Crowd Fund (Solidity Code Example)

Code example illustrating how to crowd fund with an ERC-20 token:

1. User creates a campaign.
2. Users can pledge an amount and transfer their tokens to a campaign.
3. After the campaign ends, the campaign creator can claim the funds if the total amount pledged is more than the campaign goal.
4. Otherwise, the campaign did not reach its goal and users can withdraw their pledge.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

interface IERC20 {
    function transfer(address, uint256) external returns (bool);
    function transferFrom(address, address, uint256) external returns (bool);
}

contract CrowdFund {
    event Launch(
        uint256 id,
        address indexed creator,
        uint256 goal,
        uint32 startAt,
        uint32 endAt
    );
    event Cancel(uint256 id);
    event Pledge(uint256 indexed id, address indexed caller, uint256 amount);
    event Unpledge(uint256 indexed id, address indexed caller, uint256 amount);
    event Claim(uint256 id);
    event Refund(uint256 id, address indexed caller, uint256 amount);

    struct Campaign {
        // Creator of campaign
        address creator;
        // Amount of tokens to raise
        uint256 goal;
        // Total amount pledged
        uint256 pledged;
        // Timestamp of start of campaign
        uint32 startAt;
        // Timestamp of end of campaign
        uint32 endAt;
        // True if goal was reached and creator has claimed the tokens.
        bool claimed;
    }

    IERC20 public immutable token;
    // Total count of campaigns created.
    // It is also used to generate id for new campaigns.
    uint256 public count;
    // Mapping from id to Campaign
    mapping(uint256 => Campaign) public campaigns;
    // Mapping from campaign id => pledger => amount pledged
    mapping(uint256 => mapping(address => uint256)) public pledgedAmount;

    constructor(address _token) {
        token = IERC20(_token);
    }

    function launch(uint256 _goal, uint32 _startAt, uint32 _endAt) external {
        require(_startAt >= block.timestamp, "start at < now");
        require(_endAt >= _startAt, "end at < start at");
        require(_endAt <= block.timestamp + 90 days, "end at > max duration");

        count += 1;
        campaigns[count] = Campaign({
            creator: msg.sender,
            goal: _goal,
            pledged: 0,
            startAt: _startAt,
            endAt: _endAt,
            claimed: false
        });

        emit Launch(count, msg.sender, _goal, _startAt, _endAt);
    }

    function cancel(uint256 _id) external {
        Campaign memory campaign = campaigns[_id];
        require(campaign.creator == msg.sender, "not creator");
        require(block.timestamp < campaign.startAt, "started");

        delete campaigns[_id];
        emit Cancel(_id);
    }

    function pledge(uint256 _id, uint256 _amount) external {
        Campaign storage campaign = campaigns[_id];
        require(block.timestamp >= campaign.startAt, "not started");
        require(block.timestamp <= campaign.endAt, "ended");

        campaign.pledged += _amount;
        pledgedAmount[_id][msg.sender] += _amount;
        token.transferFrom(msg.sender, address(this), _amount);

        emit Pledge(_id, msg.sender, _amount);
    }

    function unpledge(uint256 _id, uint256 _amount) external {
        Campaign storage campaign = campaigns[_id];
        require(block.timestamp <= campaign.endAt, "ended");

        campaign.pledged -= _amount;
        pledgedAmount[_id][msg.sender] -= _amount;
        token.transfer(msg.sender, _amount);

        emit Unpledge(_id, msg.sender, _amount);
    }

    function claim(uint256 _id) external {
        Campaign storage campaign = campaigns[_id];
        require(campaign.creator == msg.sender, "not creator");
        require(block.timestamp > campaign.endAt, "not ended");
        require(campaign.pledged >= campaign.goal, "pledged < goal");
        require(!campaign.claimed, "claimed");

        campaign.claimed = true;
        token.transfer(campaign.creator, campaign.pledged);

        emit Claim(_id);
    }

    function refund(uint256 _id) external {
        Campaign memory campaign = campaigns[_id];
        require(block.timestamp > campaign.endAt, "not ended");
        require(campaign.pledged < campaign.goal, "pledged >= goal");

        uint256 bal = pledgedAmount[_id][msg.sender];
        pledgedAmount[_id][msg.sender] = 0;
        token.transfer(msg.sender, bal);

        emit Refund(_id, msg.sender, bal);
    }
}