基本的なDAppの例
デプロイ済みのスマートコントラクトからデータを読み取り、Webページに表示するシンプルなDAppを構築します。
「基本的なDAppの例」はCoddyKit上の無料Web3 & DApp Development Fundamentalsレッスンです。 これはレッスン3/3です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはWeb3 & DApp Development Fundamentals学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Web3 & DApp Development Fundamentalsコースには全3レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
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!
よくある質問
「基本的なDAppの例」レッスンは無料ですか？
はい。「基本的なDAppの例」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Web3 & DApp Development Fundamentalsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Web3 & DApp Development Fundamentalsコースには全3レッスンが含まれています。
「基本的なDAppの例」で何を学びますか？
デプロイ済みのスマートコントラクトからデータを読み取り、Webページに表示するシンプルなDAppを構築します。 ブラウザで直接実行するハンズオンコードでWeb3 & DApp Development Fundamentalsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
Web3 & DApp Development Fundamentalsを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのWeb3 & DApp Development Fundamentalsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン3/3です。
「基本的なDAppの例」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このWeb3 & DApp Development Fundamentalsレッスンでコードを書いて実行できますか？
はい。すべてのWeb3 & DApp Development Fundamentalsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。