Optimism Documentation

OP Stack interop is in active development. Some features may be experimental.

We showcase cross chain composability through the implementation of contests. Leveraging the same underlying mechanism powering TicTacToe, these contests can permissionlessly integrate with the events emitted by any contract on OP Stack chains.

See the frontend documentation for how the contests UI is presented to the user.

How it works

Unlike TicTacToe which is deployed on every participating chain, the contests are deployed on a single L2, behaving like an application-specific OP Stack chain rather than a horizontally scaled app.

Implement contests

Contests.sol contains the implementation of the contests. We won’t go into the details of the implementation here, but instead focus on how the contests can leverage cross chain event reading to compose with other contracts on OP Stack chains.

Read cross-chain events

The system predeploy that enables pulling in validated cross-chain events is the CrossL2Inbox.

contract ICrossL2Inbox {
    function validateMessage(Identifier calldata _id, bytes32 _msgHash) external view;
}

Create the contest

The two contest options are detailed below: BlockHash contest and TicTacToe contest.

BlockHash contest

With the existence of an event that emits the blockhash and height of a block, we can create a contest on the parity of the blockhash being even or odd.

contract BlockHashEmitter {
    event BlockHash(uint256 blockHeight, bytes32 blockHash);

    function emitBlockHash(uint256 _blockHeight) external {
        bytes32 hash = blockhash(_blockHeight);
        require(hash != bytes32(0));

        emit BlockHash(_blockHeight, hash);
    }
}

Integrating this emitter into a contest is extremely simple. The BlockHashContestFactory is a simple factory that creates a new contest for a given chain and block height.

TicTacToe contest

A contest for TicTacToe is created on an accepted game between two players, captured by the emitted AcceptedGame event. When decoding the event, the game is uniquely identified by the chain it was created on, chainId, and the associated gameId. These identifying properties of the game are used to create the resolver for the game.

contract TicTacToeContestFactory {
    Contests  public contests;
    TicTacToe public tictactoe;

    function newContest(Identifier calldata _id, bytes calldata _data) public payable {
        // Validate Log
        require(_id.origin == address(tictactoe), "not an event from the TicTacToe contract");
        CrossL2Inbox(Predeploys.CROSS_L2_INBOX).validateMessage(_id, keccak256(_data));

        bytes32 selector = abi.decode(_data[:32], (bytes32));
        require(selector == TicTacToe.AcceptedGame.selector, "incorrect event");

        // Decode the event data
        (uint256 chainId, uint256 gameId, address creator,) = abi.decode(_data[32:], (uint256, uint256, address, address));

        IContestResolver resolver = new TicTacToeGameResolver(contests, tictactoe, chainId, gameId, creator);
        contests.newContest{ value: msg.value }(resolver, msg.sender);
    }
}

Resolve contest

A contest is identified by and has its outcome determined by the IContestResolver instance. The resolver starts in the UNDECIDED state, updated into YES or NO when resolving itself with the contest.

enum ContestOutcome {
    UNDECIDED,
    YES,
    NO
}

interface IContestResolver {
    function outcome() external returns (ContestOutcome);
}

Resolve BlockHash contest

When live, anyone can resolve the BlockHash contest by simply providing the right BlockHash event to the deployed resolver.

contract BlockHashContestFactory {
    Contests         public contests;
    BlockHashEmitter public emitter; // Same emitter deployed on every chain

    function newContest(uint256 _chainId, uint256 _blockNumber) public payable {
        IContestResolver resolver = new BlockHashResolver(contests, emitter, _chainId, _blockNumber);
        contests.newContest{ value: msg.value }(resolver, msg.sender);
    }
}

contract BlockHashResolver is IContestResolver {
    Contests         public contests;
    ContestOutcome   public outcome;
    BlockHashEmitter public emitter;

    // The target chain & block height
    uint256 public chainId;
    uint256 public blockNumber;

    function resolve(Identifier calldata _id, bytes calldata _data) external {
        require(outcome == ContestOutcome.UNDECIDED);

        // Validate Log
        require(_id.origin == address(emitter), "not an event from the emitter");
        require(_id.chainId == chainId, "must match target chain");
        CrossL2Inbox(Predeploys.CROSS_L2_INBOX).validateMessage(_id, keccak256(_data));

        bytes32 selector = abi.decode(_data[:32], (bytes32));
        require(selector == BlockHashEmitter.BlockHash.selector, "incorrect event");

        // Event should correspond to the right contest
        uint256 dataBlockNumber = abi.decode(_data[32:64], (uint256));
        require(dataBlockNumber == blockNumber, "must match target block height");

        // Resolve the contest (yes if odd, no if even)
        bytes32 blockHash = abi.decode(_data[64:], (bytes32));
        outcome = uint256(blockHash) % 2 != 0 ? ContestOutcome.YES : ContestOutcome.NO;
        contests.resolveContest(this);
    }

}

Resolve TicTacToe contest

When live, anyone can resolve the TicTacToe contest by providing the GameWon or GameDraw event of the associated game from the TicTacToe contract.

contract TicTacToeGameResolver is IContestResolver {
    Contests       public contests;
    ContestOutcome public outcome;
    TicTacToe      public tictactoe;

    // @notice Game for this resolver
    Game public game;

    constructor(Contests _contest, TicTacToe _tictactoe, uint256 _chainId, uint256 _gameId, address _creator) {
        contests = _contest;
        tictactoe = _tictactoe;

        game = Game({chainId: _chainId, gameId: _gameId, creator: _creator});
        outcome = ContestOutcome.UNDECIDED;
    }

    // @notice resolve this game by providing the game ending event
    function resolve(Identifier calldata _id, bytes calldata _data) external {
        // Validate Log
        require(_id.origin == address(tictactoe));
        CrossL2Inbox(Predeploys.CROSS_L2_INBOX).validateMessage(_id, keccak256(_data));

        // Ensure this is a finalizing event
        bytes32 selector = abi.decode(_data[:32], (bytes32));
        require(selector == TicTacToe.GameWon.selector || selector == TicTacToe.GameDraw.selector, "event not a game outcome");

        // Event should correspond to the right game
        (uint256 _chainId, uint256 gameId, address winner,,) = abi.decode(_data[32:], (uint256, uint256, address, uint8, uint8));
        require(_chainId == game.chainId && gameId == game.gameId);

        // Resolve based on if the creator has won (non-draw)
        outcome = winner == game.creator && selector != TicTacToe.GameDraw.selector ? ContestOutcome.YES : ContestOutcome.NO;
        contests.resolveContest(this);
    }
}

Takeaways

Leveraging superchain interop, contracts in the superchain can compose with each other in a similar fashion to how they would on a single chain. No restrictions are placed on the kinds of events a contract can consume via the CrossL2Inbox.
In this example, the BlockHashContestFactory and TicTacToeContestFactory can be seen as just starting points for the Contests app chain. As more contracts and apps are created in the superchain, developers can compose with them in a similar fashion without needing to change the Contests contract at all.

​How it works

​BlockHash contest

​TicTacToe contest

​Resolve BlockHash contest

​Resolve TicTacToe contest

​Takeaways

How it works

BlockHash contest

TicTacToe contest

Resolve BlockHash contest

Resolve TicTacToe contest

Takeaways