Commands and Queries

Mizer uses a Command and Query system to communicate from the ui to the main loop.

Commands

Commands are always executed as part of the main loop. They’re executed by a Processor provided by the CommandExecutorModule in the pre_process stage with priority 0.

Commands have to be reversible.

A command can have a result and if required a state which is made accessible for the revert operation. Commands are also supposed to be serializable as they might be transmitted over the network to the main instance in the session.

Commands have to be registered in crates/runtime/commander/executor/commands.rs as the command_impl! macro will generate an object safe version of every command so they can be stored and reverted.

---
title: Apply Command
---
sequenceDiagram
    Api->>CommandExecutorApi: run_command
    CommandExecutorApi->>CommandExecutor: apply
    critical In Main Loop
        CommandExecutor->>Command: apply
        Command-->>CommandExecutor: state
        Note over Command,CommandExecutor: Stored in CommandExecutor
        Command-->>CommandExecutor: result
    end
    CommandExecutor-->>CommandExecutorApi: result
    CommandExecutorApi-->>Api: result

---
title: Revert Command
---
sequenceDiagram
    Api->>CommandExecutorApi: undo
    CommandExecutorApi->>CommandExecutor: revert
    Note over CommandExecutorApi,CommandExecutor: Previous commands are stored in a history
    critical In Main Loop
        CommandExecutor->>Command: revert with state
        Command-->>CommandExecutor: ()
    end
    CommandExecutor-->>CommandExecutorApi: ()
    CommandExecutorApi-->>Api: ()

Queries

Queries can be run outside the main loop if all dependencies are available outside as well. By default, they will run as part of the main loop as well.

Queries should never modify any state but only return data. The query as well as the data a query returns should be serializable as both might be transmitted over the network.