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The schemes available for asynchronous communication in Flutter are as follows:

  • Provider
  • ValueNotifier Click to view details
  • The use of the Stream: StreamController details | StreamBuilder components used in combination
  • EventBus
  • Bloc

Stream is an abstract class that represents a sequence of asynchronous data.

  • Single Subscription streams, which have a maximum of one listener

  • Broadcast streams, which can have multiple listeners

A Stream can accept any type of data. A Stream can be a synchronous Stream or an asynchronous Stream. The difference is that a synchronous Stream sends an event immediately after executing add, addError, or close, whereas an asynchronous Stream always sends an event after executing code in the event queue.

1 Multiple subscription streams

The first step is to create a stream object, which is operated on here by a StreamController, StreamController, which will be referred to as a stream in the following articles.

StreamController<String> streamController = StreamController.broadcast();
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StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription = StreamSubscription The StreamSubscription object must be used to close the listener when the page is destroyed, otherwise the listener will not move and will leak memory. Listing 1-1 shows the core code:

/// Listing 1-1
class TestBPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState(a) {
    return_TestBPageState(); }}class _TestBPageState extends State {
  /// Message order object
  StreamSubscription _streamSubscription;
  @override
  void initState(a) {
    super.initState();
    / / / listen 2
    _streamSubscription=streamController.stream.listen((event) {
      print("Page B receives data $event");
    });
  }
  @override
  void dispose(a) {
    super.dispose();
    /// cancel the listener_streamSubscription.cancel(); }... . }Copy the code

StreamSubscription is used to handle current listener events. If there are multiple listener events in a multi-subscription stream, there should be multiple StreamSubscription events. Each individual event listener needs to be handled using a StreamSubscription object, as described in table 3 of this article.

The third step is to send data, such as clicking a button to trigger or network request data results distributed to multiple pages, of course, this is based on the actual business.

streamController.add("Test data");
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The fourth step is to close the stream after the entire operation is complete

 @override
 void dispose(a) {
   super.dispose();
   streamController.close();
 }
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2 Single subscription streams

A single subscription stream is only allowed to have one listener, which is created by default using the StreamController constructor as follows:

/// Create a single subscription flow controller
StreamController<String> streamSingController = StreamController();
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Then set the listener to be consistent with the multi-subscription stream mode:

class _TestBPageState extends State {
  /// Message order object
  StreamSubscription _streamSubscription;
  @override
  void initState(a) {
    super.initState();
    / / / listen 2
    _streamSubscription=streamSingController.stream.listen((event) {
      // Then update the data in this operation
      print("Page B receives data $event");
    });
  }
  @override
  void dispose(a) {
    super.dispose();
    /// cancel the listener_streamSubscription.cancel(); }... . }Copy the code

When a listen listener is added to it, an exception is raised:

· · · · · · · · · · · · · · · · · · · · Builder: Bad state: Stream has already been listened to.Copy the code

3 StreamSubscription

StreamSubscription is used to operate on current listening events. It provides the following operations: Unsubscribe a message. This method is used during page destruction, after which the subscription listener is not reusable

@override
void dispose(a) {
  super.dispose();
  /// unsubscribe messages
  _streamSubscription.cancel();
}
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/// suspend message subscription
if(! _streamSubscription.isPaused){ _streamSubscription.pause(); }Copy the code
/// resume message subscription
if(_streamSubscription.isPaused){
  _streamSubscription.resume();
}
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The completion of

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