Flutter：共享首选项在启动时为空 [英] Flutter: Shared Preferences null on Startup

问题描述

问题：即使启动时，如果 prefs.getBool（'myBool'）返回 null （尽管应该已经设置并保存了我的共享首选项值）。但是，它在我按下按钮时就起作用了（我想是因为它已经完成了异步代码的运行）。

Problem: Shared preference bool value is null on startup even though I have given it a value if prefs.getBool('myBool') returns null (though my shared preferences value should already be set and saved). It does, however, work by the time I press a button (I assume because it has finished running the async code).

问题：如何强制将共享首选项设置为在启动时加载（因此我的值不是 null ）而不必按打印按钮？

Question: How can I force shared preferences to load on startup (so my value is not null) without having to press the print button?

示例代码：

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

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

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);

  @override
  createState() => new MyAppState();
}

class MyAppState extends State<MyApp> {
  final padding = const EdgeInsets.all(50.0);

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

    MySharedPreferences.load();
    MySharedPreferences.printMyBool();
  }

  @override
    Widget build(BuildContext context) {
      return new MaterialApp(
        home: new Scaffold(
          body: new Padding(
            padding: padding,
            child: new Column(
              children: <Widget>[
                new Padding(
                  padding: padding,
                  child: new RaisedButton(
                    child: new Text('Save True'),
                    onPressed: () => MySharedPreferences.save(myBool: true),
                  ),
                ),
                new Padding(
                  padding: padding,
                  child: new RaisedButton(
                    child: new Text('Save False'),
                    onPressed: () => MySharedPreferences.save(myBool: false),
                  ),
                ),
                new Padding(
                  padding: padding,
                  child: new RaisedButton(
                    child: new Text('Print myBool'),
                    onPressed: () => MySharedPreferences.printMyBool(),
                ),
              ),
            ],
          ),
        ), 
      ),
    );
  }
}

class MySharedPreferences {
  static bool _myBool;

  static void load() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    _myBool = prefs.getBool('myBool') ?? false;
  }

  static void save({myBool: bool}) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    _myBool = myBool;
    await prefs.setBool('myBool', _myBool);
  }

  static void printMyBool() {
    print('myBool: ${_myBool.toString()}');
  }
}

结果：
启动时， myBool：null 打印。按下按钮后，将打印 myBool：false / true 。

Results: On startup, myBool: null is printed. Once the button is pressed, myBool: false/true is then printed.

推荐答案

您的问题是您快速连续调用load（）和printMyBool（）。由于load（）是异步调用，因此它尚未执行任何代码，因此仅对其进行了调度。因此，printMyBool在加载主体之前执行。

Your problem is that you call load() and printMyBool() in quick succession. Because load() is async calling it hasn't executed any of its code, it has only scheduled it. So, printMyBool executes before the body of load.

不需要将静态函数放在类中，只需将它们声明为顶级函数即可。另外，您并不真的希望_myBool成为全局变量-它应该是Widget状态的一部分。这样，当您更新它时，Flutter知道要重绘树的哪些部分。

There's no need to put static functions in a class - just declare them as top level functions. Also, you don't really want _myBool to be a global - it should be part of a Widget's state. That way when you update it, Flutter knows what parts of your tree to redraw.

我已经对代码进行了重组，以删除冗余的静态变量。

I've restructured your code to remove the redundant statics.

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

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

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);

  @override
  createState() => new MyAppState();
}

const EdgeInsets pad20 = const EdgeInsets.all(20.0);
const String spKey = 'myBool';

class MyAppState extends State<MyApp> {
  SharedPreferences sharedPreferences;

  bool _testValue;

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

    SharedPreferences.getInstance().then((SharedPreferences sp) {
      sharedPreferences = sp;
      _testValue = sharedPreferences.getBool(spKey);
      // will be null if never previously saved
      if (_testValue == null) {
        _testValue = false;
        persist(_testValue); // set an initial value
      }
      setState(() {});
    });
  }

  void persist(bool value) {
    setState(() {
      _testValue = value;
    });
    sharedPreferences?.setBool(spKey, value);
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Padding(
                padding: pad20,
                child: new Text(
                    _testValue == null ? 'not ready' : _testValue.toString()),
              ),
              new Padding(
                padding: pad20,
                child: new RaisedButton(
                  child: new Text('Save True'),
                  onPressed: () => persist(true),
                ),
              ),
              new Padding(
                padding: pad20,
                child: new RaisedButton(
                  child: new Text('Save False'),
                  onPressed: () => persist(false),
                ),
              ),
              new Padding(
                padding: pad20,
                child: new RaisedButton(
                  child: new Text('Print myBool'),
                  onPressed: () => print(_testValue),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

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