Smart Contracts và DApps Development

// Voting Smart Contract
pragma solidity ^0.8.0;

contract Voting {
    struct Candidate {
        uint id;
        string name;
        uint voteCount;
    }
    
    mapping(uint => Candidate) public candidates;
    mapping(address => bool) public voters;
    uint public candidatesCount;
    
    event VotedEvent(uint indexed candidateId);
    
    constructor() {
        addCandidate("Alice");
        addCandidate("Bob");
    }
    
    function addCandidate(string memory _name) private {
        candidatesCount++;
        candidates[candidatesCount] = Candidate(candidatesCount, _name, 0);
    }
    
    function vote(uint _candidateId) public {
        require(!voters[msg.sender], "Already voted");
        require(_candidateId > 0 && _candidateId <= candidatesCount, "Invalid candidate");
        
        voters[msg.sender] = true;
        candidates[_candidateId].voteCount++;
        
        emit VotedEvent(_candidateId);
    }
}

// Web3.js Integration
const Web3 = require("web3");
const contract = require("../build/contracts/Voting.json");

class VotingService {
    constructor() {
        this.web3 = new Web3(Web3.givenProvider || "http://localhost:7545");
        this.contract = null;
        this.account = null;
    }
    
    async init() {
        const networkId = await this.web3.eth.net.getId();
        const deployedNetwork = contract.networks[networkId];
        this.contract = new this.web3.eth.Contract(
            contract.abi,
            deployedNetwork && deployedNetwork.address
        );
        
        const accounts = await this.web3.eth.getAccounts();
        this.account = accounts[0];
    }
    
    async vote(candidateId) {
        return await this.contract.methods.vote(candidateId)
            .send({ from: this.account });
    }
    
    async getCandidates() {
        const candidatesCount = await this.contract.methods.candidatesCount().call();
        const candidates = [];
        
        for (let i = 1; i <= candidatesCount; i++) {
            const candidate = await this.contract.methods.candidates(i).call();
            candidates.push(candidate);
        }
        
        return candidates;
    }
}