Flutter 如何正确显示SnackBar

Scallfold.of(context).showSnackBar(SnackBar(contenxt: Text('这是一个SnackBar'));

import 'package:flutter/material.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 Code Sample for Scaffold.of.',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: MyScaffoldBody(),
        appBar: AppBar(title: Text('Scaffold.of Example')),
      ),
      color: Colors.white,
    );
  }
}

// 在Scaffold子组件里的build方法可以调用Scaffold.of方法
class MyScaffoldBody extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: RaisedButton(
        child: Text('SHOW A SNACKBAR'),
        onPressed: () {
          Scaffold.of(context).showSnackBar(
            SnackBar(
              content: Text('Have a snack!'),
            ),
          );
        },
      ),
    );
  }
}

Scaffold.of() called with a context that does not contain a Scaffold.

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: RaisedButton(
          child: Text('SHOW A SNACKBAR'),
          onPressed: () {
            ///直接在Scallfold的build方法里使用会抛异常
            Scaffold.of(context).showSnackBar(
              SnackBar(
                content: Text('Have a snack!'),
              ),
            );
          },
        ),
      ),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return Scaffold(
       ///在子组件外再包一层builder，让context不共用
      body: Builder(builder: (context) {
        return Center(
          child: RaisedButton(
            child: Text('SHOW A SNACKBAR'),
            onPressed: () {
              Scaffold.of(context).showSnackBar(
                SnackBar(
                  content: Text('Have a snack!'),
                ),
              );
            },
          ),
        );
      }),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}

import 'package:flutter/material.dart';

class ScaffoldSnackBarDemo extends StatelessWidget {
  final _scallfoldKey = GlobalKey<ScaffoldState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      ///使用GlobalKey解决
      key: _scallfoldKey,
      body: Center(
        child: RaisedButton(
          child: Text('SHOW A SNACKBAR'),
          onPressed: () {
            _scallfoldKey.currentState.showSnackBar(SnackBar(
              content: Text('Have a snack!'),
            ));
          },
        ),
      ),
      appBar: AppBar(title: Text('Scaffold.of Example')),
    );
  }
}