颤动：使用navigator.dart时失败断言

概述我是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： ‘package:flutter/src/widgets/navigator.dart’: Failed assertion: line 1846 pos 12: ‘!_debugLocked’: is not true. 这里设置： 为了指定菜单项,已经定 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：

‘package:Flutter/src/Widgets/navigator.dart’: Failed assertion: line
1846 pos 12: ‘!_deBUGLocked’: is not true.

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：

@overrIDeWidget 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,
罗杰

在特定菜单项上单击第二次时,调试控制台中的其他信息：

E/Flutter (17133): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)]
Unhandled exception: E/Flutter (17133):
‘package:Flutter/src/Widgets/routes.dart’: Failed assertion: line 177
pos 12: ‘!_TransitionCompleter.isCompleted’: Cannot install a
MaterialPageRoute after disposing it. E/Flutter (17133): #0
_AssertionError._doThrowNew (dart:core/runtime/liberrors_patch.dart:37:39) E/Flutter (17133): #1
_AssertionError._throwNew (dart:core/runtime/liberrors_patch.dart:33:5) E/Flutter (17133): #2
TransitionRoute.install (package:Flutter/src/Widgets/routes.dart)
E/Flutter (17133): #3 ModalRoute.install
(package:Flutter/src/Widgets/routes.dart:740:11) E/Flutter (17133): #4
NavigatorState.push
(package:Flutter/src/Widgets/navigator.dart:1444:11) E/Flutter
(17133): #5 RandomWordsstate.build._popupMenuSelected
(file:///D:/Flutter%20Projects/startup_namer/lib/main.dart:166:29)
E/Flutter (17133): #6
_PopupMenubuttonState.showbuttonMenu. (package:Flutter/src/material/popup_menu.dart) E/Flutter (17133): #7
_RootZone.runUnary (dart:async/zone.dart:1381:54) E/Flutter (17133): #8 _FutureListener.handleValue (dart:async/future_impl.dart:129:18) E/Flutter (17133): #9
Future._propagatetoListeners.handleValueCallback
(dart:async/future_impl.dart:633:45) E/Flutter (17133): #10
Future._propagatetoListeners (dart:async/future_impl.dart:662:32)
E/Flutter (17133): #11 Future._completeWithValue
(dart:async/future_impl.dart:477:5) E/Flutter (17133): #12
Future._asyncComplete.
(dart:async/future_impl.dart:507:7) E/Flutter (17133): #13
_microtaskLoop (dart:async/schedule_microtask.dart:41:21) E/Flutter (17133): #14 _startMicrotaskLoop
(dart:async/schedule_microtask.dart:50:5)

解决方法 你有没有尝试过：

voID _popupMenuSelected(PopupMenuChoice choice) {  Navigator.push(context,choice.pageRoute);}

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