# 智能合约案例-众筹

**Published by:** [suncle.eth](https://paragraph.com/@suncle/)
**Published on:** 2021-11-17
**URL:** https://paragraph.com/@suncle/Wm7r51RxVqbry0ImnJBR

## Content

一个众筹的智能示范，来源于学习xinyminutes 主要是展示智能合约状态机（状态机）和函数功能器（修改器）的使用 // CrowdFunder.sol pragma solidity ^0.6.6; /// @title CrowdFunder /// @author nemild contract CrowdFunder { // Variables set on create by creator address public creator; address payable public fundRecipient; // creator may be different than recipient, and must be payable uint public minimumToRaise; // required to tip, else everyone gets refund string campaignUrl; byte version = "1"; // Data structures enum State { Fundraising, ExpiredRefund, Successful } struct Contribution { uint amount; address payable contributor; } // State variables State public state = State.Fundraising; // initialize on create uint public totalRaised; uint public raiseBy; uint public completeAt; Contribution[] contributions; event LogFundingReceived(address addr, uint amount, uint currentTotal); event LogWinnerPaid(address winnerAddress); modifier inState(State _state) { require(state == _state); _; } modifier isCreator() { require(msg.sender == creator); _; } // Wait 24 weeks after final contract state before allowing contract destruction modifier atEndOfLifecycle() { require(((state == State.ExpiredRefund || state == State.Successful) && completeAt + 24 weeks < now)); _; } function crowdFund( uint timeInHoursForFundraising, string memory _campaignUrl, address payable _fundRecipient, uint _minimumToRaise) public { creator = msg.sender; fundRecipient = _fundRecipient; campaignUrl = _campaignUrl; minimumToRaise = _minimumToRaise; raiseBy = now + (timeInHoursForFundraising * 1 hours); } function contribute() public payable inState(State.Fundraising) returns(uint256 id) { contributions.push( Contribution({ amount: msg.value, contributor: msg.sender }) // use array, so can iterate ); totalRaised += msg.value; emit LogFundingReceived(msg.sender, msg.value, totalRaised); checkIfFundingCompleteOrExpired(); return contributions.length - 1; // return id } function checkIfFundingCompleteOrExpired() public { if (totalRaised > minimumToRaise) { state = State.Successful; payOut(); // could incentivize sender who initiated state change here } else if ( now > raiseBy ) { state = State.ExpiredRefund; // backers can now collect refunds by calling getRefund(id) } completeAt = now; } function payOut() public inState(State.Successful) { fundRecipient.transfer(address(this).balance); LogWinnerPaid(fundRecipient); } function getRefund(uint256 id) inState(State.ExpiredRefund) public returns(bool) { require(contributions.length > id && id >= 0 && contributions[id].amount != 0 ); uint256 amountToRefund = contributions[id].amount; contributions[id].amount = 0; contributions[id].contributor.transfer(amountToRefund); return true; } function removeContract() public isCreator() atEndOfLifecycle() { selfdestruct(msg.sender); // creator gets all money that hasn't be claimed } } 修饰器 函数修饰器(Modifiers)可以用来改变一个函数的行为。比如用于在函数执行前检查某种前置条件。这个和python的修饰器(Decorators)的作用很类似，在python中，我们也经常使用装饰器对函数执行前后增加一些逻辑。下面是solidity修饰器的简单使用，在众筹支付前需要检查合约状态是否已完成 function payOut() public inState(State.Successful) {_;} python中一个函数可以有多个装饰器，solidity中的函数也是可以有多个修饰器的。 如果同一个函数有多个修饰器，他们之间以空格隔开，修饰器会依次检查执行。 状态机 状态State在合约中本质是一个合约的全局变量，在实际合约中，状态会有很多种，各个合约方法也会对State做修改，并且根据State执行不同的逻辑。对于多个State可以通过枚举管理 enum State { Fundraising, // 筹款中 ExpiredRefund, // 过期退款 Successful // 众筹成功 }

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