Single

Limits a reaction to a single concurrent execution. Equivalent to Buffer<1>.

Syntax

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

Behavior

Only one instance of the reaction may be active (running or queued) at any time. If a new trigger arrives while the reaction is already in progress, the new task is dropped.

sequenceDiagram
    participant S as Scheduler
    participant R as Reaction

    S->>R: Trigger 1 → executes
    S--xR: Trigger 2 → dropped (reaction busy)
    S--xR: Trigger 3 → dropped (reaction busy)
    R-->>S: Trigger 1 completes
    S->>R: Trigger 4 → executes

Example

on<Trigger<Frame>, Single>().then([](const Frame& f) {
    // Only one instance runs at a time
    // If a new Frame arrives while this is processing, it's dropped
    process_frame(f);
});

A common use case is preventing work from piling up when processing is slower than the input rate (e.g. frame processing, expensive computations).

Notes

• Single is literally Buffer<1> — it is a typedef/inheritance of Buffer with size 1.
• Dropped tasks are silently discarded; no error or log is emitted.

See Also

• Buffer — generalised form allowing N concurrent executions
• Sync — mutual exclusion across different reactions sharing a group
• Once — reaction that fires only a single time then is removed