description

# Stream<T> class abstract mixin

A source of asynchronous data events.

A Stream provides a way to receive a sequence of events. Each event is either a data event, also called an element of the stream, or an error event, which is a notification that something has failed. When a stream has emitted all its events, a single "done" event notifies the listener that the end has been reached.

You produce a stream by calling an `async*` function, which then returns a stream. Consuming that stream will lead the function to emit events until it ends, and the stream closes. You consume a stream either using an `await for` loop, which is available inside an `async` or `async*` function, or by forwarding its events directly using `yield*` inside an `async*` function. Example:

    Stream<T> optionalMap<T>(
        Stream<T> source , [T Function(T)? convert]) async* {
      if (convert == null) {
        yield* source;
      } else {
        await for (var event in source) {
          yield convert(event);
        }
      }
    }

When this function is called, it immediately returns a `Stream<T>` object. Then nothing further happens until someone tries to consume that stream. At that point, the body of the `async*` function starts running. If the `convert` function was omitted, the `yield*` will listen to the `source` stream and forward all events, date and errors, to the returned stream. When the `source` stream closes, the `yield*` is done, and the `optionalMap` function body ends too. This closes the returned stream. If a `convert` is supplied, the function instead listens on the source stream and enters an `await for` loop which repeatedly waits for the next data event. On a data event, it calls `convert` with the value and emits the result on the returned stream. If no error events are emitted by the `source` stream, the loop ends when the `source` stream does, then the `optionalMap` function body completes, which closes the returned stream. On an error event from the `source` stream, the `await for` re-throws that error, which breaks the loop. The error then reaches the end of the `optionalMap` function body, since it's not caught. That makes the error be emitted on the returned stream, which then closes.

The `Stream` class also provides functionality which allows you to manually listen for events from a stream, or to convert a stream into another stream or into a future.

The forEach function corresponds to the `await for` loop, just as Iterable.forEach corresponds to a normal `for`/`in` loop. Like the loop, it will call a function for each data event and break on an error.

The more low-level listen method is what every other method is based on. You call `listen` on a stream to tell it that you want to receive events, and to register the callbacks which will receive those events. When you call `listen`, you receive a StreamSubscription object which is the active object providing the events, and which can be used to stop listening again, or to temporarily pause events from the subscription.

There are two kinds of streams: "Single-subscription" streams and "broadcast" streams.

A single-subscription stream allows only a single listener during the whole lifetime of the stream. It doesn't start generating events until it has a listener, and it stops sending events when the listener is unsubscribed, even if the source of events could still provide more. The stream created by an `async*` function is a single-subscription stream, but each call to the function creates a new such stream.

Listening twice on a single-subscription stream is not allowed, even after the first subscription has been canceled.

Single-subscription streams are generally used for streaming chunks of larger contiguous data, like file I/O.

A broadcast stream allows any number of listeners, and it fires its events when they are ready, whether there are listeners or not.

Broadcast streams are used for independent events/observers.

If several listeners want to listen to a single-subscription stream, use asBroadcastStream to create a broadcast stream on top of the non-broadcast stream.

On either kind of stream, stream transformations, such as where and skip, return the same type of stream as the one the method was called on, unless otherwise noted.

When an event is fired, the listener(s) at that time will receive the event. If a listener is added to a broadcast stream while an event is being fired, that listener will not receive the event currently being fired. If a listener is canceled, it immediately stops receiving events. Listening on a broadcast stream can be treated as listening on a new stream containing only the events that have not yet been emitted when the listen call occurs. For example the first getter listens to the stream, then returns the first event that listener receives. This is not necessarily the first even emitted by the stream, but the first of the remaining events of the broadcast stream.

When the "done" event is fired, subscribers are unsubscribed before receiving the event. After the event has been sent, the stream has no subscribers. Adding new subscribers to a broadcast stream after this point is allowed, but they will just receive a new "done" event as soon as possible.

Stream subscriptions always respect "pause" requests. If necessary they need to buffer their input, but often, and preferably they can simply request their input to pause too.

The default implementation of isBroadcast returns false. A broadcast stream inheriting from Stream must override isBroadcast to return `true` if it wants to signal that it behaves like a broadcast stream.

Implementers
  - CustomStream
  - ElementStream
  - HttpClientResponse
  - HttpRequest
  - HttpServer
  - RawDatagramSocket
  - RawSecureServerSocket
  - RawServerSocket
  - RawSocket
  - ReadPipe
  - ReceivePort
  - SecureServerSocket
  - ServerSocket
  - Socket
  - Stdin
  - StreamView
  - WebSocket
Annotations
  - @vmIsolateUnsendable

## Constructors

Stream()
  const
Stream.empty({@Since.new("3.2") bool broadcast})
  Creates an empty broadcast stream.
  const
  factory
Stream.error(Object error, [StackTrace? stackTrace])
  Creates a stream which emits a single error event before completing.
  factory
Stream.eventTransformed(Stream source, EventSink mapSink(EventSink<T> sink))
  Creates a stream where all events of an existing stream are piped through a sink-transformation.
  factory
Stream.fromFuture(Future<T> future)
  Creates a new single-subscription stream from the future.
  factory
Stream.fromFutures(Iterable<Future<T>> futures)
  Create a single-subscription stream from a group of futures.
  factory
Stream.fromIterable(Iterable<T> elements)
  Creates a stream that gets its data from `elements`.
  factory
Stream.multi(void onListen(MultiStreamController<T>), {bool isBroadcast = false})
  Creates a multi-subscription stream.
  factory
Stream.periodic(Duration period, [T computation(int computationCount)?])
  Creates a stream that repeatedly emits events at `period` intervals.
  factory
Stream.value(T value)
  Creates a stream which emits a single data event before closing.
  factory

## Properties

first → Future<T>
  The first element of this stream.
  no setter
hashCode → int
  The hash code for this object.
  no setter inherited
isBroadcast → bool
  Whether this stream is a broadcast stream.
  no setter
isEmpty → Future<bool>
  Whether this stream contains any elements.
  no setter
last → Future<T>
  The last element of this stream.
  no setter
length → Future<int>
  The number of elements in this stream.
  no setter
runtimeType → Type
  A representation of the runtime type of the object.
  no setter inherited
single → Future<T>
  The single element of this stream.
  no setter

## Methods

any(bool test(T element)) → Future<bool>
  Checks whether `test` accepts any element provided by this stream.
asBroadcastStream({void onListen(StreamSubscription<T> subscription)?, void onCancel(StreamSubscription<T> subscription)?}) → Stream<T>
  Returns a multi-subscription stream that produces the same events as this.
asyncExpand<E>(Stream<E>? convert(T event)) → Stream<E>
  Transforms each element into a sequence of asynchronous events.
asyncMap<E>(FutureOr<E> convert(T event)) → Stream<E>
  Creates a new stream with each data event of this stream asynchronously mapped to a new event.
cast<R>() → Stream<R>
  Adapt this stream to be a `Stream<R>`.
contains(Object? needle) → Future<bool>
  Returns whether `needle` occurs in the elements provided by this stream.
distinct([bool equals(T previous, T next)?]) → Stream<T>
  Skips data events if they are equal to the previous data event.
drain<E>([E? futureValue]) → Future<E>
  Discards all data on this stream, but signals when it is done or an error occurred.
elementAt(int index) → Future<T>
  Returns the value of the `index`th data event of this stream.
every(bool test(T element)) → Future<bool>
  Checks whether `test` accepts all elements provided by this stream.
expand<S>(Iterable<S> convert(T element)) → Stream<S>
  Transforms each element of this stream into a sequence of elements.
firstWhere(bool test(T element), {T orElse()?}) → Future<T>
  Finds the first element of this stream matching `test`.
fold<S>(S initialValue, S combine(S previous, T element)) → Future<S>
  Combines a sequence of values by repeatedly applying `combine`.
forEach(void action(T element)) → Future<void>
  Executes `action` on each element of this stream.
handleError(Function onError, {bool test(dynamic error)?}) → Stream<T>
  Creates a wrapper Stream that intercepts some errors from this stream.
join([String separator = ""]) → Future<String>
  Combines the string representation of elements into a single string.
lastWhere(bool test(T element), {T orElse()?}) → Future<T>
  Finds the last element in this stream matching `test`.
listen(void onData(T event)?, {Function? onError, void onDone()?, bool? cancelOnError}) → StreamSubscription<T>
  Adds a subscription to this stream.
map<S>(S convert(T event)) → Stream<S>
  Transforms each element of this stream into a new stream event.
noSuchMethod(Invocation invocation) → dynamic
  Invoked when a nonexistent method or property is accessed.
  inherited
pipe(StreamConsumer<T> streamConsumer) → Future
  Pipes the events of this stream into `streamConsumer`.
reduce(T combine(T previous, T element)) → Future<T>
  Combines a sequence of values by repeatedly applying `combine`.
singleWhere(bool test(T element), {T orElse()?}) → Future<T>
  Finds the single element in this stream matching `test`.
skip(int count) → Stream<T>
  Skips the first `count` data events from this stream.
skipWhile(bool test(T element)) → Stream<T>
  Skip data events from this stream while they are matched by `test`.
take(int count) → Stream<T>
  Provides at most the first `count` data events of this stream.
takeWhile(bool test(T element)) → Stream<T>
  Forwards data events while `test` is successful.
timeout(Duration timeLimit, {void onTimeout(EventSink<T> sink)?}) → Stream<T>
  Creates a new stream with the same events as this stream.
toList() → Future<List<T>>
  Collects all elements of this stream in a List.
toSet() → Future<Set<T>>
  Collects the data of this stream in a Set.
toString() → String
  A string representation of this object.
  inherited
transform<S>(StreamTransformer<T, S> streamTransformer) → Stream<S>
  Applies `streamTransformer` to this stream.
where(bool test(T element)) → Stream<T>
  Creates a new stream from this stream that discards some elements.

## Operators

operator ==(Object other) → bool
  The equality operator.
  inherited

## Static Methods

castFrom<S, T>(Stream<S> source) → Stream<T>
  Adapts `source` to be a `Stream<T>`.