How to make cross shard contract calls

Via the Calimero bridge, cross shard calls can be executed with a callback. Meaning that a contract on Calimero can call into contracts on NEAR and get a callback. Also, contracts on NEAR can call into Calimero contracts and get a callback.

An example DAPP that is deployed on NEAR testnet and makes cross shard calls to Calimero and back is the tic-tac-toe game whose contracts can be found here

Once both players register for playing on the contract on NEAR testnet, the game gets started on Calimero, a cross shard call is executed denoting that two players from NEAR testnet want to start a game on Calimero:

near call tictactoe.igi.testnet register_player --accountId igi.testnet
near call tictactoe.igi.testnet register_player --accountId mikimaus.testnet

Players registered for a game: playerA and playerB

Once the second player registered for a game, a CALIMERO_EVENT_CROSS_CALL event is emitted via which the arguments to call on Calimero shard are transitted. The bridge service gets this events and tries to prove on the cross shard connector on Calimero that a game of tic tac toe needs to start. If proved, start_game method is called on tic tac toe contract on Calimero. Immediatelly after that a CALIMERO_EVENT_CROSS_RESPONSE event is emitted from the cross shard connector on Calimero. Once proved on NEAR, the callback method game_started is called. Here is the transaction showing the executed callback.

Similarly, you can see how a game_ended was called from Calimero to NEAR testnet here

And now the final result of the game player on Calimero can be viewed on NEAR testnet:

near view tictactoe.igi.testnet get_finished_game --args '{"game_id":0}'
View call: tictactoe.igi.testnet.get_finished_game({"game_id":0})
{
  board: [ [ 'O', 'X', 'X' ], [ 'O', 'U', 'U' ], [ 'O', 'U', 'U' ] ],
  player_a: 'igi.testnet',
  player_b: 'igi.testnet',
  status: 'PlayerAWon',
  player_a_turn: false
}