Exemplo Básico de DApp
Crie um DApp simples que leia dados de um contrato inteligente implantado e os exiba em uma página web.
Exemplo Básico de DApp é uma aula grátis de Web3 & DApp Development Fundamentals no CoddyKit. Esta é a aula 3 de 3. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Web3 & DApp Development Fundamentals, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Web3 & DApp Development Fundamentals inclui 3 aulas no total.
Partes desta aula ainda não foram traduzidas e aparecem em inglês.
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!
Perguntas Frequentes
A aula “Exemplo Básico de DApp” é grátis?
Sim — o texto completo de “Exemplo Básico de DApp” é grátis para ler aqui na web. Para praticá-la interativamente (um editor de código integrado e um tutor de IA 24/7) e desbloquear o restante do curso de Web3 & DApp Development Fundamentals, atualize para CoddyKit PRO. O curso de Web3 & DApp Development Fundamentals inclui 3 aulas no total.
O que vou aprender em “Exemplo Básico de DApp”?
Crie um DApp simples que leia dados de um contrato inteligente implantado e os exiba em uma página web. Você pratica Web3 & DApp Development Fundamentals com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.
Preciso ter experiência prévia para começar Web3 & DApp Development Fundamentals?
Nenhuma experiência prévia é necessária. Web3 & DApp Development Fundamentals no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 3 de 3.
Quanto tempo leva a aula “Exemplo Básico de DApp”?
A maioria das aulas CoddyKit leva cerca de 5–10 minutos. Cada uma é compacta e interativa, então você faz progresso constante e retoma exatamente de onde parou entre web e app.
Posso escrever e executar código nesta aula de Web3 & DApp Development Fundamentals?
Sim. Cada aula de Web3 & DApp Development Fundamentals inclui um editor de código integrado, então você escreve e executa código real direto no navegador e recebe feedback de IA instantaneamente — nenhuma configuração local necessária.
Todas as aulas deste curso
- Front-end e Back-end na Web3
- Conexão com Ethereum (Web3.js/Ethers.js)
- Exemplo Básico de DApp