How to Implement a Hook

Hooks can be plugged into the Uniswap V4 ecosystem by extending the standard Hooks.Permissions and BaseHook contracts. The developer specifies the conditions for the custom hook at key moments like beforeSwap, afterSwap, or afterAddLiquidity (and all other base hooks).

```solidity
 function afterSwap(
        address,
        PoolKey calldata key,
        IPoolManager.SwapParams calldata params,
        BalanceDelta delta,
        bytes calldata hookData
    )
        external
        override
        onlyPoolManager
        onlyRegisteredPools(key)
        returns (bytes4, int128)
    {
        (address user, int24 tickLower, int24 tickUpper) = abi.decode(
            hookData,
            (address, int24, int24)
        );

        PoolId poolId = key.toId();
        ICustomPoolHook customHook = customPoolHooks[poolId];

        /* ------------------------------- Hooks ! the CUSTOM HOOKS -------------------------------- */

        if (address(customHook) != address(0)) {
            require(
                address(customHook).supportsInterface(
                    type(ICustomPoolHook).interfaceId
                ),
                "Custom hook does not implement ICustomPoolHook"
            );

            ICustomPoolHook.CustomHookResult memory result = customHook
                .afterSwap(user, key, params, delta, hookData);

            ICustomPoolHook.CustomHookResult memory result2 = customHook
                .whenTickRangeIsBetween(
                    user,
                    key,
                    -60,
                    60,
                    params,
                    delta,
                    hookData
                );

            ICustomPoolHook.CustomHookResult memory result3 = customHook
                .whenTheWeatherIsRainy(user, key, params, delta, hookData);

            bytes[] memory results = new bytes[](3);
            results[0] = result.extraData;
            results[1] = result2.extraData;
            results[3] = result3.extraData;
            ICustomPoolHook.CustomHookResult memory __result4 = customHook
                .whenHooksChained(user, key, params, delta, hookData, "genesis", results);
            // customResult can be used for further processing...
        }

        /* ------------------------------- Hooks ! the CUSTOM HOOKS -------------------------------- */
```