TomkeyZhang/provider

InheritedWidgets, but simple

English | Português

A wrapper around InheritedWidget to make them easier to use and more reusable.

By using provider instead of manually writing InheritedWidget, you get:

• simplified allocation/disposal of resources
• lazy-loading
• a largely reduced boilerplate over making a new class every time
• devtools friendly
• a common way to consume these [InheritedWidgets] (See Provider.of/Consumer/Selector)
• increased scalability for classes with a listening mechanism that grows exponentially in complexity (such as ChangeNotifier, which is O(N²) for dispatching notifications).

To read more about provider, see its documentation.

See also:

• the official Flutter state management documentation, which showcase how to use provider + ChangeNotifier
• flutter architecture sample, which contains an implementation of that app using provider + ChangeNotifier
• flutter_bloc and Mobx, which use provider in their architecture

Migration from v3.x.0 to v4.0.0

• The parameters builder and initialBuilder of providers are removed.

  • initialBuilder should be replaced by create.
  • builder of "proxy" providers should be replaced by update
  • builder of classical providers should be replaced by create.

• The new create/update callbacks are lazy-loaded, which means they are called the first time the value is read instead of the first time the provider is created.

  If this is undesired, you can disable lazy-loading by passing lazy: false to the provider of your choice:

  FutureProvider(
    create: (_) async => doSomeHttpRequest(),
    lazy: false,
    child: ...
  )

• ProviderNotFoundError is renamed to ProviderNotFoundException.

• The SingleChildCloneableWidget interface is removed and replaced by a new kind of widget SingleChildWidget.

  See this issue for details on how to migrate.

• Selector now deeply compares the previous and new values if they are collections.

  If this is undesired, you can revert to the old behavior by passing a shouldRebuild parameter to Selector:

  Selector<Selected, Consumed>(
    shouldRebuild: (previous, next) => previous == next,
    builder: ...,
  )

• DelegateWidget and its family is removed. Instead, for custom providers, directly subclass InheritedProvider or an existing provider.

Usage

Exposing a value

Exposing a new object instance

Providers allow to not only expose a value, but also create/listen/dispose it.

To expose a newly created object, use the default constructor of a provider. Do not use the .value constructor if you want to create an object, or you may otherwise have undesired side-effects.

See this stackoverflow answer which explains in further details why using the .value constructor to create values is undesired.

• DO create a new object inside create.

Provider(
  create: (_) => MyModel(),
  child: ...
)

• DON'T use Provider.value to create your object.

ChangeNotifierProvider.value(
  value: MyModel(),
  child: ...
)

• DON'T create your object from variables that can change over the time.

  In such a situation, your object would never be updated when the value changes.

int count;

Provider(
  create: (_) => MyModel(count),
  child: ...
)

If you want to pass variables that can change over time to your object, consider using ProxyProvider:

int count;

ProxyProvider0(
  update: (_, __) => MyModel(count),
  child: ...
)

NOTE:

When using the create/update callback of a provider, it is worth noting that this callback is called lazily by default.

What this means is, until the value is requested at least once, the create/update callbacks won't be called.

This behavior can be disabled if you want to pre-compute some logic, using the lazy parameter:

MyProvider(
  create: (_) => Something(),
  lazy: false,
)

Reusing an existing object instance:

If you already have an object instance and want to expose it, you should use the .value constructor of a provider.

Failing to do so may call the dispose method of your object when it is still in use.

• DO use ChangeNotifierProvider.value to provide an existing ChangeNotifier.

MyChangeNotifier variable;

ChangeNotifierProvider.value(
  value: variable,
  child: ...
)

• DON'T reuse an existing ChangeNotifier using the default constructor

MyChangeNotifier variable;

ChangeNotifierProvider(
  create: (_) => variable,
  child: ...
)

Reading a value

The easiest way to read a value is by using the extension methods on [BuildContext]:

• context.watch<T>(), which makes the widget listen to changes on T
• context.read<T>(), which returns T without listening to it
• context.select<T, R>(R cb(T value)), which allows a widget to listen to only a small part of T.

Or to use the static method Provider.of<T>(context), which will behave similarly to watch/read.

These methods will look up in the widget tree starting from the widget associated with the BuildContext passed, and will return the nearest variable of type T found (or throw if nothing is found).

It's worth noting that this operation is O(1). It doesn't involve actually walking in the widget tree.

Combined with the first example of exposing a value, this widget will read the exposed String and render "Hello World."

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text(
      // Don't forget to pass the type of the object you want to obtain to `watch`!
      context.watch<String>(),
    );
  }
}

Alternatively, instead of using these methods, we can use Consumer and Selector.

These can be useful for performance optimizations or when it is difficult to obtain a BuildContext descendant of the provider.

See the FAQ or the documentation of Consumer and Selector for more information.

MultiProvider

When injecting many values in big applications, Provider can rapidly become pretty nested:

Provider<Something>(
  create: (_) => Something(),
  child: Provider<SomethingElse>(
    create: (_) => SomethingElse(),
    child: Provider<AnotherThing>(
      create: (_) => AnotherThing(),
      child: someWidget,
    ),
  ),
),

To:

MultiProvider(
  providers: [
    Provider<Something>(create: (_) => Something()),
    Provider<SomethingElse>(create: (_) => SomethingElse()),
    Provider<AnotherThing>(create: (_) => AnotherThing()),
  ],
  child: someWidget,
)

The behavior of both examples is strictly the same. MultiProvider only changes the appearance of the code.

ProxyProvider

Since the 3.0.0, there is a new kind of provider: ProxyProvider.

ProxyProvider is a provider that combines multiple values from other providers into a new object, and sends the result to Provider.

That new object will then be updated whenever one of the providers it depends on updates.

The following example uses ProxyProvider to build translations based on a counter coming from another provider.

Widget build(BuildContext context) {
  return MultiProvider(
    providers: [
      ChangeNotifierProvider(create: (_) => Counter()),
      ProxyProvider<Counter, Translations>(
        update: (_, counter, __) => Translations(counter.value),
      ),
    ],
    child: Foo(),
  );
}

class Translations {
  const Translations(this._value);

  final int _value;

  String get title => 'You clicked $_value times';
}

It comes under multiple variations, such as:

• ProxyProvider vs ProxyProvider2 vs ProxyProvider3, ...

  That digit after the class name is the number of other providers that ProxyProvider depends on.

• ProxyProvider vs ChangeNotifierProxyProvider vs ListenableProxyProvider, ...

  They all work similarly, but instead of sending the result into a Provider, a ChangeNotifierProxyProvider will send its value to a ChangeNotifierProvider.

FAQ

Can I inspect the content of my objects?

Flutter comes with a devtool that shows what the widget tree is at a given moment.

Since providers are widgets, they are also visible in that devtool:

From there, if you click on one provider, you will be able to see the value it exposes:

(screenshot of the devtools using the example folder)

The devtool only shows "Instance of MyClass". What can I do?

By default, the devtool relies on toString, which defaults to "Instance of MyClass".

To have something more useful, you have two solutions:

• use the Diagnosticable API from Flutter.

  For most cases, that will be done my using DiagnosticableTreeMixin on your objects, followed by a custom implementation of debugFillProperties.

  class MyClass with DiagnosticableTreeMixin {
    MyClass({this.a, this.b});
  
    final int a;
    final String b;
  
    @override
    void debugFillProperties(DiagnosticPropertiesBuilder properties) {
      super.debugFillProperties(properties);
      // list all the properties of your class here.
      // See the documentation of debugFillProperties for more information.
      properties.add(IntProperty('a', a));
      properties.add(StringProperty('b', b));
    }
  }

• override toString.

  If you cannot use DiagnosticableTreeMixin (like if your class is in a package that does not depend on Flutter), then you can simply override toString.

  This is easier than using DiagnosticableTreeMixin but is less powerful: You will not be able to expand/collapse the details of your object.

  class MyClass with DiagnosticableTreeMixin {
    MyClass({this.a, this.b});
  
    final int a;
    final String b;
  
    @override
    String toString() {
      return '$runtimeType(a: $a, b: $b)';
    }
  }

I have an exception when obtaining Providers inside initState. What can I do?

This exception happens because you're trying to listen to a provider from a life-cycle that will never ever be called again.

It means that you either should use another life-cycle (didChangeDependencies/build), or explicitly specify that you do not care about updates.

As such, instead of:

initState() {
  super.initState();
  print(context.watch<Foo>().value);
}

you can do:

Value value;

didChangeDependencies() {
  super.didChangeDependencies();
  final value = context.watch<Foo>.value;
  if (value != this.value) {
    this.value = value;
    print(value);
  }
}

which will print value whenever it changes.

Alternatively you can do:

initState() {
  super.initState();
  print(context.read<Foo>().value);
}

Which will print value once and ignore updates.

I use ChangeNotifier and I have an exception when I update it, what happens?

This likely happens because you are modifying the ChangeNotifier from one of its descendants while the widget tree is building.

A typical situation where this happens is when starting an http request, where the future is stored inside the notifier:

initState() {
  super.initState();
  context.read<MyNotifier>().fetchSomething();
}

This is not allowed, because the modification is immediate.

Which means that some widgets may build before the mutation, while other widgets will build after the mutation. This could cause inconsistencies in your UI and is therefore not allowed.

Instead, you should perform that mutation in a place that would affect the entire tree equally:

• directly inside the create of your provider/constructor of your model:

  class MyNotifier with ChangeNotifier {
    MyNotifier() {
      _fetchSomething();
    }
  
    Future<void> _fetchSomething() async {}
  }

  This is useful when there's no "external parameter".

• asynchronously at the end of the frame:

  initState() {
    super.initState();
    Future.microtask(() =>
      context.read<MyNotifier>(context).fetchSomething(someValue);
    );
  }

  It is slightly less ideal, but allows passing parameters to the mutation.

Do I have to use ChangeNotifier for complex states?

No.

You can use any object to represent your state. For example, an alternate architecture is to use Provider.value() combined with a StatefulWidget.

Here's a counter example using such architecture:

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

  final Widget child;

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

class ExampleState extends State<Example> {
  int _count;

  void increment() {
    setState(() {
      _count++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Provider.value(
      value: _count,
      child: Provider.value(
        value: this,
        child: widget.child,
      ),
    );
  }
}

where we can read the state by doing:

return Text(context.watch<int>().toString());

and modify the state with:

return FloatingActionButton(
  onPressed: () => context.read<ExampleState>().increment(),
  child: Icon(Icons.plus_one),
);

Alternatively, you can create your own provider.

Can I make my own Provider?

Yes. provider exposes all the small components that make a fully-fledged provider.

This includes:

• SingleChildCloneableWidget, to make any widget works with MultiProvider.
• InheritedProvider, the generic InheritedWidget obtained when doing Provider.of.
• DelegateWidget/BuilderDelegate/ValueDelegate to help handle the logic of "MyProvider() that creates an object" vs "MyProvider.value() that can update over time".

Here's an example of a custom provider to use ValueNotifier as state: https://gist.github.com/rrousselGit/4910f3125e41600df3c2577e26967c91

My widget rebuilds too often, what can I do?

Instead of context.watch, you can use context.select to listen only to a specific set of properties on the obtained object.

For example, while you can write:

Widget build(BuildContext context) {
  final person = context.watch<Person>();
  return Text(person.name);
}

It may cause the widget to rebuild if something other than name changes.

Instead, you can use context.select to listen only to the name property:

Widget build(BuildContext context) {
  final name = context.select((Person p) => p.name);
  return Text(person.name);
}

This way, the widget won't unnecessarily rebuild if something other than name changes.

Similarly, you can use Consumer/Selector. Their optional child argument allows to rebuild only a very specific part of the widget tree:

Foo(
  child: Consumer<A>(
    builder: (_, a, child) {
      return Bar(a: a, child: child);
    },
    child: Baz(),
  ),
)

In this example, only Bar will rebuild when A updates. Foo and Baz won't unnecessarily rebuild.

Can I obtain two different providers using the same type?

No. While you can have multiple providers sharing the same type, a widget will be able to obtain only one of them: the closest ancestor.

Instead, you must explicitly give both providers a different type.

Instead of:

Provider<String>(
  create: (_) => 'England',
  child: Provider<String>(
    create: (_) => 'London',
    child: ...,
  ),
),

Prefer:

Provider<Country>(
  create: (_) => Country('England'),
  child: Provider<City>(
    create: (_) => City('London'),
    child: ...,
  ),
),

Existing providers

provider exposes a few different kinds of "provider" for different types of objects.

The complete list of all the objects available is here

name	description
Provider	The most basic form of provider. It takes a value and exposes it, whatever the value is.
ListenableProvider	A specific provider for Listenable object. ListenableProvider will listen to the object and ask widgets which depend on it to rebuild whenever the listener is called.
ChangeNotifierProvider	A specification of ListenableProvider for ChangeNotifier. It will automatically call ChangeNotifier.dispose when needed.
ValueListenableProvider	Listen to a ValueListenable and only expose ValueListenable.value.
StreamProvider	Listen to a Stream and expose the latest value emitted.
FutureProvider	Takes a Future and updates dependents when the future completes.