Flutter Riverpod StateNotifierProvider: only watch changes for part of a model

问题

I have a model like this:

class TimerModel {
  const TimerModel(this.timeLeft, this.buttonState);
  final String timeLeft;
  final ButtonState buttonState;
}

enum ButtonState {
  initial,
  started,
  paused,
  finished,
}

And here is the StateNotifierProvider:

class TimerNotifier extends StateNotifier<TimerModel> {
  TimerNotifier() : super(_initialState);

  static const int _initialDuration = 10;
  static final _initialState = TimerModel(
    _durationString(_initialDuration),
    ButtonState.initial,
  );

  final Ticker _ticker = Ticker();
  StreamSubscription<int> _tickerSubscription;

  void start() {
    if (state.buttonState == ButtonState.paused) {
      _tickerSubscription?.resume();
      state = TimerModel(state.timeLeft, ButtonState.started);
    } else {
      _tickerSubscription?.cancel();
      _tickerSubscription =
          _ticker.tick(ticks: _initialDuration).listen((duration) {
        state = TimerModel(_durationString(duration), ButtonState.started);
      });
      _tickerSubscription.onDone(() { 
        state = TimerModel(state.timeLeft, ButtonState.finished);
      });
      state =
          TimerModel(_durationString(_initialDuration), ButtonState.started);
    }
  }

  static String _durationString(int duration) {
    final String minutesStr =
        ((duration / 60) % 60).floor().toString().padLeft(2, '0');
    final String secondsStr =
        (duration % 60).floor().toString().padLeft(2, '0');
    return '$minutesStr:$secondsStr';
  }

  void pause() {
    _tickerSubscription?.pause();
    state = TimerModel(state.timeLeft, ButtonState.paused);
  }

  void reset() {
    _tickerSubscription?.cancel();
    state = _initialState;
  }

  @override
  void dispose() {
    _tickerSubscription?.cancel();
    super.dispose();
  }
}

class Ticker {
  Stream<int> tick({int ticks}) {
    return Stream.periodic(Duration(seconds: 1), (x) => ticks - x - 1)
        .take(ticks);
  }
}

I can listen for all changes in state like this:

final timerProvider = StateNotifierProvider<TimerNotifier>((ref) => TimerNotifier());

However I want to make another provider that only listens for changes in the ButtonState. This doesn't work:

final buttonProvider = StateProvider<ButtonState>((ref) {
  return ref.watch(timerProvider.state).buttonState;
});

because it still returns all the state changes.

This also doesn't work:

final buttonProvider = StateProvider<ButtonState>((ref) {
  return ref.watch(timerProvider.state.buttonState);
});

Because the state object doesn't have a buttonState property.

How do I only watch buttonState changes?

回答1:

Using watch gives a new state whenever the watched state changes. So can solve the problem in two parts like so:

final _buttonState = Provider<ButtonState>((ref) {
  return ref.watch(timerProvider.state).buttonState;
});

Using this provider will cause a rebuild every time the timerProvider.state changes. However, the trick is to do the following:

final buttonProvider = Provider<ButtonState>((ref) {
  return ref.watch(_buttonState);
});

Since _buttonState will be the same for most of the timerProvider.state changes, watching _buttonState will only cause rebuilds when _buttonState actually changes.

Thanks to this post for showing the answer. That post also indicates that there will be a simplified syntax soon:

final buttonState = ref.watch(timerProvider.state.select((state) => state.buttonState));

来源：https://stackoverflow.com/questions/64346019/flutter-riverpod-statenotifierprovider-only-watch-changes-for-part-of-a-model

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