A dependency injection system built with widgets for widgets. provider is mostly syntax sugar for InheritedWidget, to make common use-cases straightforward.

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 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 if 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 the Consumer widget.

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

Provider<String>.value(
  value: 'Hello World',
  child: Consumer<String>(
    builder: (context, value, child) => Text(value),
  ),
);

---

Note that you can freely use multiple providers with different types together:

Provider<int>.value(
  value: 42,
  child: Provider<String>.value(
    value: 'Hello World',
    child: // ...
  )
)

And obtain their value independently:

var value = Provider.of<int>(context);
var value2 = Provider.of<String>(context);

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.

Existing providers

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

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 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.