Example fund raising contract

RaiseFund.sol

contract RaiseFund {
    
    address public owner;
    uint public target;
    
    uint public total;
    bool public ended;
    
    struct Donation {
        address donator;
        uint value;
    }
    
    Donation[] public donations;
    
    constructor(uint _target) {
        owner = msg.sender;
        target = _target;
    }
    
    function donate() public payable {
        require(msg.value > 0);
        require(ended == false);
        
        donations.push(Donation(msg.sender, msg.value));
        total += msg.value;
        
        if (total >= target) {
            ended = true;
            owner.transfer(total);
        }
    }
    
    function refund() public {
        require(msg.sender == owner);
        require(ended == false);
        
        for (uint i = 0; i < donations.length; i += 1) {
            Donation memory donation = donations[i];
            donation.donator.transfer(donation.value);
        }
    }
}