asyncredux-observers

Set up observers for debugging and monitoring. Covers implementing actionObservers for dispatch logging, stateObserver for state change tracking, combining observers with globalWrapError, and using ob

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Setting Up Observers for Debugging and Monitoring

AsyncRedux provides several observer types for monitoring actions, state changes, errors, and widget rebuilds. These observers are configured when creating the Store.

Overview of Observer Types

Observer Type Purpose
ActionObserver Monitor action dispatch (start and end)
StateObserver Monitor state changes after actions
GlobalErrorObserver Monitor, transform, and handle action errors
ModelObserver Monitor widget rebuilds (for StoreConnector)

Store Configuration with Observers

var store = Store<AppState>(
  initialState: AppState.initialState(),
  actionObservers: [ConsoleActionObserver()],
  stateObservers: [MyStateObserver()],
  globalErrorObserver: (store) => MyGlobalErrorObserver(),
  modelObserver: DefaultModelObserver(),
);

ActionObserver

The ActionObserver monitors when actions are dispatched and when they complete. It triggers twice per action: at the start (INI) and at the end (END).

ActionObserver Abstract Class

abstract class ActionObserver<St> {
  void observe(
    ReduxAction<St> action,
    int dispatchCount, {
    required bool ini,
  });
}

Parameters

  • action: The dispatched action instance
  • dispatchCount: Sequential number of this dispatch
  • ini: true when action starts (INI phase), false when it ends (END phase)

Observation Phases

INI Phase: Action dispatch begins. The reducer hasn't modified state yet. Sync reducers may complete during this phase; async reducers start their async process.

END Phase: The reducer has finished and returned the new state. State modifications are now observable.

Important: Receiving an END observation does not guarantee all effects have finished. Async operations that were not awaited may continue running and dispatch additional actions later.

Built-in ConsoleActionObserver

AsyncRedux provides ConsoleActionObserver for development debugging:

var store = Store<AppState>(
  initialState: AppState.initialState(),
  actionObservers: kReleaseMode ? null : [ConsoleActionObserver()],
);

This prints actions in yellow to the console. Override toString() in your actions to display additional information:

class LoadUserAction extends AppAction {
  final String username;
  LoadUserAction(this.username);

  @override
  Future<AppState?> reduce() async {
    // ...
  }

  @override
  String toString() => 'LoadUserAction(username: $username)';
}

Using Log.printer for Formatted Output

var store = Store<AppState>(
  initialState: AppState.initialState(),
  actionObservers: [Log.printer(formatter: Log.verySimpleFormatter)],
);

Custom ActionObserver Implementation

class MyActionObserver implements ActionObserver<AppState> {
  @override
  void observe(
    ReduxAction<AppState> action,
    int dispatchCount, {
    required bool ini,
  }) {
    final phase = ini ? 'START' : 'END';
    print('[$phase] Action #$dispatchCount: ${action.runtimeType}');
  }
}

StateObserver

The StateObserver is notified of all state changes, allowing you to track, log, or record state history.

StateObserver Abstract Class

abstract class StateObserver<St> {
  void observe(
    ReduxAction<St> action,
    St prevState,
    St newState,
    Object? error,
    int dispatchCount,
  );
}

Parameters

  • action: The action that triggered the change
  • prevState: State before the reducer executed
  • newState: State returned by the reducer
  • error: Null if successful; contains the thrown error otherwise
  • dispatchCount: Sequential dispatch number

Detecting State Changes

Compare states using identical() to detect actual changes:

bool stateChanged = !identical(prevState, newState);

Custom StateObserver for Logging

class StateLogger implements StateObserver<AppState> {
  @override
  void observe(
    ReduxAction<AppState> action,
    AppState prevState,
    AppState newState,
    Object? error,
    int dispatchCount,
  ) {
    final changed = !identical(prevState, newState);
    print('Action #$dispatchCount: ${action.runtimeType}');
    print('  State changed: $changed');
    if (error != null) {
      print('  Error: $error');
    }
  }
}

StateObserver for Undo/Redo

A common use case is recording state history for undo/redo functionality:

class UndoRedoObserver implements StateObserver<AppState> {
  final List<AppState> _history = [];
  int _currentIndex = -1;
  final int maxHistorySize;

  UndoRedoObserver({this.maxHistorySize = 50});

  bool get canUndo => _currentIndex > 0;
  bool get canRedo => _currentIndex < _history.length - 1;

  @override
  void observe(
    ReduxAction<AppState> action,
    AppState prevState,
    AppState newState,
    Object? error,
    int dispatchCount,
  ) {
    // Skip undo/redo actions to avoid recording navigation
    if (action is UndoAction || action is RedoAction) return;

    // Skip if state didn't change
    if (identical(prevState, newState)) return;

    // Remove "future" states if we're navigating
    if (_currentIndex < _history.length - 1) {
      _history.removeRange(_currentIndex + 1, _history.length);
    }

    // Add new state
    _history.add(newState);
    _currentIndex = _history.length - 1;

    // Enforce max history size
    if (_history.length > maxHistorySize) {
      _history.removeAt(0);
      _currentIndex--;
    }
  }

  AppState? getPreviousState() {
    if (!canUndo) return null;
    _currentIndex--;
    return _history[_currentIndex];
  }

  AppState? getNextState() {
    if (!canRedo) return null;
    _currentIndex++;
    return _history[_currentIndex];
  }
}

GlobalErrorObserver

The GlobalErrorObserver monitors all errors thrown by actions. It can log them, transform them (for example, into UserExceptions), or suppress them.

Error Handling Flow

The error handling order is:

  1. wrapError() (action-level)
  2. GlobalErrorObserver (app-level transformation and monitoring/logging)

After that, UserExceptions go to the error queue (shown by UserExceptionDialog) and are not thrown, while other errors are thrown.

GlobalErrorObserver Implementation

Extend GlobalErrorObserver and override observe(). Inside it you can access error (after wrapError), originalError (before wrapError), stackTrace, action, and store. Do not use store to dispatch actions.

class MyGlobalErrorObserver extends GlobalErrorObserver<AppState> {
  @override
  Object? observe() {
    // Transform platform errors to user-friendly messages
    if (error is PlatformException) {
      return UserException('Check your internet connection').addCause(error);
    }

    // Log unexpected errors and send them to a crash reporting service.
    // Note `action` is null when the error came from the Persistor.
    if (error is! UserException) {
      var source = action?.runtimeType.toString() ?? 'persistence';
      print('Error in $source: $error');
      print(stackTrace);
      crashReporter.recordError(error, stackTrace, reason: source);
    }

    // Return the error to keep it, or null to suppress it
    return error;
  }
}

Store Configuration with GlobalErrorObserver

var store = Store<AppState>(
  initialState: AppState.initialState(),
  globalErrorObserver: (store) => MyGlobalErrorObserver(),
);

AsyncRedux also provides GlobalErrorObserverDummy (does nothing), GlobalErrorObserverForDevelopment (also shows non-UserException errors in the dialog), and SwallowGlobalErrorObserver (swallows all errors, not recommended).

ModelObserver

The ModelObserver monitors widget rebuilds when using StoreConnector. This is useful for debugging rebuild behavior and ensuring efficient state updates.

Setup

var store = Store<AppState>(
  initialState: AppState.initialState(),
  modelObserver: DefaultModelObserver(),
);

Console Output

DefaultModelObserver prints rebuild information:

Model D:1 R:1 = Rebuild:true, Connector:MyWidgetConnector, Model:MyViewModel{B}.
Model D:2 R:2 = Rebuild:false, Connector:MyWidgetConnector, Model:MyViewModel{B}.
Model D:3 R:3 = Rebuild:true, Connector:MyWidgetConnector, Model:MyViewModel{C}.
  • D: Dispatch count
  • R: Rebuild count
  • Rebuild: Whether the widget actually rebuilt
  • Connector: The StoreConnector type
  • Model: The ViewModel with current state

Configuration for Better Output

Pass debug: this to StoreConnector to enable connector type printing:

class MyWidgetConnector extends StatelessWidget with StoreConnector<AppState, MyViewModel> {
  @override
  Widget build(BuildContext context) {
    return StoreConnector<AppState, MyViewModel>(
      debug: this, // Enable for ModelObserver output
      converter: (store) => MyViewModel.fromStore(store),
      builder: (context, vm) => MyWidget(vm),
    );
  }
}

Override ViewModel.toString() for custom diagnostic information.

Using Observers for Analytics

Metrics Observer Pattern

Create a metrics observer that delegates to action-specific tracking methods:

abstract class AppAction extends ReduxAction<AppState> {
  /// Override in specific actions to track metrics
  void trackEvent(MetricsService metrics) {}
}

class MetricsObserver implements StateObserver<AppState> {
  final MetricsService metrics;

  MetricsObserver(this.metrics);

  @override
  void observe(
    ReduxAction<AppState> action,
    AppState prevState,
    AppState newState,
    Object? error,
    int dispatchCount,
  ) {
    if (action is AppAction) {
      action.trackEvent(metrics);
    }
  }
}

Then override trackEvent in specific actions:

class PurchaseAction extends AppAction {
  final Product product;
  PurchaseAction(this.product);

  @override
  Future<AppState?> reduce() async {
    await purchaseService.buy(product);
    return state.copy(purchases: state.purchases.add(product));
  }

  @override
  void trackEvent(MetricsService metrics) {
    metrics.trackPurchase(productId: product.id, price: product.price);
  }
}

Analytics ActionObserver

Track all dispatched actions for analytics:

class AnalyticsObserver implements ActionObserver<AppState> {
  final AnalyticsService analytics;

  AnalyticsObserver(this.analytics);

  @override
  void observe(
    ReduxAction<AppState> action,
    int dispatchCount, {
    required bool ini,
  }) {
    // Only track at start (ini) to avoid double-counting
    if (ini) {
      analytics.trackEvent(
        'action_dispatched',
        parameters: {'action_type': action.runtimeType.toString()},
      );
    }
  }
}

Complete Example: Store with All Observers

// observers.dart
class ConsoleStateObserver implements StateObserver<AppState> {
  @override
  void observe(
    ReduxAction<AppState> action,
    AppState prevState,
    AppState newState,
    Object? error,
    int dispatchCount,
  ) {
    final changed = !identical(prevState, newState);
    print('[$dispatchCount] ${action.runtimeType} - Changed: $changed');
    if (error != null) print('  Error: $error');
  }
}

class CrashReportingErrorObserver extends GlobalErrorObserver<AppState> {
  @override
  Object? observe() {
    // Transform errors globally
    if (error is PlatformException) {
      return UserException('Check your internet connection').addCause(error);
    }
    // Don't report UserExceptions (they're expected)
    if (error is! UserException) {
      FirebaseCrashlytics.instance.recordError(error, stackTrace);
    }
    return error; // Keep the error
  }
}

// main.dart
void main() {
  final store = Store<AppState>(
    initialState: AppState.initialState(),
    // Only enable console observers in debug mode
    actionObservers: kDebugMode ? [ConsoleActionObserver()] : null,
    stateObservers: kDebugMode ? [ConsoleStateObserver()] : null,
    // Always enable error observer (also transforms errors globally)
    globalErrorObserver: (store) => CrashReportingErrorObserver(),
  );

  runApp(StoreProvider<AppState>(
    store: store,
    child: MyApp(),
  ));
}

Multiple Observers

You can use multiple observers of the same type:

var store = Store<AppState>(
  initialState: AppState.initialState(),
  actionObservers: [
    ConsoleActionObserver(),
    AnalyticsObserver(analyticsService),
    PerformanceObserver(),
  ],
  stateObservers: [
    StateLogger(),
    UndoRedoObserver(),
    MetricsObserver(metricsService),
  ],
);

All observers will be notified in the order they are listed.

References

URLs from the documentation:

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