Last¶

Stores the last N values that were provided when the reaction triggered and provides them as an ordered list.

Syntax¶

on<Last<N, Trigger<T>>>()
on<Trigger<T1>, Last<N, With<T2>>>()

Parameters¶

Parameter	Description
`N`	The maximum number of recent values to store.
`DSLWords…`	The DSL word to wrap (`Trigger<T>` or `With<T>`).

Behavior¶

Last maintains a sliding window of the most recent N values that were provided to the reaction when it triggered. When the reaction executes, the callback receives the full window as a std::list<std::shared_ptr<const T>>, ordered oldest first.

If fewer than N triggers have occurred, the list contains only what has been collected so far.
When a new trigger occurs and the window is full, the oldest item is discarded.
When wrapping Trigger, the reaction still fires on every emission of T.
When wrapping With, the list is retrieved as supplementary data without triggering.

Last stores triggered values, not all emissions

`Last` records the value that was present each time the reaction was triggered — it does **not** track every emission of the type.

If you combine Last<N, Trigger<T>> with Single, missed emissions (those that arrived while the reaction was already running) will not appear in the window. This is a subtle but important distinction: Last records "what the reaction saw", not "what was emitted".

graph LR
    subgraph "Window (N=4)"
        direction LR
        A["oldest"] --- B["…"] --- C["…"] --- D["newest"]
    end

    E["reaction triggers"] -->|pushes out oldest| A
    E -->|current value appended| D

Example¶

on<Last<10, Trigger<SensorReading>>>().then(
    [](const std::list<std::shared_ptr<const SensorReading>>& readings) {
        // readings contains up to 10 most recent readings, oldest first
        double avg = 0;
        for (const auto& r : readings) {
            avg += r->value;
        }
        avg /= readings.size();
    });

The reaction fires on each SensorReading emission. The callback receives the last 10 readings (or fewer if not yet emitted 10 times), enabling computations like moving averages.

Notes¶

The list is ordered oldest-first — front() is the oldest item, back() is the most recent.
Items are held as std::shared_ptr<const T>, extending lifetime until they leave the window.
Useful for moving averages, buffering sensor data, and maintaining state history.
If no items have been emitted, the list is empty and the task may be dropped. Use Optional to handle this case.