Flutter 如何在 Widget 构建方法中调用异步属性

发布于12月17日

我是Ffltter和Dart的新手，我正在try 构建一款在屏幕上显示设备信息的Ffltter应用程序.为此，我try 使用此库:‘DEVICE_INFO’，此处为:https://pub.dartlang.org/packages/device_info#-readme-tab-

在MyApp类的"build"方法中，我试图实例化"device_info"包中的对象，并调用一个碰巧是异步属性的属性.由于默认的构建方法不是异步的，我如何在构建方法中调用这个属性？以下是我的代码:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    DeviceInfoPlugin deviceInfoPlugin = DeviceInfoPlugin();
    AndroidDeviceInfo androidDeviceInfo = await deviceInfoPlugin.androidInfo;
    return MaterialApp(
      title: 'My Device Info',
      home: Scaffold(
        appBar: AppBar(
          title: Text('My Device Info'),
        ),
        body: Center(
          child: Text('Device model:' + 'Moto'),
        ),
      ),
    );
  }
}

推荐答案

我建议您使用FutureBuilder:

import 'package:flutter/material.dart';

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  // save in the state for caching!
  DeviceInfoPlugin _deviceInfoPlugin;

  @override
  void initState() {
    super.initState();
    _deviceInfoPlugin = DeviceInfoPlugin();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'My Device Info',
      home: Scaffold(
        appBar: AppBar(
          title: Text('My Device Info'),
        ),
        body: FutureBuilder<AndroidDeviceInfo>(
          future: _deviceInfoPlugin.androidInfo,
          builder: (BuildContext context, AsyncSnapshot<AndroidDeviceInfo> snapshot) {
            if (!snapshot.hasData) {
              // while data is loading:
              return Center(
                child: CircularProgressIndicator(),
              );
            } else {
              // data loaded:
              final androidDeviceInfo = snapshot.data;
              return Center(
                child: Text('Android version: ${androidDeviceInfo.version}'),
              );
            }
          },
        ),
      ),
    );
  }
}

通常，当使用FutureBuilder或Futures时，您必须记住，可以随时重新构建所包含的小部件(例如，因为设备旋转了，或者显示了键盘).这意味着再次调用build方法.

在这种特殊情况下，这不是问题，因为插件缓存了值并立即返回它，但通常情况下，您永远不应该在build方法中创建或获取Future.相反，可以从initState或单击事件处理程序执行此操作:

import 'package:flutter/material.dart';

class FooWidget extends StatefulWidget {
  @override
  _FooWidgetState createState() => _FooWidgetState();
}

class _FooWidgetState extends State<FooWidget> {
  Future<int> _bar;

  @override
  void initState() {
    super.initState();
    _bar = doSomeLongRunningCalculation();
  }

  void _retry() {
    setState(() {
      _bar = doSomeLongRunningCalculation();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        FutureBuilder<int>(
          future: _bar,
          builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
            if (snapshot.hasData) {
              return Text('The answer to everything is ${snapshot.data}');
            } else {
              return Text('Calculating answer...');
            }
          },
        ),
        RaisedButton(
          onPressed: _retry,
          child: Text('Retry'),
        )
      ],
    );
  }
}

Future<int> doSomeLongRunningCalculation() async {
  await Future.delayed(Duration(seconds: 5)); // wait 5 sec
  return 42;
}