# Subscriber Exercises In this exercise, you'll learn how to write a subscriber contract for the [`PositionManager`](https://github.com/Uniswap/v4-periphery/blob/main/src/PositionManager.sol) contract. The starter code for this exercise is provided in [`foundry/src/exercises/Subscriber.sol`](https://github.com/Cyfrin/defi-uniswap-v4/blob/main/foundry/src/exercises/Subscriber.sol) Solution is in [`foundry/src/solutions/Subscriber.sol`](https://github.com/Cyfrin/defi-uniswap-v4/blob/main/foundry/src/solutions/Subscriber.sol) The `Subscriber` contract mints and burns non-transferable tokens corresponding to the amount of liquidity a user adds or removes. ## Task 1 - Notify subscribe ```solidity function notifySubscribe(uint256 tokenId, bytes memory data) external onlyPositionManager { // Write your code here } ``` - Mint the amount of liquidity locked in `tokenId`, to the owner of `tokenId`. - Store the `PoolId` and the owner of `tokenId` into the state variables `poolIds` and `ownerOf`. These data will be later used by `notifyUnsubscribe` and `notifyBurn`. ## Task 2 - Notify unsubscribe ```solidity function notifyUnsubscribe(uint256 tokenId) external onlyPositionManager { // Write your code here } ``` - Burn all the non-transferable token for `tokenId` - Delete data in `poolIds` and `ownerOf` for `tokenId`. ## Task 3 - Notify burn ```solidity function notifyBurn( uint256 tokenId, address owner, uint256 info, uint256 liquidity, int256 feesAccrued ) external onlyPositionManager { // Write your code here } ``` - Burn all the non-transferable token for `tokenId` - Delete data in `poolIds` and `ownerOf` for `tokenId`. ## Task 4 - Notify modify liquidity ```solidity function notifyModifyLiquidity( uint256 tokenId, int256 liquidityChange, int256 feesAccrued ) external onlyPositionManager { // Write your code here } ``` - Mint additional non-transferable token if `liquidityChange` is positive, otherwise burn. ## Test ```shell forge test --fork-url $FORK_URL --match-path test/Subscriber.test.sol -vvv ```