Tic-Tac-Toe

This guide covers three different implementations of the game tic-tac-toe on Sui. The first example utilizes a centralized admin that owns the board object and marks it on the users’ behalf. The second example utilizes a shared object that both users can mutate. And the third example utilizes a multisig, where instead of sharing the game board, it's in a 1-of-2 multisig of both users’ accounts.

The guide is divided into three parts that each cover a different implementation of the tic-tac-toe game board:

1. Centralized game board: An admin service that tracks player moves and updates the game board.
2. Shared game board: A shared object that allows players to directly update the game board.
3. Multisig operated game board: A multisig account that acts as the game admin, allowing either player to update the game board directly.

:::info

You can view the complete source code for this app example in the Sui repository.

:::

What the guide teaches

• Owned objects: The guide teaches you how to use owned objects, in this case to act as the game board in the centralized and multisig version of tic-tac-toe. Owned objects are objects that are owned by a single account and can only be modified by that account. In this case, the game board is owned by a game admin, who is responsible for updating the board with each player's move.
• Shared objects: The guide teaches you how to use shared objects, in this case to act as the game board in the more decentralized version of tic-tac-toe. Shared objects are objects that can be modified by multiple accounts. In this case, the game board is shared between the two players, allowing them to update the board directly.
• Multisig accounts: The guide teaches you how to use multisig accounts to share ownership of the game board between two players. Multisig accounts are accounts that require a certain threshold of signatures to authorize a transaction. In this case, the game board is owned by a 1-of-2 multisig account.
• Dynamic object fields: The guide teaches you how to use dynamic object fields, in this case to transfer the actions of the players to the game board, which will be retrieved by the game admin. See [The Move Book](https://move-book.com/programmability/dynamic-object-fields.html to learn more about dynamic object fields.

Directory structure

To begin, create a new folder on your system titled tic-tac-toe that holds all your files.

In this folder, create the following subfolders:

• move to hold the Move code for the game board.
  • sources to hold the Move source files.

Add Move.toml to tic-tac-toe/move/

:::checkpoint

• You have the latest version of Sui installed. If you run sui --version in your terminal or console, it responds with the currently installed version.
• You have a directory to place the files you create in.
• You have created a Move.toml file in the tic-tac-toe/move/ directory.

:::

owned.move {#owned.move}

Create a new file in tic-tac-toe/move/sources titled owned.move. Later, you will update this file to contain the Move code for the game board in the centralized (and multisig) version of tic-tac-toe.

In this first example of tic-tac-toe, the Game object, including the game board, is controlled by a game admin.

Ignore the admin field for now, as it is only relevant for the multisig approach.

Games are created with the new function:

Some things to note:

• MARK__ is a constant that represents an empty cell on the game board. MARK_X and MARK_O represent the two players' markers.
• The first player is sent a TurnCap, which gives them permission to take the next turn.
• This function creates and returns the Game object, it is up to its creator to send it to the game admin to own.

Because the players don’t own the game board object, they cannot directly mutate it. Instead, they indicate their move by creating a Mark object with their intended placement and send it to the game object using transfer to object:

When playing the game, the admin operates a service that keeps track of marks using events. When a request is received (send_mark), the admin tries to place the marker on the board (place_mark). Each move requires two steps (thus two transactions): one from the player and one from the admin. This setup relies on the admin's service to keep the game moving.

When a player sends a mark, a Mark object is created and is sent to the Game object. The admin then receives the mark and places it on the board. This is a use of dynamic object fields, where an object, Game, can hold other objects, Mark.

To view the entire source code, see the owned.move source file. You can find the rest of the logic, including how to check for a winner, as well as deleting the game board after the game concludes there.

owned.move

An alternative version of this game, shared tic-tac-toe, uses shared objects for a more straightforward implementation that doesn't use a centralized service. This comes at a slightly increased cost, as using shared objects is more expensive than transactions involving wholly owned objects.

shared.move {#shared.move}

In the previous version, the admin owned the game object, preventing players from directly changing the gameboard, as well as requiring two transactions for each marker placement. In this version, the game object is a shared object, allowing both players to access and modify it directly, enabling them to place markers in just one transaction. However, using a shared object generally incurs extra costs because Sui needs to sequence the operations from different transactions. In the context of this game, where players are expected to take turns, this shouldn't significantly impact performance. Overall, this shared object approach simplifies the implementation compared to the previous method.

As the following code demonstrates, the Game object in this example is almost identical to the one before it. The only differences are that it does not include an admin field, which is only relevant for the multisig version of the game, and it does not have store, because it only ever exists as a shared object (so it cannot be transferred or wrapped).

Take a look at the new function:

Instead of the game being sent to the game admin, it is instantiated as a shared object. The other notable difference is that there is no need to mint a TurnCap because the only two addresses that can play this game are x and o, and this is checked in the next function, place_mark:

shared.move

Multisig {#multisig}

Multisig tic-tac-toe uses the same Move code as the owned version of the game, but interacts with it differently. Instead of transferring the game to a third party admin account, the players create a 1-of-2 multisig account to act as the game admin, so that either player can sign on behalf of the admin. This pattern offers a way to share a resource between up to ten accounts without relying on consensus.

In this implementation of the game, the game is in a 1-of-2 multisig account that acts as the game admin. In this particular case, because there are only two players, the previous example is a more convenient use case. However, this example illustrates that in some cases, a multisig can replace shared objects, thus allowing transactions to bypass consensus when using such an implementation.

Creating a multisig account

A multisig account is defined by the public keys of its constituent key pairs, their relative weights, and the threshold -- a signature is valid if the sum of weights of constituent keys having signed the signature exceeds the threshold. In our case, there are at most two constituent keypairs, they each have a weight of 1 and the threshold is also 1. A multisig cannot mention the same public key twice, so keys are deduplicated before the multisig is formed to deal with the case where a player is playing themselves:

export function multiSigPublicKey(keys: PublicKey[]): MultiSigPublicKey {
  const deduplicated: { [key: string]: PublicKey } = {};
  for (const key of keys) {
    deduplicated[key.toSuiAddress()] = key;
  }

  return MultiSigPublicKey.fromPublicKeys({
    threshold: 1,
    publicKeys: Object.values(deduplicated).map((publicKey) => {
      return { publicKey, weight: 1 };
    }),
  });
}

MultiSig.ts

Note that an address on Sui can be derived from a public key (this fact is used in the previous example to deduplicate public keys based on their accompanying address), but the opposite is not true. This means that to start a game of multisig tic-tac-toe, players must exchange public keys, instead of addresses.

Building a multisig transaction

When creating a multisig game, we make use of owned::Game's admin field to store the multisig public key for the admin account. Later, it will be used to form the signature for the second transaction in the move. This does not need to be stored on-chain, but we are doing so for convenience so that when we fetch the Game's contents, we get the public key as well:

newMultiSigGame(player: PublicKey, opponent: PublicKey): Transaction {
  const admin = multiSigPublicKey([player, opponent]);
  const tx = new Transaction();

  const game = tx.moveCall({
    target: `${this.packageId}::owned::new`,
    arguments: [
      tx.pure.address(player.toSuiAddress()),
      tx.pure.address(opponent.toSuiAddress()),
      tx.pure(bcs.vector(bcs.u8()).serialize(admin.toRawBytes()).toBytes()),
    ],
  });

  tx.transferObjects([game], admin.toSuiAddress());

  return tx;
}

useTransactions.ts also contains functions to place, send, and receive marks, end the game, and burn completed games. These functions all return a Transaction object, which is used in the React frontend to execute the transaction with the appropriate signer.

useTransactions.ts

Placing a mark

Placing a mark requires two transactions, just like the owned example, but they are both driven by one of the players. The first transaction is executed by the player as themselves, to send the mark to the game, and the second is executed by the player acting as the admin to place the mark they just sent. In the React frontend, this is performed as follows:

function OwnedGame({
  game,
  trophy,
  invalidateGame,
  invalidateTrophy,
}: {
  game: GameData;
  trophy: Trophy;
  invalidateGame: InvalidateGameQuery;
  invalidateTrophy: InvalidateTrophyQuery;
}): ReactElement {
  const adminKey = game.admin ? new MultiSigPublicKey(new Uint8Array(game.admin)) : null;

  const client = useCurrentClient();
  const { mutate: signAndExecute } = useExecutor();
  const { mutate: multiSignAndExecute } = useExecutor({
    execute: ({ bytes, signature }) => {
      const multiSig = adminKey!!.combinePartialSignatures([signature]);
      return client.core.executeTransaction({
        transaction: bytes,
        signatures: [multiSig, signature],
      });
    },
  });

  const [turnCap, invalidateTurnCap] = useTurnCapQuery(game.id);
  const account = useCurrentAccount();
  const tx = useTransactions()!!;

  // ...

  const onMove = (row: number, col: number) => {
    signAndExecute(
      {
        tx: tx.sendMark(turnCap?.data!!, row, col),
        showObjectChanges: true,
      },
      ({ objectChanges }) => {
        const mark = objectChanges?.find(
          (c) => c.type === 'created' && c.objectType.endsWith('::Mark'),
        );

        if (mark && mark.type === 'created') {
          const recv = tx.receiveMark(game, mark);
          recv.setSender(adminKey!!.toSuiAddress());
          recv.setGasOwner(account?.address!!);

          multiSignAndExecute({ tx: recv }, () => {
            invalidateGame();
            invalidateTrophy();
            invalidateTurnCap();
          });
        }
      },
    );
  };

  // ...
}

Game.tsx

The first step is to get the multisig public key, which was written to Game.admin earlier. Then two executor hooks are created: The first is to sign and execute as the current player, and the second is to sign and execute as the multisig/admin account. After the wallet has serialized and signed the transaction the second executor creates a multisig from the wallet signature and executes the transaction with two signatures: Authorizing on behalf of the multisig and the wallet.

The reason for the two signatures is clearer when looking at the construction of the recv transaction: The multisig authorizes access to the Game, and the wallet authorizes access to the gas object. This is because the multisig account does not hold any coins of its own, so it relies on the player account to sponsor the transaction.

You can find an example React front-end supporting both the multi-sig and shared variants of the game in the ui directory, and a CLI written in Rust in the cli directory.