Stream<T> class Null safety

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 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 forwards 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 that error is (re-)thrown 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 registers 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.