Provider<State> class

A provider that exposes a read-only value.

What is a provider

Providers are the most important components of Riverpod. In short, you can think of providers as an access point to a shared state.

Providers solve the following problems:

Providers have the flexibility of global variables, without their downsides.
Providers can be accessed from anywhere, while ensuring testability and scalability.
Providers are safe to use.
As opposed to most service-locator solutions, using a provider, it is not possible to read a value in an uninitialized state.
If we can write the code to read a state, the code will execute properly. Even if the state is loaded asynchronously.
Providers allow easily and efficiently listening to a piece of state.
They can be accessed in a single line of code, and offer many ways to optimize your application.

Creating a provider

Providers come in many variants, but they all work the same way.

The most common usage is to declare them as global variables like so:

final myProvider = Provider((ref) {
  return MyValue();
});

NOTE Do not feel threatened by the fact that a provider is declared as a global. While providers are globals, the variable is fully immutable. This makes creating a provider no different from declaring a function or a class.

This snippet consist of three components:

final myProvider, the declaration of a variable.
This variable is what we will use in the future to read the state of our provider. It should always be immutable.
Provider, the provider that we decided to use.
Provider is the most basic of all providers. It exposes an object that never changes.
We could replace Provider with other providers like StreamProvider or StateNotifierProvider, to change how the value is interacted with.
A function that creates the shared state.
That function will always receive an object called ref as a parameter. This object allows us to read other providers or to perform some operations when the state of our provider will be destroyed.

The type of the object created by the function passed to a provider depends on the provider used.
For example, the function of a Provider can create any object. On the other hand, StreamProvider's callback will be expected to return a Stream.

NOTE: You can declare as many providers as you want, without limitations.
As opposed to when using package:provider, in Riverpod we can have two providers expose a state of the same "type":

final cityProvider = Provider((ref) => 'London');
final countryProvider = Provider((ref) => 'England');

The fact that both providers create a String does not cause conflicts. We will be able to read both values independently from each other without issue.

WARNING For providers to work, you need to add ProviderScope at the root of your Flutter applications:

void main() {
  runApp(ProviderScope(child: MyApp()));
}

Combining providers

We've previously seen how to create a simple provider. But the reality is, in many situations a provider will want to read the state of another provider.

To do that, we can use the ref object passed to the callback of our provider, and use its watch method.

As an example, consider the following provider:

final cityProvider = Provider((ref) => 'London');

We can now create another provider that will consume our cityProvider:

final weatherProvider = FutureProvider((ref) async {
  // We use `ref.watch` to watch another provider, and we pass it the provider
  // that we want to consume. Here: cityProvider
  final city = ref.watch(cityProvider);

  // We can then use the result to do something based on the value of `cityProvider`.
  return fetchWeather(city: city);
});

That's it. We've created a provider that depends on another provider.

One interesting aspect of this code is, if city ever changes, this will automatically call fetchWeather again and update the UI accordingly.

Creating an object that depends on a lot of providers.

Sometimes, we may want to create an object that depends on a lot of providers like so:

final cityProvider = Provider((ref) => 'London');
final countryProvider = Provider((ref) => 'England');

final weatherProvider = Provider((ref) {
  final city = ref.watch(cityProvider);
  final country = ref.watch(countryProvider);

  return Location(city: city, country: country);
});

class Location {
  Location({required this.city, required this.country});

  final String city;
  final String country;

  String get label => '$city ($country)';
}

This can quickly become tedious.

In that situation, it may be reasonable to pass the ref variable to our object directly:

final cityProvider = Provider((ref) => 'London');
final countryProvider = Provider((ref) => 'England');

final weatherProvider = Provider((ref) {
  // Pass the `ref` object to our `Location` class.
  // `Location` will then be able to call `ref.read` to read the providers.
  return Location(ref);
});

class Location {
  Location(this._ref);

  final Ref _ref;

  String get label {
    final city = _ref.read(cityProvider);
    final country = _ref.read(countryProvider);
    return '$city ($country)';
  }
}

This avoids having to implement a constructor, which makes changes on the object easier.

This is fine as, as opposed to BuildContext from Flutter, that ref object is completely independent from Flutter/the UI.
As such the object can still be shared and tested.

Disposing the resources the state is destroyed

During the lifetime of an application, the state associated with a provider may get destroyed.
In this situation, we may want to perform a clean-up before the state destruction.

This is done by using the ref object that is passed to the callback of all providers.

That ref object exposes an onDispose method, which can be used to listen to the state destruction even to perform some task.

The following example uses ref.onDispose to close a StreamController:

final example = StreamProvider.autoDispose((ref) {
  final streamController = StreamController<int>();

  ref.onDispose(() {
    // Closes the StreamController when the state of this provider is destroyed.
    streamController.close();
  });

  return streamController.stream;
});

Constructors

Provider(Create<State, ProviderRef<State>> _createFn, {String? name, Iterable<ProviderOrFamily>? dependencies, @Deprecated('Will be removed in 3.0.0') Family<Object?>? from, @Deprecated('Will be removed in 3.0.0') Object? argument, @Deprecated('Will be removed in 3.0.0') DebugGetCreateSourceHash? debugGetCreateSourceHash}): A provider that exposes a read-only value.
Provider.internal(Create<State, ProviderRef<State>> _createFn, {required String? name, required Iterable<ProviderOrFamily>? dependencies, required Iterable<ProviderOrFamily>? allTransitiveDependencies, required DebugGetCreateSourceHash? debugGetCreateSourceHash, Family<Object?>? from, Object? argument}): An implementation detail of Riverpod

Properties

allTransitiveDependencies → Iterable<ProviderOrFamily>?: All the dependencies of a provider and their dependencies too.
finalinherited
argument → Object?: If this provider was created with the .family modifier, argument is the variable that was used.
finalinherited
debugGetCreateSourceHash → DebugGetCreateSourceHash?: A debug-only fucntion for obtaining a hash of the source code of the initialization function.
finalinherited
dependencies → Iterable<ProviderOrFamily>?: The list of providers that this provider potentially depends on.
finalinherited
from → Family<Object?>?: If this provider was created with the .family modifier, from is the .family instance.
finalinherited
hashCode → int: The hash code for this object.
no setterinherited
name → String?: A custom label for providers.
finalinherited
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited

Methods

addListener(Node node, void listener(State? previous, State next), {required void onError(Object error, StackTrace stackTrace)?, required void onDependencyMayHaveChanged()?, required bool fireImmediately}) → ProviderSubscription<State>: Starts listening to this transformer
inherited
createElement() → ProviderElement<State>: An internal method that defines how a provider behaves.
override
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
overrideWith(Create<State, ProviderRef<State>> create) → Override: Override the provider with a new initialization function.
overrideWithValue(State value) → Override: Overrides a provider with a value, ejecting the default behaviour.
inherited
read(Node node) → State: Obtains the result of this provider expression without adding listener.
inherited
select<Selected>(Selected selector(State value)) → AlwaysAliveProviderListenable<Selected>: Partially listen to a provider.
inherited
toString() → String: A string representation of this object.
inherited

Operators

operator ==(Object other) → bool: The equality operator.
inherited

Constants

autoDispose → const AutoDisposeProviderBuilder: Marks the provider as automatically disposed when no longer listened to.
family → const ProviderFamilyBuilder: A group of providers that builds their value from an external parameter.