dart:async

Stream<T> class

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 event, a single "done" event will notify the listener that the end has been reached.

You listen on a stream to make it start generating events, and to set up listeners that receive the events. When you 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.

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.

Implementers

• CustomStream
• ElementStream
• HttpClientResponse
• HttpRequest
• HttpServer
• RawDatagramSocket
• RawSecureServerSocket
• RawServerSocket
• RawSocket
• ReceivePort
• SecureServerSocket
• ServerSocket
• Socket
• Stdin
• StreamView
• WebSocket

Constructors

Stream()

Stream.empty()

const

factory

Creates an empty broadcast stream. [...]

Stream.error(Object error, [StackTrace? stackTrace])

factory

Creates a stream which emits a single error event before completing. [...]

Stream.eventTransformed(Stream source, EventSink mapSink(EventSink<T> sink))

factory

Creates a stream where all events of an existing stream are piped through a sink-transformation. [...]

Stream.fromFuture(Future<T> future)

factory

Creates a new single-subscription stream from the future. [...]

Stream.fromFutures(Iterable<Future<T>> futures)

factory

Create a single-subscription stream from a group of futures. [...]

Stream.fromIterable(Iterable<T> elements)

factory

Creates a single-subscription stream that gets its data from elements. [...]

Stream.multi(void onListen(MultiStreamController<T>), {bool isBroadcast = false})

factory

Creates a multi-subscription stream. [...]

Stream.periodic(Duration period, [T computation(int computationCount)])

factory

Creates a stream that repeatedly emits events at period intervals. [...]

Stream.value(T value)

factory

Creates a stream which emits a single data event before completing. [...]

Properties

first → Future<T>

read-only

The first element of this stream. [...]

hashCode → int

read-only, inherited

The hash code for this object. [...]

isBroadcast → bool

read-only

Whether this stream is a broadcast stream.

isEmpty → Future<bool>

read-only

Whether this stream contains any elements. [...]

last → Future<T>

read-only

The last element of this stream. [...]

length → Future<int>

read-only

The number of elements in this stream. [...]

runtimeType → Type

read-only, inherited

A representation of the runtime type of the object.

single → Future<T>

read-only

The single element of this stream. [...]

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 indexth 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

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

inherited

Invoked when a non-existent method or property is accessed. [...]

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

inherited

A string representation of this object. [...]

transform<S>(StreamTransformer<T, S> streamTransformer) → Stream<S>

Applies streamTransformer to this stream. [...]

where(bool test(T event)) → Stream<T>

Creates a new stream from this stream that discards some elements. [...]

Operators

operator ==(Object other) → bool

inherited

The equality operator. [...]

Static Methods

castFrom<S, T>(Stream<S> source) → Stream<T>

Adapts source to be a Stream<T>. [...]