StreamsChannel for Flutter plugin development

StreamsChannel is inspired from EventChannel. It allows to create streams of events between Flutter and platform side.

Rationale

EventChannel allows to only create a single open stream per channel. On the first subscription, the stream will be open and it's possible to pass arguments from Flutter to platform side. Subsequent subscriptions will either re-use the open stream or override it with a new one.

In order to have multiple streams open at the same time, with different parameters for stream initialization, one has to create multiple EventChannel. And it gets complicated if the number of streams is dynamic.

Installation

Follow the instructions: https://pub.dev/packages/streams_channel#-installing-tab-

How it works

StreamsChannel works in a similar way than EventChannel. The difference is that it supports multiple open streams at the same time.

On the platform side, the channel takes a stream handler factory:

let channel = FlutterStreamsChannel(name: "my_channel", binaryMessenger: plugin.registrar.messenger())
channel.setStreamHandlerFactory { arguments in
  return MyHandler(arguments) // MyHandler is an instance FlutterStreamHandler
}

On the flutter side, each call to receiveBroadcastStream(args) will create a new stream, that won't replace any previous one that is still running.

1. The first subscription to the stream will create a new stream handler using the factory on the platform side, then trigger onListen on the handler;
2. The last subscription to the stream will trigger onCancel on the handler, then destroy the handler instance
3. If a new first subscription happens afterwards, repeat step 1.

Under the hood, StreamsChannel tracks each stream with a simple auto-incremented unique id.

Example

Flutter side

final StreamsChannel channel = new StreamsChannel('some_channel');

// continuous stream of events from platform side, match some args
channel.receiveBroadcastStream('some args')
  .listen((data) {
    // do something
  });

// another continuous stream of events from platform side, matching some other args
channel.receiveBroadcastStream('some other args')
  .listen((data) {
    // do something
  });

Platform side

Android

public class DemoPlugin {
  public static void registerWith(PluginRegistry.Registrar registrar) {
    final StreamsChannel channel = new StreamsChannel(registrar.messenger(), "streams_channel_test");
    channel.setStreamHandlerFactory(new StreamsChannel.StreamHandlerFactory() {
      @Override
      public EventChannel.StreamHandler create(Object arguments) {
        return new StreamHandler();
      }
    });
  }

  // Send "Hello" 10 times, every second, then ends the stream
  public static class StreamHandler implements EventChannel.StreamHandler {

    private final Handler handler = new Handler();
    private final Runnable runnable = new Runnable() {
      @Override
      public void run() {
        if (count > 10) {
            eventSink.endOfStream();
        } else {
            eventSink.success("Hello " + count + "/10");
        }
        count++;
        handler.postDelayed(this, 1000);
      }
    };

    private EventChannel.EventSink eventSink;
    private int count = 1;

    @Override
    public void onListen(Object o, final EventChannel.EventSink eventSink) {
      this.eventSink = eventSink;
      runnable.run();
    }

    @Override
    public void onCancel(Object o) {
      handler.removeCallbacks(runnable);
    }
  }
}

iOS

@interface StreamHandler : NSObject<FlutterStreamHandler>
  @property(strong, nonatomic) NSTimer *timer;
  @property(assign, nonatomic) NSInteger count;
@end

@implementation DemoPlugin

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {

  FlutterStreamsChannel *channel = [FlutterStreamsChannel streamsChannelWithName:@"streams_channel_test" binaryMessenger:registrar.messenger];
  [channel setStreamHandlerFactory:^NSObject<FlutterStreamHandler> *(id arguments) {
    return [StreamHandler new];
  }];
}

@end

// Send "Hello" 10 times, every second, then ends the stream
@implementation StreamHandler

- (FlutterError *)onListenWithArguments:(id)arguments eventSink:(FlutterEventSink)events {
  self.count = 1;
  self.timer = [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
    if(self.count > 10) {
      events(FlutterEndOfEventStream);
    } else {
      events([NSString stringWithFormat:@"Hello %ld/10", (long)self.count]);
      self.count++;
    }
  }];

  return nil;
}

- (FlutterError *)onCancelWithArguments:(id)arguments {
  [self.timer invalidate];
  self.timer = nil;
  return nil;
}
@end

Author

Beacons plugin is developed by Loup, a mobile development studio based in Montreal and Paris.
You can contact us at hello@loup.app

License

Apache License 2.0