deploy.py and add the following code to it.

from brownie import accounts, config, escrowNFT

def main():
    # Get the account to use
    account = accounts.from_mnemonic(config["wallets"]["from_mnemonic"])

    # Deploy the contract
    contract = escrowNFT.deploy(2, {"from": account})

    # Wait for the transaction to be mined
    contract.tx.wait(3)

    print("Contract deployed to:", contract.address)

main function is used to deploy the contract. First, we get the account to use. Then we deploy the contract and wait for the transaction to be mined. Finally, we print the contract address. Now, we need to deploy the contract. Use the following command to deploy the contract.

# Deploy the contract to the Celo Alfajores testnet
brownie run scripts/main.py --network celo-alfajores

# Deploy the contract to the Celo Mainnet
brownie run scripts/main.py --network celo-mainnet

result if the deployment is successful

Step 5: Integration with frontend​

Now that we have deployed our smart contract to the blockchain, we need to integrate it with our frontend. You can clone the frontend we have created for this tutorial. Use the following command to clone the frontend.

# Clone the frontend
git clone https://github.com/yafiabiyyu/CeloSageFE.git

# Go to the frontend directory
cd CeloSageFE

# Install the dependencies
npm install --save

After cloning the frontend, we need to update the src/utils/contract.tsx, You need to update the contract address and ABI. You can get the contract address from the deployment result and you can get the ABI from the brownie project folder build/contracts/escrowNFT.json.

after updating the contract address and ABI, You can run the frontend using the following command.

npm start

result if the frontend is running successfully

Conclusion​

In this tutorial, we have learned how to create an escrow smart contract using Solidity and Brownie. We have also learned how to deploy the smart contract to the Celo blockchain.

Next Steps​

For your next steps, if you a python developer and want to learn how to create a smart contract using python, you can check about vyper. Vyper is a python-based smart contract programming language. You can check the Vyper documentation to learn more about Vyper.

About the Author​

I am a blockchain and crypto enthusiast. I am also a software engineer. I love to learn new things and share my knowledge with others. You can find me on GitHub and LinkedIn.

Introduction​

The Celo blockchain is a proficient, fast, and lightweight platform that supports the building of innovative, complex, and client-designed mobile applications. In simple terms, it is a network that allows the development of decentralized and inventive mobile and web applications.

One of the core features of how the Celo blockchain work falls on the concept of multiple revolutionary solutions, one of which is called Proof of Stake (PoS). On completing this article, you’ll have a solid idea of the Concept of Celo’s protocols, what PoS is and how it makes Celo an efficient platform for creating indigenous, decentralized solutions.

You’ll also have all the basic information you’ll need to get started with building on the Celo blockchain.

Prerequisites​

Throughout this article, you are not expected to have any prior in-depth knowledge of any technology or intricate detail about the web3 space. If you’re reading this tutorial, it means you want to know more about what a PoS is and how the Celo Blockchain integrates this verification system.

What is Consensus Mechanism​

A Consensus Mechanism is a method of authentication adopted by blockchain platforms to ensure transactions are in sync and agree on which transaction is valid before adding the transaction to the blockchain.

Amongst others, one of the proven efficient and effective means of reaching consensus on the blockchain is using the Proof of Stack PoS consensus mechanism. Which is why the Celo blockchain uses it.

When a transaction is created on the Celo blockchain before it is added to the blockchain ledger it first needs to be validated by the chain’s miners, thus the need for consensus.

What is Proof of Stack (PoS)​

Proof of Stack is a type of consensus mechanism that adopts the idea of staking coins to earn its node runners the right to validate a transaction before adding it to the blockchain ledger.

When a transaction occurs on a blockchain platform like Celo, there is a need to first authenticate and validate the transaction before adding the transaction to the blockchain permanently. These tasks are usually carried out by the blockchain’s miners and node runners on the Celo blockchain.

Note: Node runners or validators are individuals or companies running full blockchain network nodes. They provide the backbone of the blockchain network by providing the infrastructure that allows the network to process transactions and maintain a distributed ledger. Node runners are rewarded for their services with tokens or coins from the network.

Looking back to the technology system before the web3 revolutionary breakthrough, verifying transactions would require a centralized or automated entity that was prone to either time consumption for financial and data forgery for non-monetary transactions and a lot more.

The need for a system like the PoS for the validation of transactions, therefore came as an innovative technological breakthrough.

In a PoS system, the chances of forgery or manipulation of transactions and data would be easily spotted and penalized. The PoS means of consensus is one of the important features that make the Celo Blockchain a secure and rewarding platform for developers to build on and validators to manage and get rewarded for.

The PoS algorithm also allows its validators and node runners to carry out validation while maintaining a low computational cost and manual effort.

Other types of Consensus Mechanism​

Just like the PoS is used by the Celo blockchain as a means of reaching Consensus in approving all transactions, there are also other consensus mechanisms adopted by other blockchains some of which are;

1. Proof of Work (PoW): This type of consensus algorithm requires its miners to consume a massive amount of computational power to solve the complex cryptographic puzzle that defines each transaction. Although this algorithm is effective and adopted by popular blockchain platforms, it is not quite efficient.

2. Delegated Proof of Stake (DPoS): This type of consensus algorithm is similar to the PoS algorithm. But unlike the PoS algorithm, a group of miners or node runners is tasked with achieving a distributed consensus. It serves more like a voting system where miners decide who should be responsible for validating a transaction.

3. Proof Of Importance (PoI): This algorithm is also very similar to the Proof of Stake algorithm, In the mechanism rewards are given to effective network users. The algorithm allows a hierarchy of importance based on individuals who have made more contributions to the system and gives them the right to validate transactions on the platform.

4. Proof of Activity (PoA): This algorithm is a hybrid of the Proof of Work (PoW) and the Proof of Stake (PoS) algorithms, that allow validators to stake coins as collateral, and also validate blocks using computational decryption and other resources.

5. Proof of Captivity (PoC): Thissystem of consensus neither requires a miner si solve cryptographic puzzles like the PoW or stake coins like the PoS, but rather to prove that they have a certain amount of hard drive space to contribute to storing plots of cryptographic hashes for the blockchain.

6. Proof of Authority (PoA): This algorithm simply pre-selects and authorizes validator nodes to validate a newly added block.

7. Proof of Elapsed Time (PoET): This system of consensus was designed to simply select a leader from a pool of miners and validators and assigns the task of validating the next node.

These are a few other important consensus algorithms available and adopted by other blockchain platforms.

Other Core Feature Of the Celo Blockchain​

• Scalability: The Celo blockchain was built to handle a large number of users and transactions without slowing down or becoming unresponsive. Celo is a layer-1 blockchain solution that helps to scale up blockchain technology to handle a high throughput of transactions. It does this by allowing transactions to be validated off-chain and then quickly grouping them into batches for faster processing on-chain. This helps reduce overall network congestion and improves scalability.

• Security: Transactions on the Celo Blockchain go through a set of measures taken to protect the data and transactions stored on the blockchain from tampering or unauthorized access. This includes encryption, secure protocols, and validation of transactions. Additionally, Celo's consensus protocol is designed to ensure the safety and integrity of the blockchain by maintaining a decentralized network of validators who are incentivized to keep the blockchain secure.

• Speed: The Celo blockchain is designed to offer a high level of speed, and enables users to quickly and securely transfer value across the network in a matter of seconds. It also utilizes a sharding technology called Celo Fast Finality (CFF) that allows the network to split its transactions into multiple shards to process thousands of transactions per second. It utilizes a mechanism called instant finality that also allows the network to commit transactions.

Building on Celo​

As a developer looking to build fast, secure, scalable, and innovative ideas, building on Celo is an exciting opportunity for you to create applications and services that leverage the Celo platform. Celo provides a secure, open-source platform for developers to create distributed applications and services that connect people and organizations in meaningful ways. With Celo, developers can create applications that bring new possibilities to the global economy, from unlocking financial inclusion to providing access to new markets.

Wallets​

The Celo wallet enables its users to quickly and securely send and receive payments, store digital assets, and access various financial services. It allows users to view their account balances and track their transactions.

The wallet also provides developers with the tools they require to interact with the Celo Blockchain, like signing transactions, deploying and testing contracts, calling and testing functions, etc. Additionally, the Celo wallet offers enhanced security features to ensure the safety of users’ digital assets. With its suite of features, the Celo wallet is a powerful tool for building on the Celo blockchain. Indirectly you can also interact with the celo blockchain by adding the celo network to other wallets like metamask, etc. More wallet-related information and why you need one can be found here.

Smart Contract​

The Celo blockchain also allows technological transactions like compiling, testing, debugging, deploying, and calling contracts on the network, which gives you the ability to create, a decentralized application and interact with a decentralized codebase on the blockchain.

Here, you will find more tutorials on creating, verifying, and deploying smart contracts, writing contract tests, and making contract call on the Celo blockchain using, Hardhat, Truffle, foundry, Remix, etc.

Connecting to Celo​

When connecting to the Celo alfajores or maiNet for deploying or interacting with the network, you’re advised to use the Celo configuration file below in place of the code in your .config file.

require("@nomiclabs/hardhat-waffle");
require("dotenv").config({ path: ".env" });
require("hardhat-deploy");

// You need to export an object to set up your config
// Go to https://hardhat.org/config/ to learn more

// Prints the Celo accounts associated with the mnemonic in .env
task("accounts", "Prints the list of accounts", async (taskArgs, hre) => {
  const accounts = await hre.ethers.getSigners();

  for (const account of accounts) {
    console.log(account.address);
  }
});

/**
 * @type import('hardhat/config').HardhatUserConfig
 */
module.exports = {
  defaultNetwork: "alfajores",
  networks: {
    localhost: {
      url: "http://127.0.0.1:7545",
    },
    alfajores: {
      gasPrice: 1500000000,
      gas: 4100000,
      url: "https://alfajores-forno.celo-testnet.org",
      accounts: {
        mnemonic: process.env.MNEMONIC,
        path: "m/44'/52752'/0'/0",
      },
      //chainId: 44787
    },
    celo: {
      url: "https://forno.celo.org",
      accounts: {
        mnemonic: process.env.MNEMONIC,
        path: "m/44'/52752'/0'/0",
      },
      chainId: 42220,
    },
  },
  solidity: "0.8.10",
};

Here is a link to the code sample above, you will also require your funded wallet’s Mnemonic phrase in an encrypted file. To know more about interacting with the Celo blockchain here are some tutorials you can read on.

Creating Dapps on the Celo blockchain​

The Celo blockchain provides the infrastructure for decentralized applications (DApps) to act as a bridge between users and their data privacy. The increasing number of dApps that utilize the Celo blockchain validates its usefulness in the blockchain ecosystem.

Creating the Celo Blockchain is a revolutionary way to use blockchain technology to build secure and reliable mobile and web applications. With the Celo platform, developers can easily create decentralized applications (DApps) and smart contracts that enable users to transfer value, store information, and more. This platform is designed to be user-friendly and secure, making it an attractive option for developers looking to build the next generation of applications.

Redeploying to Celo​

Redeploying your Decentralized Application (Dapp) to Celo is a necessary process that can enable you to take advantage of the Celo network’s advantages, including scalability and security. The Celo platform enables developers to create and deploy their Dapps quickly and easily, and redeploying is an important part of this process.

Redeploying your Dapp to Celo requires a Celo account, which you can create through their website or the Celo mobile app. Once you have an account, you must create a Dapp and deploy it to the Celo blockchain. This process involves setting up a smart contract, enabling the Celo network to run your Dapp. You then need to write the code for your Dapp and deploy it onto the Celo blockchain. Once your Dapp is deployed, it will be available to anyone on the Celo platform. Here you can find more information on Redeploying your Dapp to the Celo network.

Redeploying on the Celo network provides the following benefits to your decentralized application.

1. Scalability: The Celo blockchain can scale to millions of users, with each user able to transact in a matter of seconds. This makes it ideal for large-scale applications that handle large volumes of transactions.

2. Low Cost: Celo has committed to providing users with low-cost transactions, allowing developers to lower the cost of running their Dapps and making them more attractive to users.

3. Security: Celo utilizes advanced cryptography to provide a secure and reliable platform for developers and users.

4. Ease of Use: Celo has a user-friendly interface that makes it easy for developers to deploy their Dapps and for users to use them.

5. Open Source: Celo is an open-source platform that allows developers to access and customize the code to their needs and requirements.

6. Community Support: Celo has a vibrant and supportive community of developers and users who are always willing to help out and provide assistance.

Conclusion​

Now that you have completed this tutorial, you understand the concept of one of the core concepts that make up the Celo blockchain. And you now have everything you need to start building and interacting with the celo blockchain.

Next Steps​

Now that you’ve successfully grasped the lesson in this tutorial, you can also read on the following topics to help you get started on building real-world solutions and other development on Celo.

You can also consider contributing to the Celo network as a developer or as a technical writer (Celo Sage).

About the Author​​

Mayowa Julius Ogungbola

A Software Engineer and technical writer who is always open to working on new ideas. I enjoy working on GitHub, and you can also find out what I tweet about and connect with me on LinkedIn

Introduction​

When creating decentralized applications that leverage smart contracts, it is important to ensure that there are little or no vulnerabilities to prevent an attacker from compromising your application.

Unit testing helps you ensure that all functionalities in your contract are working as expected, and development environments like Truffle give you the same tools to help you write proficient tests for your contracts before final deployment.

In this tutorial, you’ll create an exemplary contract and learn how to write and run unit tests for your contract using the truffle development environment.

Prerequisites​

Throughout this tutorial you’ll need to have worked with or have a basic knowledge of the following:

• Truffle Suite: Truffle suite is a Development Environment that acts as a pipeline for interacting with the EVM and also provides essential features and valuable libraries for testing Ethereum smart contracts and makes it easy to interact with the blockchain.
• Solidity: Solidity is a high-level programming language used for creating smart contracts.
• Javascript: This tutorial will make use of Javascript, therefore you should be familiar with basic Javascript coding and algorithms.

Requirements​

This tutorial also aspects that you have the following already installed or available:

• Node & node package management npm or yarn: This tutorial will require you to use a preinstalled node package manager. You should also know about working with any package manager: npm or yarn.

Installing and setting up Truffle suite​

To install the truffle suite using your terminal. Create a workspace, head over to the directory on your terminal, and run the command npm install -g truffle.

Now, run the command npx truffle init to fire up the development environment. You’ll notice a new file structure appears in your file explorer, something like the image below:

Running a Contract Test Simulation​

To understand how unit testing works using the Truffle suite create a demo directory, different from your main directory, and run the command npx truffle unbox metacoin. The result of the successful run of the code should look like the image below.

The command starts up a demo project called <metacoin> including two contract files MetaCoin.sol and ConvertLib.sol in the contract directory and also has two testing files TestMetaCoin.sol and metacoin.js file in the test directory. For running unit tests on the metacoin contracts.

Now run the command npx truffle test and the result of the unit test should look exactly like the image below.

Truffle first compiles the contract, runs all the unit test in the test script, and returns the result of all the tests. The image above shows the result when it passes all the unit tests.

Creating the Smart Contract​

Each contract test made is composed explicitly to test a specific contract, meaning if you have four different contract files in an application, your application should likewise have four test scripts for testing each contract. In the following steps, you'll write a simple sample contract which you'll, later be writing a for.

Note: If you’re new to solidity and creating smart contracts, check out this tutorial to get started and understand solidity code. The tutorial above also has a couple of functions that will help you learn how to write solidity code.

1. Head back to the initial development environment directory you created; inside the contract folder, create a new file, Sample.sol. This will be the smart contract you’ll be writing unit tests for.

2. The Sample.sol contract will have the following functionalities:

• a. First, the contract is created, and the variables name, and age are also created and by default, have no value.

// SPDX-License-Identifier: MIT
 
pragma solidity ^0.8.17;
 
contract Sample {
    string public name;
    address public owner;
 
 
}

• b. Next, the contract’s constructor function assigns the address of the deployer of the contract to the variable owner and assigns the string "deployer" to the name variable.

     constructor() {
         owner = msg.sender;
         name = "deployer";
     }
  

• c. The next function rename accepts a string value as argument and assigns it to the variable name.

  
  function rename(string memory _name) public {
        name = _name;
    }

• d. The next function describe simply return the current values of the global variable, name.

  
    function describe() public view returns (string memory) {
        return (name);
    }

• e. Next is a modifier function ownerOnly that only allows the contract owner to call its parent function when added to any function.

    modifier ownerOnly() {
        require(
            msg.sender == owner,
            "this function requires the owner of the contract to run"
        );
        _;
    }

• f. The following function changeOwner uses the previously created ownerOnly modifier to only allow the owner of the contract to change the role of the contract owner to any address by passing as an argument to the changeOwner function.

    function changeOwner(address _newOwner) public ownerOnly {
        owner = _newOwner;
    }

• g. The next function deposit allows anyone to send a minimum of 1 ETH to the contract.

    function deposit() public payable {
        require(
            msg.value >= 0.01 * 10 ** 18,
            "you need to send at least 0.01 ETH"
        );
    }

• h. Finally, the last function in the Sample.sol contract allows anyone calling the contract to withdraw funds from the contract, as long as you pass in the number of tokens to withdraw as an argument. This transaction will also be terminated if the amount passed in exceeds than 10 ETH.

    function withdraw(uint256 _amount) public payable {
        require(_amount <= 100000000000000000);
        payable(msg.sender).transfer(_amount);
    }

If you’ve completed your Sample.sol contract, Your smart contract should look exactly like the code below; You should update your contract with the code below for uniformity sake:

// SPDX-License-Identifier: MIT
 
pragma solidity ^0.8.17;
 
contract Sample {
    string public name;
    address public owner;
 
    constructor() {
        owner = msg.sender;
        name = "deployer";
    }
 
    function rename(string memory _name) public {
        name = _name;
    }
 
    function describe() public view returns (string memory) {
        return (name);
    }
 
    modifier ownerOnly() {
        require(
            msg.sender == owner,
            "this function requires the owner of the contract to run"
        );
        _;
    }
 
    function changeOwner(address _newOwner) public ownerOnly {
        owner = _newOwner;
    }
 
    function deposit() public payable {
        require(
            msg.value >= 0.01 * 10 ** 18,
            "you need to send at least 0.01 ETH"
        );
    }
 
    function withdraw(uint256 _amount) public payable {
        require(_amount <= 100000000000000000);
        payable(msg.sender).transfer(_amount);
    }
}

To confirm you have no existing errors in your contract, run the command npx truffle compile on your terminal, and a successful result should look like the image below.

Now that you know the different functions in the Sample.sol contract and you’re familiar with what they do. Next, you’ll learn how to create a unit test script to test subsections of the contract you just made.

Writing the Unit Test Script​

Now that you have created the Sample.sol contract, you can begin writing the unit tests for the contract. After completing these tests, you’ll have a basic idea of how to create unit tests for smart contracts.

A very common pattern used when writing unit tests for smart contracts is:

a. Arrange: This is where you create dummy variables that you’ll need to run units of your test cases. They can be created globally after the contract test function s created or locally within the unit test.

b. Act: Next, is the part where you run your testing functions and store the result in a variable.

c. Assert: Since you already know the correct result of the test, then you compare your expected result with the response of the test you ran. If the test returns the expected result, it passes else, the test does not pass.

Also following the format:

 describe(<"functionName">, async function () {
  beforeEach(async function() {
<what should happen before each test is run>
})
    it("what the test is expected to do", async function () {
      const response = <what was returned>
      const result = <what should be returned>;
      expect(response).to.equal(result); // compares the response to the expected result
    });

Next, you’ll be creating a uint-test to test your Sample.sol contract using the previous format above, and you’ll learn how to create a basic unit test script on your own:

Testing a smart contract makes it easier to identify bugs and vulnerabilities and reduces the possibility of software errors that could lead to costly exploits. In the next few steps, you will learn the basic format of how to write unit tests based on your smart contract.

• First, head over to the migrations folder and create a script file called 1_deploy_contract.js and copy the code below into the script.

const Sample = artifacts.require("Sample");
// const MetaCoin = artifacts.require("MetaCoin");
 
module.exports = function (deployer) {
  // deployer.deploy(Sample);
  // deployer.link(Sample, SampleTest);
  // deployer.deploy(SamplTest);
  deployer.deploy(Sample, { gas: 1000000 });
};

The code above is created to simply deploy your Sample.sol contract. Next, navigate to the test folder and create a new test script, SampleTest.js.

1. Firstly, you’ll need to import the contract as a variable Sample in the first line of code.

const Sample = artifacts.require("Sample");

2. Next, you’ll need to initialize the contract test with the following code below. This contract - Sample will cover all the unit test functions that will be carried out on the named contract.

contract("Sample", (accounts) => {
})

3. Using the describe keyword to define a specific test for each function in the contract, you can carry out multiple tests using the it keyword for a specific function. The first test constructor tests the constructor function in the contract. Copy and add the code below.

  describe("constructor", async function () {
    it("should have the correct name", async () => {
      const sample = await Sample.deployed();
      const name = await sample.name();
      assert.equal(name, "deployer");
    });
 
    it("should have the correct owner", async () => {
const sample = await Sample.deployed();
      const owner = await sample.owner();
      assert.equal(owner, accounts[0]);
    });
  });

The function has two tests with string descriptions of what each of them is meant to do. The first test check for the initialization of the name variable and checks the value of the owner variable to the address of the deployer. The test passes if the result returns as expected and reverts with an error otherwise.

Now, run the command npx truffle test, and a successful result should look like the image below.

4. The next unit test describes the rename and describe function from the smart contract; the function carries out a single test on the rename and describe function. The test updates the name variable's value and checks the current the variable's current value if it has been updated. Copy and add the code below.

  describe("rename & describe", async function () {
    it("should be able to rename", async () => {
      const sample = await Sample.deployed();
      await sample.rename("new name");
      const name = await sample.describe();
      assert.equal(name, "new name");
    });
  });

Now, run the command npx truffle test and a successful result should look like the image below.

5. The next unit test describes the changeOwner function in the smart contract; the test first uses the right address to attempt to change the owner, which should pass successfully. And then uses another random address to change the ownership role, which is meant to be reverted. Copy and add the code below.

describe("changeOwner", async function () {
    it("should change the owner", async () => {
      const sample = await Sample.deployed();
      await sample.changeOwner(accounts[1], { from: accounts[0] });
      const owner = await sample.owner();
      assert.equal(owner, accounts[1]);
    });
 
    it("should not change the owner", async () => {
      const sample = await Sample.deployed();
      try {
        await sample.changeOwner(accounts[2], { from: accounts[1]});
      } catch (error) {
        assert.equal(
          error.message,
          "VM Exception while processing transaction: revert"
        )};
    });
  });

Now, run the command npx truffle test and a successful result should look like the image below.

6. The next function tests the deposit function of the contract. The first test will verify the deposit function works correctly which allows deposits of 0.01 ETH or greater. The second test verifies that the deposit function correctly rejects deposits of less than 0.01 ETH. Copy and paste the code below.

  describe("deposit", async function () {
    it("should allow deposits", async () => {
      const sample = await Sample.deployed();
      await sample.deposit({ value: 0.01 * 10 ** 18 });
    });
    it("should not allow deposits below 0.01 ETH", async () => {
      const sample = await Sample.deployed();
      try {
        await sample.deposit({ value: 0.009 * 10 ** 18 });
        assert.fail("deposit should have failed");
      } catch (error) {
        assert.ok(error.message.includes("revert"));
      }
    });
  });

Now, run the command npx truffle test and a successful result should look like the image below.

7. This next describe function tests the withdraw function in the contract. The first test is attempting to withdraw 0.01 ether from the contract. The second test is attempting to withdraw an amount greater than the balance to ensure that the withdrawal fails. If the test fails, it will return an error message with the word revert.

  describe("withdraw", async function () {
    it("should allow withdrawals", async () => {
      const sample = await Sample.deployed();
      await sample.withdraw(BigInt(0.01 * 10 ** 18));
    });
    it("should not allow withdrawals above balance", async () => {
      const sample = await Sample.deployed();
      try {
        await sample.withdraw(BigInt(0.01 * 10 ** 18));
        assert.fail("withdrawal should have failed");
      } catch (error) {
        assert.ok(error.message.includes("revert"));
      }
    });
  });

Finally, run the command npx truffle test and a successful result should look like the image below.

After completing your test script, your code should look exactly like the one below. For uniformity, sake replaces the entire code with this code test.

const Sample = artifacts.require("Sample");
 
contract("Sample", (accounts) => {
  describe("constructor", async function () {
    it("should have the correct name", async () => {
      const sample = await Sample.deployed();
      const name = await sample.name();
      assert.equal(name, "deployer");
    });
 
    it("should have the correct owner", async () => {
      const sample = await Sample.deployed();
      const owner = await sample.owner();
      assert.equal(owner, accounts[0]);
    });
  });
 
  describe("rename & describe", async function () {
    it("should be able to rename", async () => {
      const sample = await Sample.deployed();
      await sample.rename("new name");
      const name = await sample.describe();
      assert.equal(name, "new name");
    });
  });
  describe("changeOwner", async function () {
    it("should change the owner", async () => {
      const sample = await Sample.deployed();
      await sample.changeOwner(accounts[1], { from: accounts[0] });
      const owner = await sample.owner();
      assert.equal(owner, accounts[1]);
    });
 
    it("should not change the owner", async () => {
      const sample = await Sample.deployed();
      try {
        await sample.changeOwner(accounts[2], { from: accounts[1] });
      } catch (error) {
        assert.equal(
          error.message,
          "VM Exception while processing transaction: revert"
        );
      }
    });
  });
  describe("deposit", async function () {
    it("should allow deposits", async () => {
      const sample = await Sample.deployed();
      await sample.deposit({ value: 0.01 * 10 ** 18 });
    });
    it("should not allow deposits below 0.01 ETH", async () => {
      const sample = await Sample.deployed();
      try {
        await sample.deposit({ value: 0.009 * 10 ** 18 });
        assert.fail("deposit should have failed");
      } catch (error) {
        assert.ok(error.message.includes("revert"));
      }
    });
  });
  describe("withdraw", async function () {
      it("should allow withdrawals", async () => {
        const sample = await Sample.deployed();
        await sample.withdraw(BigInt(0.01 * 10 ** 18));
      });
      it("should not allow withdrawals above balance", async () => {
        const sample = await Sample.deployed();
        try {
          await sample.withdraw(BigInt(0.01 * 10 ** 18));
          assert.fail("withdrawal should have failed");
        } catch (error) {
          assert.ok(error.message.includes("revert"));
        }
      });
    });
});

Conclusion​

Writing unit tests for smart contracts can help a great deal in ensuring a secure and proficient contract, by suggesting fixes and improvements after discovering errors, issues, and security vulnerabilities in your contract. You have successfully created your unit test script for a simple sample contract using truffle. Now that you understand how unit tests are written, you can move on to writing more complex test scripts for other smart contracts. You can also read about how to run the unit test for smart contracts using Truffle.

Next Steps​

Here is some other tutorial article.

Unit testing with Hardhat and Celo

How to create and Test contract calls with Celo and Hardhat

About the Author​

Mayowa Julius Ogungbola

A Software Engineer and technical writer who is always open to working on new ideas. I enjoy working on GitHub, and you could also find out what I tweet about and connect with me on LinkedIn

References​

Here is a link to the complete tutorial sample code on my GitHub, Leave a ⭐on the repository if you find it helpful.

Introduction​​

This tutorial will show you how to create a simple Android app that allows users to make payments using the Celo Java SDK. The app, called "Buy Me A Coffee", will allow users to make a donation to any one of their choice using their Celo account. We will walk you through the process of setting up the Celo Java SDK and integrating it into your app, so you can start accepting payments in no time. By the end of this tutorial, you will have a working Android app that allows users to make payments using the Celo network.

He is a quick look at what we will build in this tutorial.

Fig: 0-1 dApp UI

Prerequisites​​

To successfully follow this tutorial, you will need basic knowledge of blockchain technology and Android development.

• You will need to create five generated accounts using the celocli. To set this up, this guide will be helpful.

• To test the functionality of the Celo Java SDK you will need to have some test tokens. This can be gotten from the faucet.

Requirements​​

• Alfajores Testnet Account - required to connect to the dApp and make test transactions.
• Android Studio - required to build an app that runs on an emulator or a physical android phone. To run the completed code, the minimum version of Android Studio 5.0 or newer and
• Java SDK version 11 or higher

Workspace Setup and Configuration in Android Studio​

To get started, your Android Studio should be up and running. Add all necessary dependencies to your app/build.gradle file. To start using the Celo Java SDK, here are the dependencies you would need.

implementation 'io.github.gconnect:celo-sdk-java:0.0.2'
implementation 'org.web3j:core:5.0.0'
implementation 'de.hdodenhof:circleimageview:3.1.0'

Celo Java SDK - is a Java library for working with the Celo Blockchain, Contractkit and Celo Core Contracts.

Web3j is a highly modular, reactive, type-safe Java and Android library for working with Smart Contracts and integrating with clients (nodes) on the Ethereum network:

Circleimageview library is used for making an image circular.

Create an Android project​

Start up Android Studio and create a new Project. Select basic activity.

Fig 1- 0: Create New Project

Fig 1-1: Create New Basic Activity

Note: Constant.java file holds the constants for the app.

Account Creation​

Celo provides us with several options to create an account. These include downloading the mobile wallet (Valora or Alfajores), and using Ganache and Celo CLI. For development purposes, any of these options will be good to easily create or generate Celo accounts. For this tutorial, we will use the Celo CLI to generate 5 Celo Testnet accounts. For more details on account creation, you can check the Celo Developer Doc for Testnet Wallet Setup.

To set up the Celo CLI on your machine, follow this guide. After successful setup, from your terminal, run this command to create a new account.

celocli account:new

Funding an Account​

To fund your Celo Testnet account, copy the address of the wallet or from your generated account after running the above command. Simply go to the faucet and paste the address to get it funded. If you need more funds, paste your address in the input field and check the captcha and click on the get started button. That should get your account funded immediately.

Fig 1-3: Celo Faucet

Checking Account Balance​

There are several ways to check your account balance. You can use the Celocli command, the Celo Java SDK or simply enter your account address in the Alfajores Celo Scan explorer.

Using the Celocli run this command to check account balance

celocli account:balance 0x93312861A9844E5E62F3730CAA913DD11adE75b9

Output should look like this 👇

Fig 2-1: Celocli Output

Checking balance using the Alfajores Celo Scan explorer output should look like this 👇

Fig 2-2: Alfajores Celo Scan explorer.

Checking balance using using the Celo Java-SDK required code snippet should look like this.👇

ContractKit contractKit = ContractKit.build(new HttpService("https://alfajores-forno.celo-testnet.org"));
Web3j web3j = Web3j.build(new HttpService(ContractKit.ALFAJORES_TESTNET));

ContractKitOptions config = new ContractKitOptions.Builder()
       .setFeeCurrency(CeloContract.StableToken)
       .setGasPrice(BigInteger.valueOf(21_000))
       .build();
contractKit = new ContractKit(web3j, config);

// Multiple accounts can be added to the kit wallet. The first added account will be used by default.
contractKit.addAccount(Constant.somePrivateKey);

// To change default account to sign transactions
contractKit.setDefaultAccount(Constant.publicKey);

AccountBalance balance = contractKit.getTotalBalance(Constant.myAddress);
System.out.println("cUSD balance " + balance.cUSD);
System.out.println("Celo balance " + balance.CELO);

The above code snippet can be found in the FirstFragment.java file in the complete code of the project repository.

Note: This code should not be run on the main UI thread otherwise, the app will scratch. This is so because the above code requires making network calls. To keep things simple, we are running the above code in a runnable thread.

The output will look like this 👇

Manifest.xml​

Inside your manifest.xml file add network permission and network state. This will prevent the app from crashing. And it also notifies the app that internet connection is required.

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

Data Model, Data Source, Constant and Recyclerview Adapter​

Data Model​

The BeneficiaryModel.java file handles the data model popularly called pojo. This defines the initial data object.

package com.example.celocoffeeapp;

import android.os.Parcel;
import android.os.Parcelable;

public class BeneficiaryModel implements Parcelable {
   private int imgId;
   private String name;
   private String description;
   private String walletAddress;

   public BeneficiaryModel(int imgId, String name, String description,
                            String walletAddress) {
       this.imgId = imgId;
       this.name = name;
       this.description = description;
       this.walletAddress = walletAddress;
   }

   protected BeneficiaryModel(Parcel in) {
       imgId = in.readInt();
       name = in.readString();
       description = in.readString();
       walletAddress = in.readString();
   }

   public static final Creator<BeneficiaryModel> CREATOR = new Creator<BeneficiaryModel>() {
       @Override
       public BeneficiaryModel createFromParcel(Parcel in) {
           return new BeneficiaryModel(in);
       }

       @Override
       public BeneficiaryModel[] newArray(int size) {
           return new BeneficiaryModel[size];
       }
   };

   public String getDescription() {
       return description;
   }
   public void setDescription(String description) {
       this.description = description;
   }
   public int getImgId() {
       return imgId;
   }
   public void setImgId(int imgId) {
       this.imgId = imgId;
   }

   public String getName() {
       return name;
   }

   public void setName(String name) {
       this.name = name;
   }

   public String getWalletAddress() {
       return walletAddress;
   }

   public void setWalletAddress(String walletAddress) {
       this.walletAddress = walletAddress;
   }

   @Override
   public int describeContents() {
       return 0;
   }

   @Override
   public void writeToParcel(Parcel dest, int flags) {
       dest.writeInt(imgId);
       dest.writeString(name);
       dest.writeString(description);
       dest.writeString(walletAddress);
   }
}

Constant​

This handles constants and can be found in constant.java file.

package com.example.celocoffeeapp;

public class Constant {
   static final String address1 = "0xCB1b7902253a02ac7F977d470681F518AcA362Fa";
   static final String address2 = "0x9799a1530Fc0f0A8FD4Ce036e8B0D99d22Ef9d5a";
   static final String address3 = "0xE1f35f42b09482C30e114C16b9F54d2033d79980";
   static final String address4 = "0x70E7249F304062cACc03A018f574B5C3d0b43466";
   static final String address5 = "0x2f3d7F1804c6A4A8dC8Df271848aA94240F9cdD5";
   static final String myAddress = "0xCb1fe4ABF5E3218f749095D4E950f39264F0485d";
   static final String somePrivateKey = "4dcc51d372e5a8303ddace99a625ecce8b7fcee4e6a88cbab4acc8f5fb39fea9";
   static final String publicKey = "03cbc12902c23a2287365d15a3a2f102f1e2ed5b8712dd0a7b44f797336bd7b50e";
   static final String someAddress = "0x93312861A9844E5E62F3730CAA913DD11adE75b9";
}

Data Source​

This is the local data source, used to populate the data in the recyclerview. The data source can be found in the DataSource.java file.

package com.example.celocoffeeapp;

public class DataSource {
 public static BeneficiaryModel[] myListData = new BeneficiaryModel[] {
           new BeneficiaryModel(
                   R.drawable.img1,
                   "John Doe",
                   "Software Engineer",
                   Constant.address1
          ),
           new BeneficiaryModel(
                   R.drawable.img2,
                   "Ronald Clit",
                   "Product Designer",
                   Constant.address2
           ),
           new BeneficiaryModel(
                   R.drawable.img3,
                   "Allen Smith",
                   " Technical Writer",
                   Constant.address3
           ),
           new BeneficiaryModel(
                   R.drawable.img4,
                   "Justin Cooper",
                   "Celo Developer",
                   Constant.address4
           ),

           new BeneficiaryModel(
                   R.drawable.img5,
                   "Jane Doe",
                   "Dev Rel",
                   Constant.address5
           ),
   };
}

Recyclerview Adapter​

This handles the populating of the list. The Recyclerview adapter can be found in the BeneficiaryAdapter.java file.

package com.example.celocoffeeapp;

import android.annotation.SuppressLint;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import androidx.constraintlayout.widget.ConstraintLayout;
import androidx.recyclerview.widget.RecyclerView;


public class BeneficiaryAdapter extends RecyclerView.Adapter<BeneficiaryAdapter.ViewHolder>{
   private BeneficiaryModel[] listdata;
   private Context context;
   private ItemClickListener itemClickListener;

   public BeneficiaryAdapter(ItemClickListener clicklistener, Context context, BeneficiaryModel[] listdata) {
       this.listdata = listdata;
       this.context = context;
       this.itemClickListener = clicklistener;
   }

   @Override
   public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
       LayoutInflater layoutInflater = LayoutInflater.from(parent.getContext());
       View listItem = layoutInflater.inflate(R.layout.beneficiary_list_item, parent, false);
       ViewHolder viewHolder = new ViewHolder(listItem);
       return viewHolder;
   }

   @Override
   public void onBindViewHolder(ViewHolder holder, @SuppressLint("RecyclerView") int position) {
       final BeneficiaryModel myListData = listdata[position];
       holder.imageView.setImageResource(listdata[position].getImgId());
       holder.name.setText(listdata[position].getName());
       holder.description.setText(listdata[position].getDescription());
holder.donation_received.setText(String.valueOf(String.format("Donations Received: %s", BeneficiaryAccountBalance.balance(position, listdata))));

       holder.button.setText(String.format(listdata[position].getWalletAddress().substring(0, 5) + "..."  + "%s" , "Donate"));
       holder.button.setOnClickListener(new View.OnClickListener() {
           @Override
           public void onClick(View view) {
               Toast.makeText(view.getContext(), "click on item: " + myListData.getName(), Toast.LENGTH_LONG).show();
               itemClickListener.onItemClicked(myListData);
           }
       });
   }

   @Override
   public int getItemCount() {
       return listdata.length;
   }

   public static class ViewHolder extends RecyclerView.ViewHolder {
       public ImageView imageView;
       public TextView name, description, donation_received;
       public Button button;
       public ConstraintLayout constraintLayout;

       public ViewHolder(View itemView) {
           super(itemView);
           this.imageView = (ImageView) itemView.findViewById(R.id.profile_image);
           this.name = (TextView) itemView.findViewById(R.id.name);
           this.description = (TextView) itemView.findViewById(R.id.description);
           this.donation_received = (TextView) itemView.findViewById(R.id.donations_received);
           this.button = (Button) itemView.findViewById(R.id.donateBtn);
           constraintLayout = (ConstraintLayout) itemView.findViewById(R.id.constraint);

       }
   }

   public interface ItemClickListener {
       public void onItemClicked(BeneficiaryModel listData);
   }
}