Errors, events and views

Errors

Return an ActionError to fail a transaction:

Constructor	Use it when
`ActionError::invalid(msg)`	the op is malformed or illegal right now (cell taken, bad stake)
`ActionError::unauthorized(msg)`	the sender may not do this (not your turn, not a player)
`ActionError::not_found(msg)`	the thing the op names does not exist (unknown game id)

There are also Insufficient (raised by the chain when a balance is too low), Conflict and Internal. The error text goes into the receipt, so write messages a player can act on: "cell is taken", not "bad".

The transaction is still in the block with ok: false; its state changes are gone and its nonce is spent.

Events

Emit an event for every change a UI cares about, with a namespaced kind:

tictactoe.created {id, x, stake, opponent}
tictactoe.joined {id, o}
tictactoe.moved {id, by, cell, board, turn}
tictactoe.finished {id, result, winner, reason, payout}

Document them at the top of the file, as tic-tac-toe does, because the browser game is written against them.

Views

view receives the read-only state and the query string as a map. Tic-tac-toe supports:

?id=<id>: one game,
?open=1: open games anyone may join (at most 50),
?player=<address>: that player's games,
no parameters: a small description of the module (timeout_blocks, max_stake).

Lists scan open/ or player/<address>/ keys with state.scan_raw, always with a limit, and sort by created_height so the order never depends on storage.