在Flutter中显示SnackBar [英] Display SnackBar in Flutter

问题描述

我想在Flutter的有状态小部件内显示一个简单的SnackBar.我的应用程序使用名为 MyHomePage 的有状态小部件创建了 MaterialApp 的新实例.

I want to display a simple SnackBar inside Flutter's stateful widget. My application creates new instance of MaterialApp with a stateful widget called MyHomePage.

我尝试在showSnackBar()方法中显示SnackBar.但是它失败，显示为<在空值上调用了'showSnackBar''.

I try to show the SnackBar in showSnackBar() method. But it fails with 'The method 'showSnackBar' was called on null'.

此代码有什么问题?

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {

  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();

  @override
  void initState() {
    super.initState();
    showInSnackBar("Some text");
  }

  @override
  Widget build(BuildContext context) {
    return new Padding(
            key: _scaffoldKey,
            padding: const EdgeInsets.all(16.0),
            child: new Text("Simple Text")
    );
  }

  void showInSnackBar(String value) {
    _scaffoldKey.currentState.showSnackBar(new SnackBar(
        content: new Text(value)
    ));
  }
}

解决方案:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        title: 'Flutter',
        theme: new ThemeData(
            primarySwatch: Colors.blue,
        ),
        home: new Scaffold(body: new MyHomePage()),
      );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {

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

  @override
  Widget build(BuildContext context) {
    showInSnackBar("Some text");
    return new Padding(
        padding: const EdgeInsets.all(16.0),
        child: new Scaffold(
          body: new Text("Simple Text")
        )
    );
  }

  void showInSnackBar(String value) {
    Scaffold.of(context).showSnackBar(new SnackBar(
        content: new Text(value)
    ));
  }
}

推荐答案

存在三个问题.首先是您在任何地方都没有脚手架，而脚手架小部件是知道如何显示小吃店的小部件.第二个是您有一把握住脚手架的钥匙，但是您将其放在填充物上(填充物对小吃店一无所知).第三个是因为与之关联的小部件有机会被初始化之前就已经使用了密钥，因为initState是在构建之前被调用的.

There's three problems. The first is that you don't have a Scaffold anywhere, and the Scaffold widget is the one that knows how to show snack bars. The second is that you have a key for getting a hold of the scaffold, but you've put it on a Padding instead (and Paddings don't have any knowledge of snack bars). The third is that you've used the key before the widget that it's associated with has had a chance to be initialised, since initState is called before build.

最简单的解决方案是将MyApp小部件中的home行更改为:

The simplest solution is to change the home line in your MyApp widget to:

home: new Scaffold(body: new MyHomePage()),

...然后删除所有提及_scaffoldKey的地方，而在您当前拥有_scaffoldKey.currentState的地方使用Scaffold.of(context).

...and then remove all mention of _scaffoldKey and instead use Scaffold.of(context) where you currently have _scaffoldKey.currentState.

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