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

Question

我有这样一个模型:

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

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

这是 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);
  }
}

我可以像这样监听状态的所有变化:

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

但是我想创建另一个只监听 ButtonState 变化的提供者。这不起作用:

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

因为它仍然返回所有的状态变化。

这也行不通:

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

因为 state 对象没有 buttonState 属性。

我如何只观察 buttonState 的变化？

Answer 1

使用 watch 会在监视状态发生变化时提供新状态。所以可以像这样分两部分解决问题:

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

每次 timerProvider.state 更改时，使用此提供程序都会导致重建。但是，诀窍是执行以下操作:

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

由于 _buttonState 对于大多数 timerProvider.state 更改都是相同的，所以观看 _buttonState 只会在 _buttonState 时导致重建 实际上改变了。

感谢this post用于显示答案。该帖子还表明很快就会有一个简化的语法:

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