State management

The state of a Flutter app refers to all the objects it uses to display its UI or manage system resources. State management is how we organize our app to most effectively access these objects and share them between different widgets.

This page explores many aspects of state management, including:

• Using a StatefulWidget
• Sharing state between widgets using constructors, InheritedWidgets, and callbacks
• Using Listenables to notify other widgets when something changes
• Using Model-View-ViewModel (MVVM) for your application's architecture

For other introductions to state management, check out these resources:

• Video: Managing state in Flutter. This video shows how to use the riverpod package.

flutter_dash Tutorial: State management. This shows how to use ChangeNotifer with the provider package.

This guide doesn't use third-party packages like provider or Riverpod. Instead, it only uses primitives available in the Flutter framework.

The simplest way to manage state is to use a StatefulWidget, which stores state within itself. For example, consider the following widget:

class MyCounter extends StatefulWidget {
  const MyCounter({super.key});

  @override
  State<MyCounter> createState() => _MyCounterState();
}

class _MyCounterState extends State<MyCounter> {
  int count = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text('Count: $count'),
        TextButton(
          onPressed: () {
            setState(() {
              count++;
            });
          },
          child: Text('Increment'),
        )
      ],
    );
  }
}

This code illustrates two important concepts when thinking about state management:

• Encapsulation : The widget that uses MyCounter has no visibility into the underlying count variable and no means to access or change it.
• Object lifecycle : The _MyCounterState object and its count variable are created the first time that MyCounter is built, and exist until it's removed from the screen. This is an example of ephemeral state.

You might find the following resources to be useful:

• Article: Ephemeral state and app state
• API docs: StatefulWidget

Some scenarios where an app needs to store state include the following:

• To update the shared state and notify other parts of the app
• To listen for changes to the shared state and rebuild the UI when it changes

This section explores how you can effectively share state between different widgets in your app. The most common patterns are:

• Using widget constructors (sometimes called "prop drilling" in other frameworks)
• Using InheritedWidget (or a similar API, such as the provider package).
• Using callbacks to notify a parent widget that something has changed

Since Dart objects are passed by reference, it's very common for widgets to define the objects they need to use in their constructor. Any state you pass into a widget's constructor can be used to build its UI:

class MyCounter extends StatelessWidget {
  final int count;
  const MyCounter({super.key, required this.count});

  @override
  Widget build(BuildContext context) {
    return Text('$count');
  }
}

This makes it obvious for other users of your widget to know what they need to provide in order to use it:

Column(
  children: [
    MyCounter(
      count: count,
    ),
    MyCounter(
      count: count,
    ),
    TextButton(
      child: Text('Increment'),
      onPressed: () {
        setState(() {
          count++;
        });
      },
    )
  ],
)

Passing the shared data for your app through widget constructors makes it clear to anyone reading the code that there are shared dependencies. This is a common design pattern called dependency injection and many frameworks take advantage of it or provide tools to make it easier.

Manually passing data down the widget tree can be verbose and cause unwanted boilerplate code, so Flutter provides InheritedWidget, which provides a way to efficiently host data in a parent widget so that child widgets can get access them without storing them as a field.

To use InheritedWidget, extend the InheritedWidget class and implement the static method of() using dependOnInheritedWidgetOfExactType. A widget calling of() in a build method creates a dependency that is managed by the Flutter framework, so that any widgets that depend on this InheritedWidget rebuild when this widget re-builds with new data and updateShouldNotify returns true.

class MyState extends InheritedWidget {
  const MyState({
    super.key,
    required this.data,
    required super.child,
  });

  final String data;

  static MyState of(BuildContext context) {
    // This method looks for the nearest `MyState` widget ancestor.
    final result = context.dependOnInheritedWidgetOfExactType<MyState>();

    assert(result != null, 'No MyState found in context');

    return result!;
  }

  @override
  // This method should return true if the old widget's data is different
  // from this widget's data. If true, any widgets that depend on this widget
  // by calling `of()` will be re-built.
  bool updateShouldNotify(MyState oldWidget) => data != oldWidget.data;
}

Next, call the of() method from the build()method of the widget that needs access to the shared state:

class HomeScreen extends StatelessWidget {
  const HomeScreen({super.key});

  @override
  Widget build(BuildContext context) {
    var data = MyState.of(context).data;
    return Scaffold(
      body: Center(
        child: Text(data),
      ),
    );
  }
}

You can notify other widgets when a value changes by exposing a callback. Flutter provides the ValueChanged type, which declares a function callback with a single parameter:

typedef ValueChanged<T> = void Function(T value);

By exposing onChanged in your widget's constructor, you provide a way for any widget that is using this widget to respond when your widget calls onChanged.

class MyCounter extends StatefulWidget {
  const MyCounter({super.key, required this.onChanged});

  final ValueChanged<int> onChanged;

  @override
  State<MyCounter> createState() => _MyCounterState();
}

For example, this widget might handle the onPressed callback, and call onChanged with its latest internal state for the count variable:

TextButton(
  onPressed: () {
    widget.onChanged(count++);
  },
),

For more information on sharing state between widgets, check out the following resources:

• Article: Flutter Architectural Overview—State management
• Video: Pragmatic state management
• Video: InheritedWidgets
• Video: A guide to Inherited Widgets
• Sample: Provider shopper
• Sample: Provider counter
• API Docs: InheritedWidget

Now that you've chosen how you want to share state in your app, how do you update the UI when it changes? How do you change the shared state in a way that notifies other parts of the app?

Flutter provides an abstract class called Listenable that can update one or more listeners. Some useful ways to use listenables are:

• Use a ChangeNotifier and subscribe to it using a ListenableBuilder
• Use a ValueNotifier with a ValueListenableBuilder

To use ChangeNotifier, create a class that extends it, and call notifyListeners whenever the class needs to notify its listeners.

class CounterNotifier extends ChangeNotifier {
  int _count = 0;
  int get count => _count;

  void increment() {
    _count++;
    notifyListeners();
  }
}

Then pass it to ListenableBuilder to ensure that the subtree returned by the builder function is re-built whenever the ChangeNotifier updates its listeners.

Column(
  children: [
    ListenableBuilder(
      listenable: counterNotifier,
      builder: (context, child) {
        return Text('counter: ${counterNotifier.count}');
      },
    ),
    TextButton(
      child: Text('Increment'),
      onPressed: () {
        counterNotifier.increment();
      },
    ),
  ],
)

A ValueNotifier is a simpler version of a ChangeNotifier, that stores a single value. It implements the ValueListenable and Listenable interfaces, so it's compatible with widgets such as ListenableBuilder and ValueListenableBuilder. To use it, create an instance of ValueNotifier with the initial value:

ValueNotifier<int> counterNotifier = ValueNotifier(0);

Then use the value field to read or update the value, and notify any listeners that the value has changed. Because ValueNotifier extends ChangeNotifier, it is also a Listenable and can be used with a ListenableBuilder. But you can also use ValueListenableBuilder, which provides the value in the builder callback:

Column(
  children: [
    ValueListenableBuilder(
      valueListenable: counterNotifier,
      builder: (context, child, value) {
        return Text('counter: $value');
      },
    ),
    TextButton(
      child: Text('Increment'),
      onPressed: () {
        counterNotifier.value++;
      },
    ),
  ],
)

To learn more about Listenable objects, check out the following resources:

• API Docs: Listenable
• API Docs: ValueNotifier
• API Docs: ValueListenable
• API Docs: ChangeNotifier
• API Docs: ListenableBuilder
• API Docs: ValueListenableBuilder
• API Docs: InheritedNotifier

Now that we understand how to share state and notify other parts of the app when its state changes, we're ready to start thinking about how to organize the stateful objects in our app.

This section describes how to implement a design pattern that works well with reactive frameworks like Flutter, called Model-View-ViewModel or MVVM.

The Model is typically a Dart class that does low-level tasks such as making HTTP requests, caching data, or managing system resources such as a plugin. A model doesn't usually need to import Flutter libraries.

For example, consider a model that loads or updates the counter state using an HTTP client:

import 'package:http/http.dart';

class CounterData {
  CounterData(this.count);

  final int count;
}

class CounterModel {
  Future<CounterData> loadCountFromServer() async {
    final uri = Uri.parse('https://myfluttercounterapp.net/count');
    final response = await get(uri);

    if (response.statusCode != 200) {
      throw ('Failed to update resource');
    }

    return CounterData(int.parse(response.body));
  }

  Future<CounterData> updateCountOnServer(int newCount) async {
    // ...
  }
}

This model doesn't use any Flutter primitives or make any assumptions about the platform it's running on; its only job is to fetch or update the count using its HTTP client. This allows the model to be implemented with a Mock or Fake in unit tests, and defines clear boundaries between your app's low-level components and the higher-level UI components needed to build the full app.

The CounterData class defines the structure of the data and is the true "model" of our application. The model layer is typically responsible for the core algorithms and data structures needed for your app. If you are interested in other ways to define the model, such as using immutable value types, check out packages like freezed or build_collection on pub.dev.

A ViewModel binds the View to the Model. It protects the model from being accessed directly by the View, and ensures that data flow starts from a change to the model. Data flow is handled by the ViewModel, which uses notifyListeners to inform the View that something changed. The ViewModel is like a waiter in a restaurant that handles the communication between the kitchen (model) and the customers (views).

import 'package:flutter/foundation.dart';

class CounterViewModel extends ChangeNotifier {
  final CounterModel model;
  int? count;
  String? errorMessage;
  CounterViewModel(this.model);

  Future<void> init() async {
    try {
      count = (await model.loadCountFromServer()).count;
    } catch (e) {
      errorMessage = 'Could not initialize counter';
    }
    notifyListeners();
  }

  Future<void> increment() async {
    var count = this.count;
    if (count == null) {
      throw('Not initialized');
    }
    try {
      await model.updateCountOnServer(count + 1);
      count++;
    } catch(e) {
      errorMessage = 'Count not update count';
    }
    notifyListeners();
  }
}

Notice that the ViewModel stores an errorMessage when it receives an error from the Model. This protects the View from unhandled runtime errors, which could lead to a crash. Instead, the errorMessage field can be used by the view to show a user-friendly error message.

Since our ViewModel is a ChangeNotifier, any widget with a reference to it can use a ListenableBuilder to rebuild its widget tree when the ViewModel notifies its listeners:

ListenableBuilder(
  listenable: viewModel,
  builder: (context, child) {
    return Column(
      children: [
        if (viewModel.errorMessage != null)
          Text(
            'Error: ${viewModel.errorMessage}',
            style: Theme.of(context)
                .textTheme
                .labelSmall
                ?.apply(color: Colors.red),
          ),
        Text('Count: ${viewModel.count}'),
        TextButton(
          onPressed: () {
            viewModel.increment();
          },
          child: Text('Increment'),
        ),
      ],
    );
  },
)

This pattern allows the business logic of your application to be separate from the UI logic and low-level operations performed by the Model layer.

This page touches the surface of state management as there are many ways to organize and manage the state of your Flutter application. If you would like to learn more, check out the following resources:

• Article: List of state management approaches
• Repository: Flutter Architecture Samples

As this section of the website is evolving, we welcome your feedback!

除非另有说明，本文档之所提及适用于 Flutter 的最新稳定版本，本页面最后更新时间: 2024-10-23。 查看文档源码 或者 为本页面内容提出建议。