颤动：使用navigator.dart时失败断言
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我是Flutter的新手并且玩弄它.所以,请耐心等待我.
单击PopupMenuButton的特定菜单项时会抛出以下异常,但始终只是第二次：
这里设置： 为了指定菜单项,已经定义了以下类： class PopupMenuChoice {
const PopupMenuChoice({this.title,this.pageRoute});
final String title;
final MaterialPageRoute pageRoute;
}
在AppBar的actions属性中定义PopupMenuButton： new PopupMenuButton<PopupMenuChoice>(
itemBuilder: (BuildContext context) {
return _popupMenus.map((PopupMenuChoice choice) {
return new PopupMenuItem<PopupMenuChoice>(
value: choice,child: new Text(choice.title),);
}).toList();
},onSelected: _popupMenuSelected,),
相应的小部件在下面的类中定义(AppBar在此类的“return new Scaffold”中创建)： class RandomWordsState extends State<RandomWords> {
final _suggestions = <WordPair>[];
final _saved = new Set<WordPair>();
final _popupMenus = <PopupMenuChoice>[];
...
}
正如您所看到的,有用于保存WordPair对象的私有变量,还有用于菜单选项的私有变量. _popupMenus列表在“构建覆盖”中设置： @override
Widget build(BuildContext context) {
// Setup page routes
if (_popupMenus.where((p) => p.title == 'Saved Suggestions').length == 0) {
final _pageRouteSavedSuggestions = new MaterialPageRoute(
builder: (context) {
final tiles = _saved.map(
(pair) {
return new ListTile(
title: new Text(
pair.asPascalCase,style: _biggerFont,);
},);
final divided = ListTile
.divideTiles(
context: context,tiles: tiles,)
.toList();
return new Scaffold(
appBar: new AppBar(
title: new Text('Saved Suggestions'),body: new ListView(children: divided),);
},);
_popupMenus.add(new PopupMenuChoice(
title: 'Saved Suggestions',pageRoute: _pageRouteSavedSuggestions));
}
if (_popupMenus.where((p) => p.title == 'TEST Page').length == 0) {
final _pageRouteTest = new MaterialPageRoute(
builder: (context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('TEST Page'),body: new Text('Some content...'),);
_popupMenus.add(
new PopupMenuChoice(title: 'TEST Page',pageRoute: _pageRouteTest));
}
...
在PopupMenuChoice定义的MaterialPageRoute中,私有变量可能是访问权限(例如_saved). 这里是onSelected的PopupMenuButton的相应事件处理程序： void _popupMenuSelected(PopupMenuChoice choice) {
Navigator.of(context).push(choice.pageRoute);
}
任何人都可以解释为什么抛出这个异常？它怎么能被阻止？ 谢谢, 在特定菜单项上单击第二次时,调试控制台中的其他信息：
解决方法
你有没有尝试过：
void _popupMenuSelected(PopupMenuChoice choice) {
Navigator.push(context,choice.pageRoute);
}
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