今天和大家分享有关于“在Android中的Flutter要怎么正确显示SnackBar?”这个问题的相关内容分享,希望小编的小伙伴们有所帮助!
简介
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)
官方示例
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!'),
),
);
},
),
);
}
}
错误示例
但是若直接在构建Scallfold的build方法中调用会报异常:
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')),
);
}
}
解决方法一:Scaffold的子组件通过Builder构建
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')),
);
}
}
解决方法二:使用GlobalKey存储ScaffoldState
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')),
);
}
}
Flutter学习Github代码仓库
https://github.com/AiguangLi/Flutter
那么以上的内容就是今天有关于“在Android中的Flutter要怎么正确显示SnackBar?”这个问题的相关内容分享,更多有关于Android的相关知识内容我们都可以在W3Cschool中进行学习和了解!