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 anelement 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 anasync* function, which then returnsa stream. Consuming that stream will lead the function to emit eventsuntil it ends, and the stream closes.You consume a stream either using anawait for loop, which is availableinside anasync orasync* function, or by forwarding its events directlyusingyield* inside anasync* 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 aStream<T> object.Then nothing further happens until someone tries to consume that stream.At that point, the body of theasync* function starts running.If theconvert function was omitted, theyield* will listen to thesource stream and forward all events, date and errors, to the returnedstream. When thesource stream closes, theyield* is done,and theoptionalMap function body ends too. This closes the returnedstream.If aconvertis supplied, the function instead listens on the sourcestream and enters anawait for loop whichrepeatedly waits for the next data event.On a data event, it callsconvert with the value and emits the resulton the returned stream.If no error events are emitted by thesource stream,the loop ends when thesource stream does,then theoptionalMap function body completes,which closes the returned stream.On an error event from thesource stream,theawait for re-throws that error, which breaks the loop.The error then reaches the end of theoptionalMap function body,since it's not caught.That makes the error be emitted on the returned stream, which then closes.

TheStream class also provides functionality which allows you tomanually listen for events from a stream, or to convert a streaminto another stream or into a future.

TheforEach function corresponds to theawait for loop,just asIterable.forEach corresponds to a normalfor/in loop.Like the loop, it will call a function for each data event and break on anerror.

The more low-levellisten method is what every other method is based on.You calllisten on a stream to tell it that you want to receiveevents, and to register the callbacks which will receive those events.When you calllisten, you receive aStreamSubscription objectwhich 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 wholelifetime 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 anasync* 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 afterthe first subscription has been canceled.

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

A broadcast stream allows any number of listeners, and it firesits 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,useasBroadcastStream to create a broadcast stream on top of thenon-broadcast stream.

On either kind of stream, stream transformations, such aswhere andskip, 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 streamcontaining only the events that have not yet been emitted when thelistencall occurs.For example thefirst getter listens to the stream, then returns the firstevent that listener receives.This is not necessarily the first even emitted by the stream, but the firstof theremaining events of the broadcast stream.

When the "done" event is fired, subscribers are unsubscribed beforereceiving the event. After the event has been sent, the stream has nosubscribers. Adding new subscribers to a broadcast stream after this pointis allowed, but they will just receive a new "done" event as soonas possible.

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

The default implementation ofisBroadcast returns false.A broadcast stream inheriting fromStream must overrideisBroadcastto returntrue if it wants to signal that it behaves like a broadcaststream.

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")boolbroadcast})

Creates an empty broadcast stream.

const

factory

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

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

factory

Stream.eventTransformed(Streamsource,EventSinkmapSink(EventSink<T>sink))

Creates a stream where all events of an existing stream are piped througha 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 fromelements.

factory

Stream.multi(voidonListen(MultiStreamController<T>), {boolisBroadcast =false})

Creates a multi-subscription stream.

factory

Stream.periodic(Durationperiod, [Tcomputation(intcomputationCount)?])

Creates a stream that repeatedly emits events atperiod intervals.

factory

Stream.value(Tvalue)

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 setterinherited

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 setterinherited

single→Future<T>

The single element of this stream.

no setter

Methods

any(booltest(Telement))→Future<bool>

Checks whethertest accepts any element provided by this stream.

asBroadcastStream({voidonListen(StreamSubscription<T>subscription)?,voidonCancel(StreamSubscription<T>subscription)?})→Stream<T>

Returns a multi-subscription stream that produces the same events as this.

asyncExpand<E>(Stream<E>?convert(Tevent))→Stream<E>

Transforms each element into a sequence of asynchronous events.

asyncMap<E>(FutureOr<E>convert(Tevent))→Stream<E>

Creates a new stream with each data event of this stream asynchronouslymapped to a new event.

cast<R>()→Stream<R>

Adapt this stream to be aStream<R>.

contains(Object?needle)→Future<bool>

Returns whetherneedle occurs in the elements provided by this stream.

distinct([boolequals(Tprevious,Tnext)?])→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 erroroccurred.

elementAt(intindex)→Future<T>

Returns the value of theindexth data event of this stream.

every(booltest(Telement))→Future<bool>

Checks whethertest accepts all elements provided by this stream.

expand<S>(Iterable<S>convert(Telement))→Stream<S>

Transforms each element of this stream into a sequence of elements.

firstWhere(booltest(Telement), {TorElse()?})→Future<T>

Finds the first element of this stream matchingtest.

fold<S>(SinitialValue,Scombine(Sprevious,Telement))→Future<S>

Combines a sequence of values by repeatedly applyingcombine.

forEach(voidaction(Telement))→Future<void>

Executesaction on each element of this stream.

handleError(FunctiononError, {booltest(dynamicerror)?})→Stream<T>

Creates a wrapper Stream that intercepts some errors from this stream.

join([Stringseparator =""])→Future<String>

Combines the string representation of elements into a single string.

lastWhere(booltest(Telement), {TorElse()?})→Future<T>

Finds the last element in this stream matchingtest.

listen(voidonData(Tevent)?, {Function?onError,voidonDone()?,bool?cancelOnError})→StreamSubscription<T>

Adds a subscription to this stream.

map<S>(Sconvert(Tevent))→Stream<S>

Transforms each element of this stream into a new stream event.

noSuchMethod(Invocationinvocation)→ dynamic

Invoked when a nonexistent method or property is accessed.

inherited

pipe(StreamConsumer<T>streamConsumer)→Future

Pipes the events of this stream intostreamConsumer.

reduce(Tcombine(Tprevious,Telement))→Future<T>

Combines a sequence of values by repeatedly applyingcombine.

singleWhere(booltest(Telement), {TorElse()?})→Future<T>

Finds the single element in this stream matchingtest.

skip(intcount)→Stream<T>

Skips the firstcount data events from this stream.

skipWhile(booltest(Telement))→Stream<T>

Skip data events from this stream while they are matched bytest.

take(intcount)→Stream<T>

Provides at most the firstcount data events of this stream.

takeWhile(booltest(Telement))→Stream<T>

Forwards data events whiletest is successful.

timeout(DurationtimeLimit, {voidonTimeout(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 aList.

toSet()→Future<Set<T>>

Collects the data of this stream in aSet.

toString()→String

A string representation of this object.

inherited

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

AppliesstreamTransformer to this stream.

where(booltest(Tevent))→Stream<T>

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

Operators

operator ==(Objectother)→bool

The equality operator.

inherited

Static Methods

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

Adaptssource to be aStream<T>.