如何在颤动中生成流

Question

I am making some experiment with flutter stream. I have a class for generating stream of int . Here is the class :

class CounterRepository {
  int _counter = 123;

  void increment() {
    _counter++;
  }

  void decrement() {
    _counter--;
  }

  Stream<int> watchCounter() async* {
    yield _counter;
  }
}

I expect with the change of _counter , watchCounter() will yield updated counter value. When I call increment() or decrement() from UI, it seems the value of _counter is changing but watchCounter doesn't yield the updated _counter value. How to yield updated _counter value here? I am using StreamBuilder from UI to get the streamed data.

Answer 1

You have created your streams using -

Stream<int> watchCounter() async* {
    yield _counter;
}

But to reflect the changes of your stream, you need to receive those stream events. You can control those stream events using a StreamController

Creating a stream

Future<void> main() async {
  var stream = watchCounter();
}

Using that stream

stream.listen

Subscribes to the stream by calling the listen function and supplys it with a Function to call back to when there's a new value available.

 stream.listen((value) { print('Value from controller: $value'); });

There are many other approaches to control and manage streams but for your particular question .listen will do the job.

Answer 2

You are missing an infinite while loop, it's a manual stream.

Stream<dynamic> watchCounter() async* {
   while (true) {
      // you can change it to 5 seconds or higher
      await Future.delayed(const Duration(seconds: 1));
      yield _counter;
   }
}

And after that you just need to call listen:

watchCounter().listen((value) {
  // hear can be empty if you want.
});

You can put it in your init state to run, don't put it in your widget build, please.

This should work fine