Stream Controller In Flutter

StreamController :

Stream Type
stream controller’s onListen, onPause and onResume callbacks.

The callback may return either void or a future.

Controller :

• A controller with the stream it controls.
• Controller allows sending data, error and done events on its [stream].
• Possible to check whether the stream is paused or not, and whether it has subscribers or not, as well as getting a callback when either of these change.

A controller with a [stream] that supports only one single subscriber.

• If [sync] is true, the returned stream controller is a [SynchronousStreamController], and must be used with the care and attention necessary to not break the [Stream] contract. If in doubt, use the non-sync version.
• Using an asynchronous controller will never give the wrong behavior, but using a synchronous controller incorrectly can cause otherwise correct programs to break.
• A synchronous controller is only intended for optimizing event propagation when one asynchronous event immediately triggers another. It should not be used unless the calls to [add] or [addError] are guaranteed to occur in places where it won’t break Stream invariants.
• Use synchronous controllers only to forward (potentially transformed) events from another stream or a future.
• A Stream should be inert until a subscriber starts listening on it (using the [onListen] callback to start producing events). Streams should not leak resources (like websockets) when no user ever listens on the stream.
• The controller buffers all incoming events until a subscriber is registered, but this feature should only be used in rare circumstances.
• The [onPause] function is called when the stream becomes paused. [onResume] is called when the stream resumed.
• The [onListen] callback is called when the stream receives its listener and [onCancel] when the listener ends its subscription. If [onCancel] needs to perform an asynchronous operation, [onCancel] should return a future that completes when the cancel operation is done.
• If the stream is canceled before the controller needs new data the [onResume] call might not be executed.

A controller where [stream] can be listened to more than once.

• The [Stream] returned by [stream] is a broadcast stream. It can be listened to more than once.
• Broadcast streams do not buffer events when there is no listener.
• Each listener subscription is handled independently, and if one pauses, only the pausing listener is affected. A paused listener will buffer events internally until unpaused or canceled.

Receive Stream

• Receives events from [source] and puts them into this controller’s stream.
• Events must not be added directly to this controller using [add], [addError], [close] or [addStream], until the returned future is complete.
• Data and error events are forwarded to this controller’s stream. A done event on the source will end the addStream operation and complete the returned future.
• If [cancelOnError] is true, only the first error on [source] is forwarded to the controller’s stream, and the addStream ends after this. If [cancelOnError] is false, all errors are forwarded and only a done event will end the addStream. If [cancelOnError] is omitted, it defaults to false.