存储键值对数据

如果你要存储的键值集合相对较少，则可以用 shared_preferences 插件。

If you have a relatively small collection of key-values to save, you can use the shared_preferences plugin.

通常你需要在两个平台用原生的方式存储数据。幸运的是 shared_preferences 插件可以把 key-value 保存到磁盘中。它通过封装 iOS 上的 NSUserDefaults 和 Android 上的 SharedPreferences 为简单数据提供持久化存储。

Normally, you would have to write native platform integrations for storing data on both iOS and Android. Fortunately, the shared_preferences plugin can be used to persist key-value data on disk. The shared preferences plugin wraps NSUserDefaults on iOS and SharedPreferences on Android, providing a persistent store for simple data.

这个教程包含以下步骤：

This recipe uses the following steps:

1. 添加依赖

   Add the dependency.

2. 保存数据

   Save Data.

3. 读取数据

   Read Data.

4. 移除数据

   Remove Data.

1. 添加依赖

1. Add the dependency

在开始之前，你需要在 pubspec.yaml 文件中添加 shared_preferences 插件：

Before starting, add the shared_preferences plugin to the pubspec.yaml file:

dependencies:
  flutter:
    sdk: flutter
  shared_preferences: "<newest version>"

2. 保存数据

2. Save data

要存储数据，请使用 SharedPreferences 类的 setter 方法。 Setter方法可用于各种基本数据类型，例如 setInt、setBool 和 setString。

To persist data, use the setter methods provided by the SharedPreferences class. Setter methods are available for various primitive types, such as setInt, setBool, and setString.

Setter 方法做两件事：首先，同步更新 key-value 到内存中，然后保存到磁盘中。

Setter methods do two things: First, synchronously update the key-value pair in-memory. Then, persist the data to disk.

// obtain shared preferences
final prefs = await SharedPreferences.getInstance();

// set value
prefs.setInt('counter', counter);

3. 读取数据

3. Read data

要读取数据，请使用 SharedPreferences 类相应的 getter 方法。对于每一个 setter 方法都有对应的 getter 方法。例如，你可以使用 getInt、getBool 和 getString 方法。

To read data, use the appropriate getter method provided by the SharedPreferences class. For each setter there is a corresponding getter. For example, you can use the getInt, getBool, and getString methods.

final prefs = await SharedPreferences.getInstance();

// Try reading data from the counter key. If it doesn't exist, return 0.
final counter = prefs.getInt('counter') ?? 0;

4. 移除数据

4. Remove data

使用 remove() 方法删除数据。

To delete data, use the remove() method.

final prefs = await SharedPreferences.getInstance();

prefs.remove('counter');

支持类型

Supported types

虽然使用 key-value 存储非常简单方便，但是它也有以下局限性：

Although key-value storage is easy and convenient to use, it has limitations:

• 只能用于基本数据类型： int、double、bool、string 和 stringList。

  Only primitive types can be used: int, double, bool, string, and stringList.

• 不适用于大量数据的存储。

  It’s not designed to store a lot of data.

关于 Android 平台上 Shared Preferences 的更多信息，请前往 Android 开发者网站上查看 Shared preferences 文档 。

For more information about shared preferences on Android, see the shared preferences documentation on the Android developers website.

测试支持

Testing support

使用 shared_preferences 存储数据来测试代码是一个不错的思路。为此，你需要模拟出 shared_preferences 库的 MethodChannel 方法。

It’s a good idea to test code that persists data using shared_preferences. You can do this by mocking out the MethodChannel used by the shared_preferences library.

在你的测试中，你可以通过在测试文件的 setupAll() 方法中添加运行以下代码，对 SharedPreferences 的值进行初始：

Populate SharedPreferences with initial values in your tests by running the following code in a setupAll() method in your test files:

const MethodChannel('plugins.flutter.io/shared_preferences')
  .setMockMethodCallHandler((MethodCall methodCall) async {
    if (methodCall.method == 'getAll') {
      return <String, dynamic>{}; // set initial values here if desired
    }
    return null;
  });

完整示例

Complete example

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of the application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Shared preferences demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Shared preferences demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  @override
  void initState() {
    super.initState();
    _loadCounter();
  }

  //Loading counter value on start
  _loadCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = (prefs.getInt('counter') ?? 0);
    });
  }

  //Incrementing counter after click
  _incrementCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = (prefs.getInt('counter') ?? 0) + 1;
      prefs.setInt('counter', _counter);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}