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Context

For developers looking to support custom liquidity mining, Subscriber contracts can be used to receive notifications about position modifications or transfers.

Guide

1. Implement the ISubscriber interface​

Can also refer to MockSubscriber for an actual implementation example.

import {ISubscriber} from "v4-periphery/src/interfaces/ISubscriber.sol";

contract MySubscriber is ISubscriber {
    uint256 public notifySubscribeCount;
    uint256 public notifyUnsubscribeCount;
    uint256 public notifyModifyLiquidityCount;
    uint256 public notifyBurnCount;
    // other implementations...

    function notifySubscribe(uint256, bytes memory) external onlyByPosm {
        notifySubscribeCount++;
    }

    function notifyUnsubscribe(uint256) external onlyByPosm {
        notifyUnsubscribeCount++;
    }

    function notifyModifyLiquidity(uint256, int256, BalanceDelta) external onlyByPosm {
        notifyModifyLiquidityCount++;
    }

    function notifyBurn(uint256, address, PositionInfo, uint256, BalanceDelta)
        external
        onlyByPosm
    {
        notifyBurnCount++;
    }
}

2. A caveat on unsubscribe()​

There is a variable unsubscribeGasLimit specifically set at deployment of PositionManager to prevent gas griefing on unsubscribe() - which could result in the subscriber contract not being notified during unsubscribe().

If notifying the subscriber contract is not necessary users can still specify a gas limit where notifyUnsubscribe() hits OutOfGas and reverts yet the unsubscription will still succeed.

From _unsubscribe() on Notifier:

if (address(_subscriber).code.length > 0) {
    // require that the remaining gas is sufficient to notify the subscriber
    // otherwise, users can select a gas limit where .notifyUnsubscribe hits OutOfGas yet the
    // transaction/unsubscription can still succee
    if (gasleft() < unsubscribeGasLimit) GasLimitTooLow.selector.revertWith();
    try _subscriber.notifyUnsubscribe{gas: unsubscribeGasLimit}(tokenId) {} catch {}
}

3. Opt-in to a subscriber contract​

To opt-in to a subscriber contract, call subscribe() on PositionManager.

import {IPositionManager} from "v4-periphery/src/interfaces/IPositionManager.sol";

IPositionManager posm = IPositionManager(<address>);
ISubscriber mySubscriber = ISubscriber(<address>);

bytes memory optionalData = ...;
posm.subscribe(tokenId, mySubscriber, optionalData);
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