Set Up Your Contract

Setting up the Contract

This guide is an example of a custodial contract Uniswap V3 positions, which allows interaction with the Uniswap V3 Periphery by minting a position, adding liquidity to a position, decreasing liquidity, and collecting fees.

First, declare the solidity version used to compile the contract and abicoder v2 to allow arbitrary nested arrays and structs to be encoded and decoded in calldata, a feature we use when transacting with a pool.

// SPDX-License-Identifier: GPL-2.0-or-later
pragma solidity =0.7.6;
pragma abicoder v2;

Import the contracts needed from the npm package installation.

import '@uniswap/v3-core/contracts/interfaces/IUniswapV3Pool.sol';
import '@uniswap/v3-core/contracts/libraries/TickMath.sol';
import '@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol';
import '@uniswap/v3-periphery/contracts/interfaces/ISwapRouter.sol';
import '@uniswap/v3-periphery/contracts/interfaces/INonfungiblePositionManager.sol';
import '@uniswap/v3-periphery/contracts/libraries/TransferHelper.sol';
import '@uniswap/v3-periphery/contracts/base/LiquidityManagement.sol';

Create a contract called LiquidityExamples and inherit both IERC721Receiver and LiquidityManagement.

We've chosen to hardcode the token contract addresses and pool fee tiers for our example. In production, you would likely use an input parameter for this, allowing you to change the pools and tokens you are interacting with on a per transaction basis.

contract LiquidityExamples is IERC721Receiver {

    address public constant DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
    address public constant USDC = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;

    uint24 public constant poolFee = 3000;

Declare an immutable public variable nonfungiblePositionManager of type INonfungiblePositionManager.

    INonfungiblePositionManager public immutable nonfungiblePositionManager;

Allowing ERC721 Interactions

Every NFT is identified by a unique uint256 ID inside the ERC-721 smart contract, declared as the tokenId

To allow deposits of ERC721 expressions of liquidity, create a struct called Deposit, a mapping of uint256 to the Deposit struct, then declare that mapping as a public variable deposits.

    struct Deposit {
        address owner;
        uint128 liquidity;
        address token0;
        address token1;
    }

    mapping(uint256 => Deposit) public deposits;

The Constructor

Declare the constructor here, which is executed once when the contract is deployed. Our constructor hard codes the address of the nonfungible position manager interface, V3 router, and the periphery immutable state constructor, which requires the factory and the address of weth9 (the ERC-20 wrapper for ether).

    constructor(
        INonfungiblePositionManager _nonfungiblePositionManager,
        address _factory,
        address _WETH9
    ) PeripheryImmutableState(_factory, _WETH9) {
        nonfungiblePositionManager = _nonfungiblePositionManager;
    }

Allowing custody of ERC721 tokens

To allow the contract to custody ERC721 tokens, implement the onERC721Received function within the inherited IERC721Receiver.sol contract.

The from identifier may be omitted because it is not used.

    function onERC721Received(
        address operator,
        address,
        uint256 tokenId,
        bytes calldata
    ) external override returns (bytes4) {
        // get position information
        _createDeposit(operator, tokenId);
        return this.onERC721Received.selector;
    }

Creating a Deposit

To add a Deposit instance to the deposits mapping, create an internal function called _createDeposit that destructures the positions struct returned by positions in nonfungiblePositionManager.sol. Pass the relevant variables token0 token1 and liquidity to the deposits mapping.

    function _createDeposit(address owner, uint256 tokenId) internal {
        (, , address token0, address token1, , , , uint128 liquidity, , , , ) =
            nonfungiblePositionManager.positions(tokenId);

        // set the owner and data for position
        // operator is msg.sender
        deposits[tokenId] = Deposit({owner: owner, liquidity: liquidity, token0: token0, token1: token1});
    }

The Full Contract Setup

// SPDX-License-Identifier: GPL-2.0-or-later
pragma solidity =0.7.6;
pragma abicoder v2;

import '@uniswap/v3-core/contracts/interfaces/IUniswapV3Pool.sol';
import '@uniswap/v3-core/contracts/libraries/TickMath.sol';
import '@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol';
import '../libraries/TransferHelper.sol';
import '../interfaces/INonfungiblePositionManager.sol';
import '../base/LiquidityManagement.sol';

contract LiquidityExamples is IERC721Receiver {
    address public constant DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
    address public constant USDC = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;

    uint24 public constant poolFee = 3000;

    INonfungiblePositionManager public immutable nonfungiblePositionManager;

    /// @notice Represents the deposit of an NFT
    struct Deposit {
        address owner;
        uint128 liquidity;
        address token0;
        address token1;
    }

    /// @dev deposits[tokenId] => Deposit
    mapping(uint256 => Deposit) public deposits;

    constructor(
        INonfungiblePositionManager _nonfungiblePositionManager
    ) {
        nonfungiblePositionManager = _nonfungiblePositionManager;
    }

    // Implementing `onERC721Received` so this contract can receive custody of erc721 tokens
    function onERC721Received(
        address operator,
        address,
        uint256 tokenId,
        bytes calldata
    ) external override returns (bytes4) {
        // get position information

        _createDeposit(operator, tokenId);

        return this.onERC721Received.selector;
    }

    function _createDeposit(address owner, uint256 tokenId) internal {
        (, , address token0, address token1, , , , uint128 liquidity, , , , ) =
            nonfungiblePositionManager.positions(tokenId);

        // set the owner and data for position
        // operator is msg.sender
        deposits[tokenId] = Deposit({owner: owner, liquidity: liquidity, token0: token0, token1: token1});
    }
}