如何写个众筹合约

编写众筹合约涉及使用 Solidity 语言来定义智能合约。以下是一个简单的众筹合约示例，基于以太坊的 ERC-20 代币标准。请注意，这只是一个基础示例，实际应用中可能需要更多的安全性和功能。

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

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contract Crowdfunding {
    address public owner;
    IERC20 public token; // 使用的代币合约地址
    uint256 public goal; // 众筹目标
    uint256 public deadline; // 截止日期
    uint256 public raisedAmount; // 已筹集金额
    mapping(address => uint256) public contributions; // 参与者的贡献记录
    bool public fundingGoalReached = false; // 众筹目标是否达成
    bool public crowdsaleClosed = false; // 众筹是否关闭

    event Contribution(address indexed contributor, uint256 amount);
    event FundingGoalReached(uint256 amountRaised);
    event CrowdsaleClosed();

    modifier onlyOwner() {
        require(msg.sender == owner, "Only the owner can call this function");
        _;
    }

    modifier notClosed() {
        require(!crowdsaleClosed, "Crowdsale is closed");
        _;
    }

    modifier afterDeadline() {
        require(block.timestamp >= deadline, "Crowdsale deadline has not passed");
        _;
    }

    constructor(
        address _token,
        uint256 _goal,
        uint256 _durationInMinutes
    ) {
        owner = msg.sender;
        token = IERC20(_token);
        goal = _goal;
        deadline = block.timestamp + _durationInMinutes * 1 minutes;
    }

    function contribute(uint256 _amount) external notClosed {
        require(block.timestamp < deadline, "Crowdsale has ended");
        require(raisedAmount + _amount <= goal, "Contribution exceeds the goal");

        token.transferFrom(msg.sender, address(this), _amount);
        contributions[msg.sender] += _amount;
        raisedAmount += _amount;

        emit Contribution(msg.sender, _amount);

        if (raisedAmount >= goal) {
            fundingGoalReached = true;
            emit FundingGoalReached(raisedAmount);
        }
    }

    function withdraw() external afterDeadline {
        require(fundingGoalReached, "Funding goal not reached");
        uint256 amount = contributions[msg.sender];
        contributions[msg.sender] = 0;
        token.transfer(msg.sender, amount);
    }

    function closeCrowdsale() external onlyOwner {
        require(!crowdsaleClosed, "Crowdsale already closed");
        crowdsaleClosed = true;
        emit CrowdsaleClosed();
    }
}
```

上述合约使用了 OpenZeppelin 的 ERC20 合约库，确保了在代币交互方面的安全性。在实际使用中，你需要引入相关的库文件，可以通过 [OpenZeppelin GitHub](https://github.com/OpenZeppelin/openzeppelin-contracts) 获取。

此众筹合约支持以下功能：

1. 设置众筹目标、截止日期和代币。
2. 参与者可以通过 `contribute` 函数贡献代币。
3. 在达到众筹目标后，参与者可以通过 `withdraw` 函数提取代币。
4. 合约拥有者可以通过 `closeCrowdsale` 函数关闭众筹。

请确保在实际使用前仔细测试和审查合约，以确保其符合项目需求并且安全可靠。

具体可以参照下面的合约适当修改，仅供学习参考。

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
 
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 
contract Crowdfunding {
    address public owner;
    IERC20 public token; // 使用的代币合约地址
    uint256 public goal; // 众筹目标
    uint256 public deadline; // 截止日期
    uint256 public raisedAmount; // 已筹集金额
    mapping(address => uint256) public contributions; // 参与者的贡献记录
    bool public fundingGoalReached = false; // 众筹目标是否达成
    bool public crowdsaleClosed = false; // 众筹是否关闭
 
    event Contribution(address indexed contributor, uint256 amount);
    event FundingGoalReached(uint256 amountRaised);
    event CrowdsaleClosed();
 
    modifier onlyOwner() {
        require(msg.sender == owner, "Only the owner can call this function");
        _;
    }
 
    modifier notClosed() {
        require(!crowdsaleClosed, "Crowdsale is closed");
        _;
    }
 
    modifier afterDeadline() {
        require(block.timestamp >= deadline, "Crowdsale deadline has not passed");
        _;
    }
 
    constructor(
        address _token,
        uint256 _goal,
        uint256 _durationInMinutes
    ) {
        owner = msg.sender;
        token = IERC20(_token);
        goal = _goal;
        deadline = block.timestamp + _durationInMinutes * 1 minutes;
    }
 
    function contribute(uint256 _amount) external notClosed {
        require(block.timestamp < deadline, "Crowdsale has ended");
        require(raisedAmount + _amount <= goal, "Contribution exceeds the goal");
 
        token.transferFrom(msg.sender, address(this), _amount);
        contributions[msg.sender] += _amount;
        raisedAmount += _amount;
 
        emit Contribution(msg.sender, _amount);
 
        if (raisedAmount >= goal) {
            fundingGoalReached = true;
            emit FundingGoalReached(raisedAmount);
        }
    }
 
    function withdraw() external afterDeadline {
        require(fundingGoalReached, "Funding goal not reached");
        uint256 amount = contributions[msg.sender];
        contributions[msg.sender] = 0;
        token.transfer(msg.sender, amount);
    }
 
    function closeCrowdsale() external onlyOwner {
        require(!crowdsaleClosed, "Crowdsale already closed");
        crowdsaleClosed = true;
        emit CrowdsaleClosed();
    }
}