Flutter的两种本地存储方式之 SharedPreferences（1）

SharedPreferences 存储

缓存少量的键值对信息（比如记录用户是否阅读了公告，或是简单的计数），可以使用 SharedPreferences。

SharedPreferences 会以原生平台相关的机制，为简单的键值对数据提供持久化存储，即在 iOS 上使用 NSUserDefaults，在 Android 使用 SharedPreferences。

SharedPreferences 的使用方式非常简单方便。不过需要注意的是，以键值对的方式只能存储基本类型的数据，比如 int、double、bool 和 string。

1. pubspec.yaml引入

shared_preferences: 0.5.12+4

2. 代码

import 'dart:developer';

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 your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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;

//涉及到耗时的文件读写，以异步的方式对这些操作进行包装

//读取SharedPreferences中key为counter的值
  Future<int> _loadCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    int counter = (prefs.getInt('counter') ?? 0);
    return counter;
  }

//递增写入SharedPreferences中key为counter的值
  Future<void> _incrementCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    int counter = (prefs.getInt('counter') ?? 0) + 1;
    print("当前值: " + counter.toString());
    prefs.setInt('counter', counter);  //setter（setInt）方法会同步更新内存中的键值对，然后将数据保存至磁盘，因此无需再调用更新方法强制刷新缓存
  }

  _fresh() {
    // _incrementCounter();

    _incrementCounter().then((_) => {
          _loadCounter().then((value) {
            setState(() {
              _counter = value;
            });
            print("before:$value");
          })
        });
  }

  // void _incrementCounter() {
  //   setState(() {

  //     _counter++;
  //   });
  // }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        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: _fresh,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}