TransferHelper.sol

// SPDX-License-Identifier: BUSL-1.1
pragma solidity 0.7.6;

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

/**
 * @title TransferHelper
 * @dev Wrappers around ERC20 operations that returns the value received by recipent and the actual allowance of approval.
 * To use this library you can add a `using TransferHelper for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
 library TransferHelper{
    // using SafeMath for uint;

    function safeTransfer(IERC20 _token, address _to, uint _amount) internal returns (uint amountReceived){
        if (_amount > 0){
            uint balanceBefore = _token.balanceOf(_to);
            address(_token).call(abi.encodeWithSelector(_token.transfer.selector, _to, _amount));
            uint balanceAfter = _token.balanceOf(_to);
            require(balanceAfter > balanceBefore, "TF");
            amountReceived = balanceAfter - balanceBefore;
        }
    }

    function safeTransferFrom(IERC20 _token, address _from, address _to, uint _amount) internal returns (uint amountReceived){
        if (_amount > 0){
            uint balanceBefore = _token.balanceOf(_to);
            address(_token).call(abi.encodeWithSelector(_token.transferFrom.selector, _from, _to, _amount));
            // _token.transferFrom(_from, _to, _amount);
            uint balanceAfter = _token.balanceOf(_to);
            require(balanceAfter > balanceBefore, "TFF");
            amountReceived = balanceAfter - balanceBefore;
        }
    }

    function safeApprove(IERC20 _token, address _spender, uint256 _amount) internal returns (uint) {
        bool success;
        if (_token.allowance(address(this), _spender) != 0){
            (success, ) = address(_token).call(abi.encodeWithSelector(_token.approve.selector, _spender, 0));
            require(success, "AF");
        }
        (success, ) = address(_token).call(abi.encodeWithSelector(_token.approve.selector, _spender, _amount));
        require(success, "AF");

        return _token.allowance(address(this), _spender);
    }

    // function safeIncreaseAllowance(IERC20 _token, address _spender, uint256 _amount) internal returns (uint) {
    //     uint256 allowanceBefore = _token.allowance(address(this), _spender);
    //     uint256 allowanceNew = allowanceBefore.add(_amount);
    //     uint256 allowanceAfter = safeApprove(_token, _spender, allowanceNew);
    //     require(allowanceAfter == allowanceNew, "AF");
    //     return allowanceNew;
    // }

    // function safeDecreaseAllowance(IERC20 _token, address _spender, uint256 _amount) internal returns (uint) {
    //     uint256 allowanceBefore = _token.allowance(address(this), _spender);
    //     uint256 allowanceNew = allowanceBefore.sub(_amount);
    //     uint256 allowanceAfter = safeApprove(_token, _spender, allowanceNew);
    //     require(allowanceAfter == allowanceNew, "AF");
    //     return allowanceNew;
    // }
}