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小編給大家分享一下在Android中的Flutter要如何正確顯示SnackBar,希望大家閱讀完這篇文章之后都有所收獲,下面讓我們一起去探討吧!
官方API文檔Scaffold的of方法說明有說明調(diào)用Scaffold.of方法是在Scallfold的子組件的Build方法中,也就是不能直接在構(gòu)建Scaffold的build方法里調(diào)用,否則會拋異常。
Typical usage of the Scaffold.of function is to call it from within the build method of a child of a Scaffold.
通常為顯示一個SnackBar需要構(gòu)建一個Builder,通過Builder的context調(diào)用(原因是)
Scallfold.of(context).showSnackBar(SnackBar(contenxt: Text('這是一個SnackBar'));
顯示SnackBar,官方典型示例代碼如下所示:
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子組件里的build方法可以調(diào)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!'), ), ); }, ), ); } }
但是若直接在構(gòu)建Scallfold的build方法中調(diào)用會報異常:
Scaffold.of() called with a context that does not contain a Scaffold.
錯誤代碼如下所示:
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方法里使用會拋異常 Scaffold.of(context).showSnackBar( SnackBar( content: Text('Have a snack!'), ), ); }, ), ), appBar: AppBar(title: Text('Scaffold.of Example')), ); } }
這時候要不就是按官方的,將需要顯示SnackBar的代碼另外抽離一個自定義子組件,在子組件的build方法再顯示SnackBar,要不就是在Scaffold的build方法體對該子組件再包一層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( ///在子組件外再包一層builder,讓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解決 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')), ); } }
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