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FxDart 101 › Раздел 6 · Объединение › fork

fork

Branches one buffered iteration of a source into independent, replayable readers.

Lecture

Iterating the same Dart Iterable object twice normally runs its source twice — a sync* generator restarts from scratch every time you ask for a fresh .iterator. That's wasteful (or outright wrong) when producing a value is expensive: a network fetch, a slow computation, a stream you can only read once. fork fixes this: every call to fork(iterable) with the same iterable object returns an independent cursor over one shared, lazily-growing buffer. The underlying source is walked exactly once, no matter how many forks read from it or in what order.

The sharing is keyed by the identity of the iterable you pass in (via an internal Expando), so you must fork the same object — not two separately-constructed iterables that happen to look alike. Each fork can be consumed at its own pace: reading ahead on one fork pulls new values from the source and appends them to the shared buffer; a fork that's behind just replays values already in the buffer, at no extra cost. forkAsync works the same way for FxAsyncIterable, and additionally lets concurrent downstream demand from multiple forks pull the shared async source in parallel.

Demo 1 · One source, two branches, proven with a counter

source() increments calls every time it produces a value. Both evens and doubled fork the exact same shared object — if the source ran twice, calls would end up at 10, not 5:

Demo 2 · Forks at different paces share one buffer

Branch a races ahead and pulls two fresh values; when branch b asks for its first two, it simply replays what a already buffered — no new calls to source() until b needs a third value neither fork has seen yet:

Try it yourself

Exercise: right now readings is iterated twice with no fork, so sensor() runs twice and reads ends up at 6. Fork readings for each consumer so the sensor is only read once (reads should be 3).