官方API文档Scaffold的of方法说明有说明调用Scaffold.of方法是在Scallfold的子组件的Build方法中，也就是不能直接在构建Scaffold的build方法里调用，否则会抛异常。
Typical usage of the Scaffold.of function is to call it from within the build method of a child of a Scaffold.
通常为显示一个SnackBar需要构建一个Builder，通过Builder的context调用（原因是）
Scallfold.of(context).showSnackBar(SnackBar(contenxt: Text('这是一个SnackBar'));
显示SnackBar，官方典型示例代码如下所示：
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!'),
),
);
},
),
);
}
}
但是若直接在构建Scallfold的build方法中调用会报异常:
Scaffold.of() called with a context that does not contain a Scaffold.
错误代码如下所示：
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')),
);
}
}
这时候要不就是按官方的，将需要显示SnackBar的代码另外抽离一个自定义子组件，在子组件的build方法再显示SnackBar，要不就是在Scaffold的build方法体对该子组件再包一层Builder，如下所示。
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')),
);
}
}
https://github.com/AiguangLi/Flutter
