aave-protocol-v2/contracts/flashloan/base/FlashLoanReceiverBase.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.8;

import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "../interfaces/IFlashLoanReceiver.sol";
import "../../interfaces/ILendingPoolAddressesProvider.sol";
import "../../libraries/UniversalERC20.sol";

abstract contract FlashLoanReceiverBase is IFlashLoanReceiver {

    using UniversalERC20 for IERC20;
    using SafeMath for uint256;

    ILendingPoolAddressesProvider public addressesProvider;

    constructor(ILendingPoolAddressesProvider _provider) public {
        addressesProvider = _provider;
    }

    receive() external payable {}

    function transferFundsBackToPoolInternal(address _reserve, uint256 _amount) internal {

        address payable pool = payable(addressesProvider.getLendingPool());

        transferInternal(pool,_reserve, _amount);
    }

    function transferInternal(address payable _destination, address _reserve, uint256  _amount) internal {
        if(IERC20(_reserve).isETH()) {
            //solium-disable-next-line
            _destination.call{value: _amount}("");
            return;
        }

        IERC20(_reserve).universalTransfer(_destination, _amount);


    }

    function getBalanceInternal(address _target, address _reserve) internal view returns(uint256) {
        if(IERC20(_reserve).isETH()) {

            return _target.balance;
        }

        return IERC20(_reserve).balanceOf(_target);

    }
}