`sampleOn`

Emits the newest source value each time the trigger stream fires — the source sets the values, the trigger sets the rhythm.

FxEvents<T> FxEvents<T>.sampleOn(Stream<void> trigger) // chain (events)

Lecture

A sensor updates two hundred times a second; the display repaints sixty. Processing every update is wasted work — what the consumer actually wants is the newest value, on its own schedule. sampleOn(trigger) is exactly that split: the source stream supplies values, the trigger stream supplies moments, and each trigger event emits the latest source value.

Two details keep it honest. A trigger that fires when nothing new has arrived stays silent — you never see the same value twice in a row just because the clock ticked. And values that were superseded between triggers are dropped, not queued: this is a lossy operator by design, for state-like streams where only the newest reading matters.

Lifetime follows the source: when the source closes, the chain closes and the trigger subscription is cancelled — an endless Stream.periodic tick makes a perfectly good trigger. Compare the neighbors: throttle rate-limits with a fixed window measured from the source's own events; sampleOn hands the schedule to a second stream entirely. fxdart events layer, after Rx's sample.

Demo 1 · The trigger sets the rhythm

Demo 2 · Silent when nothing is new, closes with the source

Try it yourself

Exercise: one drag position per frame.

Related: throttle — rate limiting from the source's own timing · debounce — waiting for quiet instead of sampling · withLatestFrom — the same "latest value" idea, but combining two data streams