import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

/// A duplicate of the ChangeNotifier class which has the dispose method
/// renamed to disposeListeners. This is done so that it can be used
/// as a mixin with a State class which also has a dispose method
class DisposableChangeNotifier implements Listenable {
  ObserverList<VoidCallback> _listeners = ObserverList<VoidCallback>();

  bool _debugAssertNotDisposed() {
    assert(() {
      if (_listeners == null) {
        throw FlutterError('A $runtimeType was used after being disposed.\n'
            'Once you have called dispose() on a $runtimeType, it can no longer be used.');
      }
      return true;
    }());
    return true;
  }

  /// Whether any listeners are currently registered.
  ///
  /// Clients should not depend on this value for their behavior, because having
  /// one listener's logic change when another listener happens to start or stop
  /// listening will lead to extremely hard-to-track bugs. Subclasses might use
  /// this information to determine whether to do any work when there are no
  /// listeners, however; for example, resuming a [Stream] when a listener is
  /// added and pausing it when a listener is removed.
  ///
  /// Typically this is used by overriding [addListener], checking if
  /// [hasListeners] is false before calling `super.addListener()`, and if so,
  /// starting whatever work is needed to determine when to call
  /// [notifyListeners]; and similarly, by overriding [removeListener], checking
  /// if [hasListeners] is false after calling `super.removeListener()`, and if
  /// so, stopping that same work.
  @protected
  bool get hasListeners {
    assert(_debugAssertNotDisposed());
    return _listeners.isNotEmpty;
  }

  /// Register a closure to be called when the object changes.
  ///
  /// This method must not be called after [dispose] has been called.
  @override
  void addListener(VoidCallback listener) {
    assert(_debugAssertNotDisposed());
    _listeners.add(listener);
  }

  /// Remove a previously registered closure from the list of closures that are
  /// notified when the object changes.
  ///
  /// If the given listener is not registered, the call is ignored.
  ///
  /// This method must not be called after [dispose] has been called.
  ///
  /// If a listener had been added twice, and is removed once during an
  /// iteration (i.e. in response to a notification), it will still be called
  /// again. If, on the other hand, it is removed as many times as it was
  /// registered, then it will no longer be called. This odd behavior is the
  /// result of the [ChangeNotifier] not being able to determine which listener
  /// is being removed, since they are identical, and therefore conservatively
  /// still calling all the listeners when it knows that any are still
  /// registered.
  ///
  /// This surprising behavior can be unexpectedly observed when registering a
  /// listener on two separate objects which are both forwarding all
  /// registrations to a common upstream object.
  @override
  void removeListener(VoidCallback listener) {
    assert(_debugAssertNotDisposed());
    _listeners.remove(listener);
  }

  /// Discards any resources used by the object. After this is called, the
  /// object is not in a usable state and should be discarded (calls to
  /// [addListener] and [removeListener] will throw after the object is
  /// disposed).
  ///
  /// This method should only be called by the object's owner.
  @mustCallSuper
  void disposeListenables() {
    assert(_debugAssertNotDisposed());
    _listeners = null;
  }

  /// Call all the registered listeners.
  ///
  /// Call this method whenever the object changes, to notify any clients the
  /// object may have. Listeners that are added during this iteration will not
  /// be visited. Listeners that are removed during this iteration will not be
  /// visited after they are removed.
  ///
  /// Exceptions thrown by listeners will be caught and reported using
  /// [FlutterError.reportError].
  ///
  /// This method must not be called after [dispose] has been called.
  ///
  /// Surprising behavior can result when reentrantly removing a listener (i.e.
  /// in response to a notification) that has been registered multiple times.
  /// See the discussion at [removeListener].
  @protected
  @visibleForTesting
  void notifyListeners() {
    assert(_debugAssertNotDisposed());
    if (_listeners != null) {
      final List<VoidCallback> localListeners =
          List<VoidCallback>.from(_listeners);
      for (final VoidCallback listener in localListeners) {
        try {
          if (_listeners.contains(listener)) {
            listener();
          }
        } catch (exception, stack) {
          FlutterError.reportError(FlutterErrorDetails(
            exception: exception,
            stack: stack,
            library: 'foundation library',
            context: ErrorDescription(
                'while dispatching notifications for $runtimeType'),
            informationCollector: () sync* {
              yield DiagnosticsProperty<DisposableChangeNotifier>(
                'The $runtimeType sending notification was',
                this,
                style: DiagnosticsTreeStyle.errorProperty,
              );
            },
          ));
        }
      }
    }
  }
}