Once

Ensures a reaction runs exactly once, then permanently unbinds itself.

Syntax

on<Trigger<T>, Once>().then([](const T& data) {
    // ...
});

Once can be combined with any triggering word.

Behavior

When a reaction with Once executes for the first time, it automatically unbinds the reaction handle immediately after completion. The reaction is fully unbound — not merely disabled. It will never trigger again, even if the triggering type is re-emitted.

Example

on<Trigger<InitData>, Once>().then([](const InitData& data) {
    // Runs exactly once, then this reaction is removed
    initialize_system(data);
});

Notes

• The reaction is unbound, not disabled. It cannot be re-enabled via its handle.
• Useful for one-time initialization, lazy setup, or responding to a single event.
• Equivalent to manually calling unbind() on the reaction handle at the end of the callback, but declarative.

See Also

• Single — restrict a reaction to one concurrent execution
• Startup — trigger once when the system starts
• Managing Reactions — manually unbind or enable/disable reactions via ReactionHandle