Creating a Flash Loan using Aave

Flashloan.sol

pragma solidity ^0.6.6;

import "./FlashLoanReceiverBase.sol";
import "./ILendingPoolAddressesProvider.sol";
import "./ILendingPool.sol";

contract Flashloan is FlashLoanReceiverBase {

    constructor(address _addressProvider) FlashLoanReceiverBase(_addressProvider) public {}

    /**
        This function is called after your contract has received the flash loaned amount
     */
    function executeOperation(
        address _reserve,
        uint256 _amount,
        uint256 _fee,
        bytes calldata _params
    )
        external
        override
    {
        require(_amount <= getBalanceInternal(address(this), _reserve), "Invalid balance, was the flashLoan successful?");

        //
        // Your logic goes here.
        // !! Ensure that *this contract* has enough of `_reserve` funds to payback the `_fee` !!
        //

        uint totalDebt = _amount.add(_fee);
        transferFundsBackToPoolInternal(_reserve, totalDebt);
    }

    /**
        Flash loan 1000000000000000000 wei (1 ether) worth of `_asset`
     */
    function flashloan(address _asset) public onlyOwner {
        bytes memory data = "";
        uint amount = 1 ether;

        ILendingPool lendingPool = ILendingPool(addressesProvider.getLendingPool());
        lendingPool.flashLoan(address(this), _asset, amount, data);
    }
}

FlashLoanReceiverBase.sol

pragma solidity ^0.6.6;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/math/SafeMath.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/token/ERC20/IERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/token/ERC20/SafeERC20.sol";
import "./IFlashLoanReceiver.sol";
import "./ILendingPoolAddressesProvider.sol";
import "./Withdrawable.sol";

abstract contract FlashLoanReceiverBase is IFlashLoanReceiver, Withdrawable {

    using SafeERC20 for IERC20;
    using SafeMath for uint256;

    address constant ethAddress = 0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE;
    ILendingPoolAddressesProvider public addressesProvider;

    constructor(address _addressProvider) public {
        addressesProvider = ILendingPoolAddressesProvider(_addressProvider);
    }

    receive() payable external {}

    function transferFundsBackToPoolInternal(address _reserve, uint256 _amount) internal {
        address payable core = addressesProvider.getLendingPoolCore();
        transferInternal(core, _reserve, _amount);
    }

    function transferInternal(address payable _destination, address _reserve, uint256 _amount) internal {
        if(_reserve == ethAddress) {
            (bool success, ) = _destination.call{value: _amount}("");
            require(success == true, "Couldn't transfer ETH");
            return;
        }
        IERC20(_reserve).safeTransfer(_destination, _amount);
    }

    function getBalanceInternal(address _target, address _reserve) internal view returns(uint256) {
        if(_reserve == ethAddress) {
            return _target.balance;
        }
        return IERC20(_reserve).balanceOf(_target);
    }
}

IFlashLoanReceiver.sol

pragma solidity ^0.6.6;

/**
* @title IFlashLoanReceiver interface
* @notice Interface for the Aave fee IFlashLoanReceiver.
* @author Aave
* @dev implement this interface to develop a flashloan-compatible flashLoanReceiver contract
**/
interface IFlashLoanReceiver {
    function executeOperation(address _reserve, uint256 _amount, uint256 _fee, bytes calldata _params) external;
}

ILendingPool.sol

pragma solidity ^0.6.6;

interface ILendingPool {
  function addressesProvider () external view returns ( address );
  function deposit ( address _reserve, uint256 _amount, uint16 _referralCode ) external payable;
  function redeemUnderlying ( address _reserve, address _user, uint256 _amount ) external;
  function borrow ( address _reserve, uint256 _amount, uint256 _interestRateMode, uint16 _referralCode ) external;
  function repay ( address _reserve, uint256 _amount, address _onBehalfOf ) external payable;
  function swapBorrowRateMode ( address _reserve ) external;
  function rebalanceFixedBorrowRate ( address _reserve, address _user ) external;
  function setUserUseReserveAsCollateral ( address _reserve, bool _useAsCollateral ) external;
  function liquidationCall ( address _collateral, address _reserve, address _user, uint256 _purchaseAmount, bool _receiveAToken ) external payable;
  function flashLoan ( address _receiver, address _reserve, uint256 _amount, bytes calldata _params ) external;
  function getReserveConfigurationData ( address _reserve ) external view returns ( uint256 ltv, uint256 liquidationThreshold, uint256 liquidationDiscount, address interestRateStrategyAddress, bool usageAsCollateralEnabled, bool borrowingEnabled, bool fixedBorrowRateEnabled, bool isActive );
  function getReserveData ( address _reserve ) external view returns ( uint256 totalLiquidity, uint256 availableLiquidity, uint256 totalBorrowsFixed, uint256 totalBorrowsVariable, uint256 liquidityRate, uint256 variableBorrowRate, uint256 fixedBorrowRate, uint256 averageFixedBorrowRate, uint256 utilizationRate, uint256 liquidityIndex, uint256 variableBorrowIndex, address aTokenAddress, uint40 lastUpdateTimestamp );
  function getUserAccountData ( address _user ) external view returns ( uint256 totalLiquidityETH, uint256 totalCollateralETH, uint256 totalBorrowsETH, uint256 availableBorrowsETH, uint256 currentLiquidationThreshold, uint256 ltv, uint256 healthFactor );
  function getUserReserveData ( address _reserve, address _user ) external view returns ( uint256 currentATokenBalance, uint256 currentUnderlyingBalance, uint256 currentBorrowBalance, uint256 principalBorrowBalance, uint256 borrowRateMode, uint256 borrowRate, uint256 liquidityRate, uint256 originationFee, uint256 variableBorrowIndex, uint256 lastUpdateTimestamp, bool usageAsCollateralEnabled );
  function getReserves () external view;
}

ILendingPoolAddressesProvider.sol

pragma solidity ^0.6.6;

/**
    @title ILendingPoolAddressesProvider interface
    @notice provides the interface to fetch the LendingPoolCore address
 */

interface ILendingPoolAddressesProvider {
    function getLendingPoolCore() external view returns (address payable);
    function getLendingPool() external view returns (address);
}

Withdrawable.sol

pragma solidity ^0.6.6 <= 0.9.0;

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/token/ERC20/ERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/token/ERC20/SafeERC20.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/docs-v3.x/contracts/access/Ownable.sol";

/**
    Ensures that any contract that inherits from this contract is able to
    withdraw funds that are accidentally received or stuck.
 */
 
contract Withdrawable is Ownable {
    using SafeERC20 for ERC20;
    address constant ETHER = address(0);

    event LogWithdraw(
        address indexed _from,
        address indexed _assetAddress,
        uint amount
    );

    /**
     * @dev Withdraw asset.
     * @param _assetAddress Asset to be withdrawn.
     */
    function withdraw(address _assetAddress) public onlyOwner {
        uint assetBalance;
        if (_assetAddress == ETHER) {
            address self = address(this); // workaround for a possible solidity bug
            assetBalance = self.balance;
            msg.sender.transfer(assetBalance);
        } else {
            assetBalance = ERC20(_assetAddress).balanceOf(address(this));
            ERC20(_assetAddress).safeTransfer(msg.sender, assetBalance);
        }
        emit LogWithdraw(msg.sender, _assetAddress, assetBalance);
    }
}