Providers

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Now that we have installed Riverpod, let's talk about "providers".

Providers are the most important part of a Riverpod application. A provider is an object that encapsulates a piece of state and allows listening to that state.

Why use providers?

Wrapping a piece of state in a provider:

• Allows easily accessing that state in multiple locations. Providers are a complete replacement for patterns like Singletons, Service Locators, Dependency Injection or InheritedWidgets.

• Simplifies combining this state with others. Ever struggled to merge multiple objects into one? This scenario is built directly inside providers.

• Enables performance optimizations. Whether for filtering widget rebuilds or for caching expensive state computations; providers ensure that only what is impacted by a state change is recomputed.

• Increases the testability of your application. With providers, you do not need complex setUp/tearDown steps. Furthermore, any provider can be overridden to behave differently during a test, which allows easily testing a very specific behavior.

• Allows easy integration with advanced features, such as logging or pull-to-refresh.

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 constants like so:

@riverpod
MyValue my(Ref ref) {
  return MyValue();
}

备注

Do not be frightened by the global aspect of providers. Providers are fully immutable. Declaring a provider is no different from declaring a function, and providers are testable and maintainable.

This snippet consists 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. Providers should always be final.

• 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 NotifierProvider, 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, perform some operations when the state of our provider will be destroyed, and much more.

The type of the object returned 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.

信息

You can declare as many providers as you want without limitations. As opposed to when using package:provider, Riverpod allows creating multiple providers exposing a state of the same "type":

@riverpod
String city(Ref ref) => 'London';
@riverpod
String country(Ref ref) => 'England';

The fact that both providers create a String does not cause any problem.

警告

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

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

Different Types of Providers

There are multiple types of providers for multiple different use cases.

With all of these providers available, it is sometimes difficult to understand when to use one provider type over another. Use the table below to choose a provider that fits what you want to provide to the widget tree.

Provider Type	Provider Create Function	Example Use Case
Provider	Returns any type	A service class / computed property (filtered list)
StateProvider	Returns any type	A filter condition / simple state object
FutureProvider	Returns a Future of any type	A result from an API call
StreamProvider	Returns a Stream of any type	A stream of results from an API
NotifierProvider	Returns a subclass of (Async)Notifier	A complex state object that is immutable except through an interface
StateNotifierProvider	Returns a subclass of StateNotifier	A complex state object that is immutable except through an interface. Prefer using a notifierProvider
ChangeNotifierProvider	Returns a subclass of ChangeNotifier	A complex state object that requires mutability

警告

While all providers have their purpose, ChangeNotifierProviders are not recommended for scalable applications. See Why Immutability. It exists in the flutter_riverpod package to provide an easy migration path from package:provider, and allows for some flutter specific use-cases such as integration with some Navigator 2 packages. :::

Provider Modifiers

All Providers have a built-in way to add extra functionalities to your different providers.

They may add new features to the ref object or change slightly how the provider is consumed. Modifiers can be used on all providers, with a syntax similar to named constructor:

final myAutoDisposeProvider = StateProvider.autoDispose<int>((ref) => 0);
final myFamilyProvider = Provider.family<String, int>((ref, id) => '$id');

At the moment, there are two modifiers available:

• .autoDispose, which will make the provider automatically destroy its state when it is no longer being listened to.
• .family, which allows creating a provider from external parameters.

A provider can use multiple modifiers at once:

final userProvider = FutureProvider.autoDispose.family<User, int>((ref, userId) async {
  return fetchUser(userId);
});

:::

That's it for this guide!

You can continue with Reading a Provider. Alternatively, you can see Combining Provider States.