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Getting Started

In this guide, we will write a smart contract that calls flash on a V3 pool and swaps the full amount withdrawn of token0 and token1 in the corresponding pools with the same token pair - but different fee tiers. After the swap, the contract will pay back the first pool and transfer profits to the original calling address.

Flash Transactions Overview​

Flash transactions are an approach to transferring tokens on Ethereum that transfer token balances before the necessary conditions are met for those balances to be transferred. In the context of a swap, this would mean the output is sent from the swap before the input is received.

Uniswap V3 introduces a new function, flash, within the Pool contract. Flash withdraws a specified amount of both token0 and token1 to the recipient address. The withdrawn amount, plus the swap fees, will be due to the pool at the end of the transaction. flash includes a fourth parameter, data, which allows the caller to abi.encode any necessary data to be passed through the function and decoded later.

    function flash(
        address recipient,
        uint256 amount0,
        uint256 amount1,
        bytes calldata data
    ) external override lock noDelegateCall {

The Flash Callback​

flash will withdraw the tokens, but how are they paid back? To understand this, we must look inside the flash function code. midway through the flash function, we see this:

IUniswapV3FlashCallback(msg.sender).uniswapV3FlashCallback(fee0, fee1, data);

This step calls the FlashCallback function on msg.sender - which passes the fee data needed to calculate the balances due to the pool, as well as any data encoded into the data parameter.

In V3 there are three separate callback functions, uniswapV3SwapCallback, uniswapV3MintCallback, and uniswapV3FlashCallback, each available to be overridden with custom logic. To write our arbitrage contract, we'll be calling flash and overriding the uniswapV3FlashCallback with the steps needed to finish executing our transaction.

Inheriting The V3 Contracts​

Inherit IUniswapV3FlashCallback and PeripheryPayments, as we will use each in our program. Note these two inherited contracts already extend many other contracts that we will be using, such as LowGasSafeMath which we attach, to types uint256 and int256.

contract PairFlash is IUniswapV3FlashCallback, PeripheryPayments {
    using LowGasSafeMath for uint256;
    using LowGasSafeMath for int256;

Declare an immutable public variable swapRouter of type ISwapRouter:

    ISwapRouter public Immutable swapRouter;

Declare the constructor here, which is executed once when the contract is deployed. Our constructor hardcodes the address of the V3 router, factory, and the address of weth9, the ERC-20 wrapper for ether.

    constructor(
        ISwapRouter _swapRouter,
        address _factory,
        address _WETH9
    ) PeripheryImmutableState(_factory, _WETH9) {
        swapRouter = _swapRouter;
    }

The full import section and contract declaration:

pragma solidity =0.7.6;
pragma abicoder v2;

import '@uniswap/v3-core/contracts/interfaces/callback/IUniswapV3FlashCallback.sol';
import '@uniswap/v3-core/contracts/libraries/LowGasSafeMath.sol';

import '@uniswap/v3-periphery/contracts/base/PeripheryPayments.sol';
import '@uniswap/v3-periphery/contracts/base/PeripheryImmutableState.sol';
import '@uniswap/v3-periphery/contracts/libraries/PoolAddress.sol';
import '@uniswap/v3-periphery/contracts/libraries/CallbackValidation.sol';
import '@uniswap/v3-periphery/contracts/libraries/TransferHelper.sol';
import '@uniswap/v3-periphery/contracts/interfaces/ISwapRouter.sol';



contract PairFlash is IUniswapV3FlashCallback, PeripheryPayments {
    using LowGasSafeMath for uint256;
    using LowGasSafeMath for int256;

    ISwapRouter public immutable swapRouter;

    constructor(
        ISwapRouter _swapRouter,
        address _factory,
        address _WETH9
    ) PeripheryImmutableState(_factory, _WETH9) {
        swapRouter = _swapRouter;
    }
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