Flutter - InheritedWidget - dispose

Question

I was wondering if anybody knew a way to know when an InheritedWidget was disposed?

The reason of this question is that I am doing some experiments and I am using an InheritedWidget as a provider of a BLoC. This BLoC is initialized at the InheritedWidget level, and uses a StreamController.

As it is more than recommended to close a StreamController, I am trying to find a solution.

Here is a piece of code (silly code just for experiments) to illustrate the question:

///
/// ApplicationProvider
/// 
/// A provider of ApplicationBloc
/// 
class ApplicationProvider extends InheritedWidget {
  //
  // Initialization of the BLoC
  //
  final ApplicationBloc bloc = new ApplicationBloc();

  ApplicationProvider({Key key, Widget child}) : super(key: key, child: child);

  @override
  bool updateShouldNotify(_) => true;

  static ApplicationBloc of(BuildContext context, [bool redraw = true]) {
    return redraw ? (context.inheritFromWidgetOfExactType(ApplicationProvider) as ApplicationProvider).bloc
                  : (context.ancestorWidgetOfExactType(ApplicationProvider) as ApplicationProvider).bloc;
  }
}

//
// The BLoC
//   
class ApplicationBloc {

  int _counter;
  StreamController<int> _counterController = new StreamController<int>.broadcast();
  Sink get inCounter => _counterController;

  Stream<int> get outCounter => _counterController.stream;

  ApplicationBloc(){
    _counter = 0;
  }

  void increment(){
    _counter++;
    inCounter.add(_counter);
  }

  int get counter => _counter;

  //
  // How could I call this method ???
  //
  void dispose(){
    _counterController.close();
  }
}

So the main question is "how can I call the dispose() method of my BLoC" ?

Many thanks for your help.

Answer 1

InheritedWidget behaves the same way as other Widget do. Their lifetime is really short: Usually not longer than one build call.

If you want to store data for longer, InheritedWidget is not what you want. You'll need a State for that.

Which also means that ultimately, you can use State's dispose for your bloc dispose.

class BlocHolder extends StatefulWidget {
  final Widget child;

  BlocHolder({this.child});

  @override
  _BlocHolderState createState() => _BlocHolderState();
}

class _BlocHolderState extends State<BlocHolder> {
  final _bloc = new MyBloc();

  @override
  Widget build(BuildContext context) {
    return MyInherited(bloc: _bloc, child: widget.child,);
  }


  @override
  void dispose() {
    _bloc.dispose();
    super.dispose();
  }
}

class MyInherited extends InheritedWidget {
  final MyBloc bloc;

  MyInherited({this.bloc, Widget child}): super(child: child);

  @override
  bool updateShouldNotify(InheritedWidget oldWidget) {
    return oldWidget != this;
  }
}


class MyBloc {
  void dispose() {

  }
}

Answer 2

Inherited widgets behave very much like stateless widgets, which also do not have a dispose method. Inherited widgets are getting rebuilt frequently, and all the values stored inside of it would be lost (and without a proper updateShouldNotify implementation, the dependent widget trees will also be rebuilt frequently!).

To solve this problem, you can utilize a StatefulWidget:

import 'dart:async';

import 'package:flutter/widgets.dart';

class ApplicationProvider extends StatefulWidget {
  const ApplicationProvider({Key key, this.child}) : super(key: key);

  final Widget child;

  @override
  State<StatefulWidget> createState() => _ApplicationProviderState();
}

class _ApplicationProviderState extends State<ApplicationProvider> {
  final ApplicationBloc bloc = new ApplicationBloc();

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return _ApplicationProvider(
      bloc: bloc,
      child: widget.child,
    );
  }
}

class _ApplicationProvider extends InheritedWidget {
  _ApplicationProvider({
    Key key,
    this.bloc,
    Widget child,
  }) : super(key: key, child: child);

  final ApplicationBloc bloc;

  @override
  bool updateShouldNotify(_ApplicationProvider oldWidget) {
    return bloc != oldWidget.bloc;
  }
}

class ApplicationBloc {
  ApplicationBloc of(BuildContext context) {
    final _ApplicationProvider provider = context.inheritFromWidgetOfExactType(_ApplicationProvider);
    return provider.bloc;
  }

  int _counter;
  StreamController<int> _counterController = new StreamController<int>.broadcast();
  Sink get inCounter => _counterController;

  Stream<int> get outCounter => _counterController.stream;

  ApplicationBloc() {
    _counter = 0;
  }

  void increment() {
    _counter++;
    inCounter.add(_counter);
  }

  int get counter => _counter;

  void dispose() {
    _counterController.close();
  }
}