TokenSwap.sol

// SPDX-License-Identifier: MIT
//
// https://cryptomarketpool.com/trustless-token-swap-in-a-solidity-smart-contract/

pragma solidity ^0.8.0;

interface IERC20 {
    function totalSupply() external view returns (uint256);

    function balanceOf(address account) external view returns (uint256);

    function transfer(address recipient, uint256 amount) external returns (bool);

    function allowance(address owner, address spender) external view returns (uint256);

    function approve(address spender, uint256 amount) external returns (bool);

    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) external returns (bool);

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract TokenSwap {
    IERC20 public token1;
    IERC20 public token2;
    address public owner1;
    address public owner2;

    constructor(
        address _token1,
        address _owner1,
        address _token2,
        address _owner2
    ) {
        token1 = IERC20(_token1);
        owner1 = _owner1;
        token2 = IERC20(_token2);
        owner2 = _owner2;
    }

    //this function will allow 2 people to trade 2 tokens as the same time (atomic) and swap them between accounts
    //Bob holds token 1 and needs to send to alice
    //Alice holds token 2 and needs to send to Bob
    //this allows them to swap an amount of both tokens at the same time

    //*** Important ***
    //this contract needs an allowance to send tokens at token 1 and token 2 that is owned by owner 1 and owner 2

    function swap(uint256 _amount1, uint256 _amount2) public {
        require(msg.sender == owner1 || msg.sender == owner2, "Not authorized");
        require(token1.allowance(owner1, address(this)) >= _amount1, "Token 1 allowance too low");
        require(token2.allowance(owner2, address(this)) >= _amount2, "Token 2 allowance too low");

        _safeTransferFrom(token1, owner1, owner2, _amount1);
        _safeTransferFrom(token2, owner2, owner1, _amount2);
    }

    function _safeTransferFrom(
        IERC20 token,
        address sender,
        address recipient,
        uint256 amount
    ) private {
        bool sent = token.transferFrom(sender, recipient, amount);
        require(sent, "Token transfer failed");
    }
}