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.
-
The parameters
builder
andinitialBuilder
of providers are removed.initialBuilder
should be replaced bycreate
.builder
of "proxy" providers should be replaced byupdate
builder
of classical providers should be replaced bycreate
.
-
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 toProviderNotFoundException
. -
The
SingleChildCloneableWidget
interface is removed and replaced by a new kind of widgetSingleChildWidget
.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 toSelector
:Selector<Selected, Consumed>( shouldRebuild: (previous, next) => previous == next, builder: ..., )
-
DelegateWidget
and its family is removed. Instead, for custom providers, directly subclassInheritedProvider
or an existing provider.
Providers allows 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: (_) => new MyModel(),
child: ...
)
- DON'T use
Provider.value
to create your object.
ChangeNotifierProvider.value(
value: new MyModel(),
child: ...
)
-
DON'T create your object from variables that can change over the time.
In such situation, your object would never be updated when the value changes.
int count;
Provider(
create: (_) => new MyModel(count),
child: ...
)
If you want to pass variables that can change over time to your object,
consider using ProxyProvider
:
int count;
ProxyProvider0(
update: (_, __) => new MyModel(count),
child: ...
)
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 existingChangeNotifier
.
MyChangeNotifier variable;
ChangeNotifierProvider.value(
value: variable,
child: ...
)
- DON'T reuse an existing
ChangeNotifier
using the default constructor
MyChangeNotifier variable;
ChangeNotifierProvider(
create: (_) => variable,
child: ...
)
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.
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.
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
vsProxyProvider2
vsProxyProvider3
, ...That digit after the class name is the number of other providers that
ProxyProvider
depends on. -
ProxyProvider
vsChangeNotifierProxyProvider
vsListenableProxyProvider
, ...They all work similarly, but instead of sending the result into a
Provider
, aChangeNotifierProxyProvider
will send its value to aChangeNotifierProvider
.
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.
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
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(() => Provider.of<Foo>(context).fetchSomething(someValue); ); }
It is slightly less ideal, but allows passing parameters to the mutation.
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.
Yes. provider
exposes all the small components that makes a fully fledged provider.
This includes:
SingleChildCloneableWidget
, to make any widget works withMultiProvider
.InheritedProvider
, the genericInheritedWidget
obtained when doingProvider.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
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.
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: ...,
),
),
provider
exposes a few different kinds of "provider" for different types of objects.
The complete list of all the objects availables 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. |