Web3 & DApp Development Fundamentals · 课时

DApp 基础示例

构建一个简单的 DApp，从已部署的智能合约读取数据，并将其显示在网页上。

DApp 基础示例 是 CoddyKit 上的免费 Web3 & DApp Development Fundamentals 课时。这是第 3 节课，共 3 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Web3 & DApp Development Fundamentals 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web3 & DApp Development Fundamentals 课程共包含 3 节课。

本课时的部分内容尚未翻译，以英文显示。

Building Our First DApp

Welcome to building your first Decentralized Application (DApp)! In this lesson, we'll create a simple DApp that reads a message stored on a smart contract and displays it on a web page.

This example will tie together concepts you've learned about smart contracts and front-end interaction.

The Simple Message Contract

First, let's define the smart contract. This contract will be very basic: it just stores a single string variable, myMessage, which we'll make public so its value can be easily read from outside the contract.

Remember, this contract would be compiled and deployed to a blockchain (like Ethereum).

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleMessage {
    string public myMessage = "Hello CoddyKit DApp!";
}

Contract Address & ABI

Once our SimpleMessage contract is deployed to a blockchain, we get two critical pieces of information:

Contract Address: This is like the contract's unique street address on the blockchain.
ABI (Application Binary Interface): This is a JSON description of the contract's functions and variables. It tells our front-end how to 'talk' to the contract.

We'll use these in our web application to connect and read data.

Front-end HTML Structure

Our DApp needs a basic web page to display the message. We'll create an index.html file with a heading and a paragraph where our contract's message will appear. We'll also link our JavaScript file (app.js) and the Web3.js library.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CoddyKit DApp</title>
</head>
<body>
    <h1>DApp Message Reader</h1>
    <p>Message from contract: <span id="contractMessage">Loading...</span></p>
    <script src="https://cdn.jsdelivr.net/npm/web3@1.7.0/dist/web3.min.js"></script>
    <script src="app.js"></script>
</body>
</html>

Connecting with Web3.js

Now for the JavaScript (app.js)! We need to connect our web page to the Ethereum blockchain. The Web3.js library helps us do this. It will look for a provider, like MetaMask, in the user's browser.

// app.js
let web3;

async function initWeb3() {
    if (window.ethereum) {
        web3 = new Web3(window.ethereum);
        try {
            await window.ethereum.request({ method: 'eth_requestAccounts' });
            console.log("Connected to MetaMask!");
        } catch (error) {
            console.error("User denied account access");
        }
    } else if (window.web3) {
        web3 = new Web3(window.web3.currentProvider);
    } else {
        console.log('Non-Ethereum browser detected. Try MetaMask!');
        // Fallback for read-only access (e.g., Infura)
        // web3 = new Web3(new Web3.providers.HttpProvider("YOUR_INFURA_URL"));
    }
}

Instantiating the Contract Object

With Web3.js connected to a provider, we can now create a JavaScript object that represents our deployed SimpleMessage smart contract. We do this using its contract address and ABI.

The ABI tells Web3.js which functions are available on the contract.

// app.js (continued)
const contractAddress = "0xYourDeployedContractAddressHere"; // REPLACE THIS!
const contractABI = [
    {
        "inputs": [],
        "name": "myMessage",
        "outputs": [
            {
                "internalType": "string",
                "name": "",
                "type": "string"
            }
        ],
        "stateMutability": "view",
        "type": "function"
    }
];

let simpleMessageContract;

async function loadContract() {
    if (!web3) await initWeb3();
    simpleMessageContract = new web3.eth.Contract(contractABI, contractAddress);
    console.log("Contract loaded:", simpleMessageContract);
}

Reading Contract Data

Now that we have our contract object, reading the myMessage variable is straightforward. Since myMessage is a public view variable, we can call its auto-generated getter function directly without sending a transaction (which means no gas cost).

// app.js (continued)
async function readMessage() {
    if (!simpleMessageContract) await loadContract();
    try {
        // Call the auto-generated getter for 'myMessage'
        const message = await simpleMessageContract.methods.myMessage().call();
        console.log("Message from contract:", message);
        return message;
    } catch (error) {
        console.error("Error reading message:", error);
        return "Error loading message.";
    }
}

Full DApp JavaScript (app.js)

Here's the complete app.js file that ties everything together. It connects to the blockchain, loads the contract, reads the message, and then updates the HTML to display it when the page loads.

Remember to replace 0xYourDeployedContractAddressHere with your actual deployed contract address!

// app.js
let web3; // Declare web3 globally
const contractAddress = "0xYourDeployedContractAddressHere"; // REPLACE THIS!
const contractABI = [
    {
        "inputs": [],
        "name": "myMessage",
        "outputs": [
            {
                "internalType": "string",
                "name": "",
                "type": "string"
            }
        ],
        "stateMutability": "view",
        "type": "function"
    }
];
let simpleMessageContract;

async function initWeb3() {
    if (window.ethereum) { web3 = new Web3(window.ethereum); await window.ethereum.request({ method: 'eth_requestAccounts' }); }
    else if (window.web3) { web3 = new Web3(window.web3.currentProvider); }
    else { console.log('Non-Ethereum browser detected. Try MetaMask!'); }
}

async function loadContract() {
    if (!web3) await initWeb3();
    simpleMessageContract = new web3.eth.Contract(contractABI, contractAddress);
}

async function readMessage() {
    if (!simpleMessageContract) await loadContract();
    try {
        const message = await simpleMessageContract.methods.myMessage().call();
        return message;
    } catch (error) {
        console.error("Error reading message:", error);
        return "Error loading message.";
    }
}

document.addEventListener('DOMContentLoaded', async () => {
    const messageElement = document.getElementById('contractMessage');
    await initWeb3();
    await loadContract();
    const msg = await readMessage();
    messageElement.innerText = msg;
});

DApp Interaction Check

Which of the following are essential pieces of information needed by a front-end DApp to interact with a deployed smart contract?

Recap: Your First DApp

Congratulations! You've just walked through the process of building a basic DApp.

We defined a simple Solidity smart contract.
We learned about the importance of the contract's address and ABI.
We created an HTML page and used Web3.js to connect to the blockchain.
Finally, we instantiated our contract object and read data from it, displaying it on our web page.

This fundamental pattern of connecting, interacting, and displaying is at the heart of most DApps!

常见问题解答

「DApp 基础示例」课时是免费的吗？

是的 — 「DApp 基础示例」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Web3 & DApp Development Fundamentals 课程的其余内容，请升级到 CoddyKit PRO。 Web3 & DApp Development Fundamentals 课程共包含 3 节课。

「DApp 基础示例」这节课中我会学到什么？

构建一个简单的 DApp，从已部署的智能合约读取数据，并将其显示在网页上。你通过在浏览器中直接运行的动手代码来练习 Web3 & DApp Development Fundamentals，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Web3 & DApp Development Fundamentals 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Web3 & DApp Development Fundamentals 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 3 节课，共 3 节。

「DApp 基础示例」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Web3 & DApp Development Fundamentals 课中编写并运行代码吗？

能。每节 Web3 & DApp Development Fundamentals 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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