A mixture between dependency injection (DI) and state management, built with widgets for widgets

A mixture between dependency injection (DI) and state management, built with
widgets for widgets.

It purposefully uses widgets for DI/state management instead of dart-only
classes like Stream.
The reason is, widgets are very simple yet robust and scalable.

By using widgets for state management, provider can guarantee:

• maintainability, through a forced uni-directional data-flow
• testability/composability, since it is always possible to mock/override a
  value
• robustness, as it is harder to forget to handle the update scenario of a
  model/widget

To read more about provider, see the documentation.

Migration from v2.0.0 to v3.0.0

• Providers can no longer be instantiated with const.
• Provider now throws if used with a Listenable/Stream.
  Consider using ListenableProvider/StreamProvider instead. Alternatively,
  this exception can be disabled by setting
  Provider.debugCheckInvalidValueType to null like so:

void main() {
  Provider.debugCheckInvalidValueType = null;

  runApp(MyApp());
}

• All XXProvider.value constructors now use value as parameter name.

Before:

ChangeNotifierProvider.value(notifier: myNotifier),

After:

ChangeNotifierProvider.value(value: myNotifier),

• StreamProvider‘s default constructor now builds a Stream instead of a
  StreamController. The previous behavior has been moved to the named
  constructor StreamProvider.controller.

Before:

StreamProvider(builder: (_) => StreamController<int>()),

After:

StreamProvider.controller(builder: (_) => StreamController<int>()),

Usage

Exposing a value

To expose a variable using provider, wrap any widget into one of the provider
widgets from this package and pass it your variable. Then, all descendants of
the newly added provider widget can access this variable.

A simple example would be to wrap the entire application into a Provider
widget and pass it our variable:

Provider<String>.value(
  value: 'Hello World',
  child: MaterialApp(
    home: Home(),
  )
)

Alternatively, for complex objects, most providers expose a constructor that
takes a function to create the value. The provider will call that function only
once, when inserting the widget in the tree, and expose the result. This is
perfect for exposing a complex object that never changes over time without
writing a StatefulWidget.

The following creates and exposes a MyComplexClass. And in the event where
Provider is removed from the widget tree, the instantiated MyComplexClass
will be disposed.

Provider<MyComplexClass>(
  builder: (context) => MyComplexClass(),
  dispose: (context, value) => value.dispose()
  child: SomeWidget(),
)

Reading a value

The easiest way to read a value is by using the static method
Provider.of<T>(BuildContext context).

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

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 `Provider.of`!
      Provider.of<String>(context)
    );
  }
}

Alternatively instead of using Provider.of, 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<Foo>.value(
  value: foo,
  child: Provider<Bar>.value(
    value: bar,
    child: Provider<Baz>.value(
      value: baz,
      child: someWidget,
    )
  )
)

In that situation, we can use MultiProvider to improve the readability:

MultiProvider(
  providers: [
    Provider<Foo>.value(value: foo),
    Provider<Bar>.value(value: bar),
    Provider<Baz>.value(value: baz),
  ],
  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(builder: (_) => Counter()),
      ProxyProvider<Counter, Translations>(
        builder: (_, 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

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(Provider.of<Foo>(context).value);
}

you can do:

Value value;

didChangeDependencies() {
  super.didChangeDependencies();
  final value = Provider.of<Foo>(context).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(Provider.of<Foo>(context, listen: false).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();
  Provider.of<Foo>(context).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 builder 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(() =>
      Provider.of<Foo>(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(Provider.of<int>(context).toString());

and modify the state with:

return FloatingActionButton(
  onPressed: Provider.of<ExampleState>(context).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 makes 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 Provider.of, 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
unnecesseraly rebuild.

To go one step further, it is possible to use Selector to ignore changes if
they don’t have an impact on the widget-tree:

Selector<List, int>(
  selector: (_, list) => list.length,
  builder: (_, length, __) {
    return Text('$length');
  }
);

This snippet will rebuild only if the length of the list changes. But it won’t
unnecessarily update if an item is updated.

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

Prefer:

Provider<Country>(
  builder: (_) => Country('England'),
  child: Provider<City>(
    builder: (_) => 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 availables is here

GitHub

View Github