ProviderElement<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 offers many way 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 creates 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 situation 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 expose 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

ProviderElement(ProviderBase<State> provider): A ProviderElementBase for Provider

Properties

container → ProviderContainer: The ProviderContainer that owns this ProviderElementBase.
no setterinherited
debugAssertDidSetStateEnabled → bool: Whether the assert that prevents requireState from returning if the state was not set before is enabled.
no setterinherited
hashCode → int: The hash code for this object.
no setterinherited
hasListeners → bool: Whether this ProviderElementBase is currently listened or not.
no setterinherited
mounted → bool: Whether the element was disposed or not
no setterinherited
origin → ProviderBase<State>: The provider associated to this ProviderElementBase, before applying overrides.
no setterinherited
provider → ProviderBase<State>: The provider associated to this ProviderElementBase, after applying overrides.
no setterinherited
requireState → State: no setterinherited
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited
state ↔ State: Obtains the state currently exposed by this provider.
getter/setter pairoverride

Methods

addListener(ProviderBase<State> provider, void listener(State? previous, State next), {required bool fireImmediately, required void onError(Object error, StackTrace stackTrace)?}) → ProviderSubscription<State>: inherited
dispose() → void: Called on ProviderContainer.dispose.
inherited
flush() → void: inherited
getState() → Result<State>?: inherited
listen<T>(ProviderListenable<T> listenable, void listener(T? previous, T value), {bool fireImmediately = false, void onError(Object error, StackTrace stackTrace)?}) → void Function(): Listen to a provider and call listener whenever its value changes.
inherited
markMustRecomputeState() → void: inherited
mayNeedDispose() → void: Life-cycle for when a listener is removed.
inherited
mount() → void: Called the first time a provider is obtained.
inherited
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
onDispose(void listener()) → void: Adds a listener to perform an operation right before the provider is destroyed.
inherited
read<T>(ProviderBase<T> provider) → T: Read the state associated with a provider, without listening to that provider.
inherited
readSelf() → State: Returns the currently exposed by a provider
inherited
refresh<T>(ProviderBase<T> provider) → T: Re-create the state of a provider and return the new state.
inherited
setState(State newState) → void: inherited
toString() → String: A string representation of this object.
inherited
update(ProviderBase<State> newProvider) → void: Called when the override of a provider changes.
inherited
visitAncestors(void visitor(ProviderElementBase element)) → void: Visit the ProviderElementBases that this provider is listening to.
inherited
visitChildren(void visitor(ProviderElementBase element)) → void: Visit the ProviderElements of providers that are listening to this element.
inherited
watch<T>(ProviderListenable<T> listenable) → T: Obtains the state of a provider and cause the state to be re-evaluated when that provider emits a new value.
inherited

Operators

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