`LiveValue`

A live "current value" with subscribers: a late subscriber immediately receives the latest value, then every update after it.

class LiveValue<T> { LiveValue(); // empty — subscribers wait for the first add LiveValue.seeded(T value); // starts with a current value LiveValue.from(Stream<T> source); // hot, fed by source LiveValue.seededFrom(T seed, Stream<T> src); // hot, seeded T get value; // latest value; StateError when none set bool get hasValue; bool get isClosed; void add(T value); // set + deliver to subscribers FxEvents<T> get live; // replay the latest, then live updates Stream<T> get stream; // plain-Stream view of live Future<void> close(); } LiveValue<T> Stream<T>.fxLive // = LiveValue.from(stream) LiveValue<T> Stream<T>.fxLiveSeeded(T seed) // = LiveValue.seededFrom(seed, stream)

Lecture

A plain Stream has no memory: subscribe late and you get nothing until the next event, which for state — the current user, the current temperature, the current zoom — means every new screen starts blank. LiveValue<T> is state done as an event source: it holds a current value, add updates it and notifies subscribers, and every late subscriber replays the latest value first, then rides the live updates. No gap, no blank start, no "wait for the next tick".

The API is deliberately small. Construct empty (LiveValue()) or with a seed (LiveValue.seeded(value)). Read synchronously with .value — which throws a StateError when nothing has been set, so check .hasValue or seed it; there is no silent null pretending to be state. Subscribe through .live, which is an FxEvents chain (map it, debounce it, combine it), or .stream for the plain-Stream view of the same feed.

close() ends the feed: subscribers' streams close, and a later add throws — though even a closed LiveValue still replays its last value to a late subscriber before closing their stream. If you know Rx, this is BehaviorSubject reduced to its defining behavior; fxdart events layer, not part of FxTS.

Demo 1 · Late subscribers start from the latest value

Demo 2 · value, hasValue, and close

Method spelling

A Stream reaches both constructors as members: source.fxLive is LiveValue.from(source), and source.fxLiveSeeded(v) is LiveValue.seededFrom(v, source). Both are still hot — the subscription opens on the spot.

final price = ticker.fxLive;
final count = taps.fxLiveSeeded(0);   // has a value before the first tap

Try it yourself

Exercise: derive a label feed from live state.

Related: fxEvents — .live speaks this chain natively · combineLatest — deriving state from two live feeds · Stream bridges — carrying the feed into the pull world