The Full Contract

Below we have the complete functioning code example: a contract that can custody Uniswap V3 position NFT's and manipulate the positions and liquidity therein by collecting fees, increasing or decreasing liquidity, and minting new positions. View on github here.

// 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});
    }

    /// @notice Calls the mint function defined in periphery, mints the same amount of each token.
    /// For this example we are providing 1000 DAI and 1000 USDC in liquidity
    /// @return tokenId The id of the newly minted ERC721
    /// @return liquidity The amount of liquidity for the position
    /// @return amount0 The amount of token0
    /// @return amount1 The amount of token1
    function mintNewPosition()
        external
        returns (
            uint256 tokenId,
            uint128 liquidity,
            uint256 amount0,
            uint256 amount1
        )
    {
        // For this example, we will provide equal amounts of liquidity in both assets.
        // Providing liquidity in both assets means liquidity will be earning fees and is considered in-range.
        uint256 amount0ToMint = 1000;
        uint256 amount1ToMint = 1000;

        // transfer tokens to contract
        TransferHelper.safeTransferFrom(DAI, msg.sender, address(this), amount0ToMint);
        TransferHelper.safeTransferFrom(USDC, msg.sender, address(this), amount1ToMint);

        // Approve the position manager
        TransferHelper.safeApprove(DAI, address(nonfungiblePositionManager), amount0ToMint);
        TransferHelper.safeApprove(USDC, address(nonfungiblePositionManager), amount1ToMint);

        INonfungiblePositionManager.MintParams memory params =
            INonfungiblePositionManager.MintParams({
                token0: DAI,
                token1: USDC,
                fee: poolFee,
                tickLower: TickMath.MIN_TICK,
                tickUpper: TickMath.MAX_TICK,
                amount0Desired: amount0ToMint,
                amount1Desired: amount1ToMint,
                amount0Min: 0,
                amount1Min: 0,
                recipient: address(this),
                deadline: block.timestamp
            });

        // Note that the pool defined by DAI/USDC and fee tier 0.3% must already be created and initialized in order to mint
        (tokenId, liquidity, amount0, amount1) = nonfungiblePositionManager.mint(params);

        // Create a deposit
        _createDeposit(msg.sender, tokenId);

        // Remove allowance and refund in both assets.
        if (amount0 < amount0ToMint) {
            TransferHelper.safeApprove(DAI, address(nonfungiblePositionManager), 0);
            uint256 refund0 = amount0ToMint - amount0;
            TransferHelper.safeTransfer(DAI, msg.sender, refund0);
        }

        if (amount1 < amount1ToMint) {
            TransferHelper.safeApprove(USDC, address(nonfungiblePositionManager), 0);
            uint256 refund1 = amount1ToMint - amount1;
            TransferHelper.safeTransfer(USDC, msg.sender, refund1);
        }
    }

    /// @notice Collects the fees associated with provided liquidity
    /// @dev The contract must hold the erc721 token before it can collect fees
    /// @param tokenId The id of the erc721 token
    /// @return amount0 The amount of fees collected in token0
    /// @return amount1 The amount of fees collected in token1
    function collectAllFees(uint256 tokenId) external returns (uint256 amount0, uint256 amount1) {
        // Caller must own the ERC721 position, meaning it must be a deposit

        // set amount0Max and amount1Max to uint256.max to collect all fees
        // alternatively can set recipient to msg.sender and avoid another transaction in `sendToOwner`
        INonfungiblePositionManager.CollectParams memory params =
            INonfungiblePositionManager.CollectParams({
                tokenId: tokenId,
                recipient: address(this),
                amount0Max: type(uint128).max,
                amount1Max: type(uint128).max
            });

        (amount0, amount1) = nonfungiblePositionManager.collect(params);

        // send collected feed back to owner
        _sendToOwner(tokenId, amount0, amount1);
    }

    /// @notice A function that decreases the current liquidity by half. An example to show how to call the `decreaseLiquidity` function defined in periphery.
    /// @param tokenId The id of the erc721 token
    /// @return amount0 The amount received back in token0
    /// @return amount1 The amount returned back in token1
    function decreaseLiquidityInHalf(uint256 tokenId) external returns (uint256 amount0, uint256 amount1) {
        // caller must be the owner of the NFT
        require(msg.sender == deposits[tokenId].owner, 'Not the owner');
        // get liquidity data for tokenId
        uint128 liquidity = deposits[tokenId].liquidity;
        uint128 halfLiquidity = liquidity / 2;

        // amount0Min and amount1Min are price slippage checks
        // if the amount received after burning is not greater than these minimums, transaction will fail
        INonfungiblePositionManager.DecreaseLiquidityParams memory params =
            INonfungiblePositionManager.DecreaseLiquidityParams({
                tokenId: tokenId,
                liquidity: halfLiquidity,
                amount0Min: 0,
                amount1Min: 0,
                deadline: block.timestamp
            });

        (amount0, amount1) = nonfungiblePositionManager.decreaseLiquidity(params);

        //send liquidity back to owner
        _sendToOwner(tokenId, amount0, amount1);
    }

    /// @notice Increases liquidity in the current range
    /// @dev Pool must be initialized already to add liquidity
    /// @param tokenId The id of the erc721 token
    /// @param amount0 The amount to add of token0
    /// @param amount1 The amount to add of token1
    function increaseLiquidityCurrentRange(
        uint256 tokenId,
        uint256 amountAdd0,
        uint256 amountAdd1
    )
        external
        returns (
            uint128 liquidity,
            uint256 amount0,
            uint256 amount1
        ) {

        TransferHelper.safeTransferFrom(deposits[tokenId].token0, msg.sender, address(this), amountAdd0);
        TransferHelper.safeTransferFrom(deposits[tokenId].token1, msg.sender, address(this), amountAdd1);

        TransferHelper.safeApprove(deposits[tokenId].token0, address(nonfungiblePositionManager), amountAdd0);
        TransferHelper.safeApprove(deposits[tokenId].token1, address(nonfungiblePositionManager), amountAdd1);

        INonfungiblePositionManager.IncreaseLiquidityParams memory params = INonfungiblePositionManager.IncreaseLiquidityParams({
            tokenId: tokenId,
            amount0Desired: amountAdd0,
            amount1Desired: amountAdd1,
            amount0Min: 0,
            amount1Min: 0,
            deadline: block.timestamp
        });

        (liquidity, amount0, amount1) = nonfungiblePositionManager.increaseLiquidity(params);

    }

    /// @notice Transfers funds to owner of NFT
    /// @param tokenId The id of the erc721
    /// @param amount0 The amount of token0
    /// @param amount1 The amount of token1
    function _sendToOwner(
        uint256 tokenId,
        uint256 amount0,
        uint256 amount1
    ) internal {
        // get owner of contract
        address owner = deposits[tokenId].owner;

        address token0 = deposits[tokenId].token0;
        address token1 = deposits[tokenId].token1;
        // send collected fees to owner
        TransferHelper.safeTransfer(token0, owner, amount0);
        TransferHelper.safeTransfer(token1, owner, amount1);
    }

    /// @notice Transfers the NFT to the owner
    /// @param tokenId The id of the erc721
    function retrieveNFT(uint256 tokenId) external {
        // must be the owner of the NFT
        require(msg.sender == deposits[tokenId].owner, 'Not the owner');
        // transfer ownership to original owner
        nonfungiblePositionManager.safeTransferFrom(address(this), msg.sender, tokenId);
        //remove information related to tokenId
        delete deposits[tokenId];
    }
}