標籤頁行爲導致Xamarin.Forms應用程序崩潰

Question

我想實現一個事件的命令行爲（這表現在Xamarin：https://github.com/xamarin/xamarin-forms-samples/tree/master/Behaviors/EventToCommandBehavior）一旦

，我嘗試將TabbedPage.Behaviors部分添加到我的TabbedPage .XAML應用程序會崩潰上推出iOS中，以下情況例外：

Foundation.MonoTouchException: Objective-C exception thrown. Name: NSInternalInconsistencyException Reason: Application windows are expected to have a root view controller at the end of application launch 
Native stack trace: 
    0 CoreFoundation      0x000000010b48434b __exceptionPreprocess + 171 
    1 libobjc.A.dylib      0x0000000115be221e objc_exception_throw + 48 
    2 CoreFoundation      0x000000010b488442 +[NSException raise:format:arguments:] + 98 
    3 Foundation       0x000000010c065e4d -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195 
    4 UIKit        0x000000010ecb0619 -[UIApplication _runWithMainScene:transitionContext:completion:] + 3827 
    5 UIKit        0x000000010ecacf69 -[UIApplication workspaceDidEndTransaction:] + 188 
    6 FrontBoardServices     0x000000011890b723 __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 24 
    7 FrontBoardServices     0x000000011890b59c -[FBSSerialQueue _performNext] + 189 
    8 FrontBoardServices     0x000000011890b925 -[FBSSerialQueue _performNextFromRunLoopSource] + 45 
    9 CoreFoundation      0x000000010b429311 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17 
    10 CoreFoundation      0x000000010b40e59c __CFRunLoopDoSources0 + 556 
    11 CoreFoundation      0x000000010b40da86 __CFRunLoopRun + 918 
    12 CoreFoundation      0x000000010b40d494 CFRunLoopRunSpecific + 420 
    13 UIKit        0x000000010ecab7e6 -[UIApplication _run] + 434 
    14 UIKit        0x000000010ecb1964 UIApplicationMain + 159 
    15 ???         0x000000012e4eb58c 0x0 + 5071877516 
    16 ???         0x000000012e4eb1fd 0x0 + 5071876605 

    at at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr) 
    at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Users/builder/data/lanes/3969/44931ae8/source/xamarin-macios/src/UIKit/UIApplication.cs:79 
    at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00038] in /Users/builder/data/lanes/3969/44931ae8/source/xamarin-macios/src/UIKit/UIApplication.cs:63 
    at TabDemo.iOS.Application.Main (System.String[] args) [0x00008] in /Users/Shared/git/Experiments/TabDemo/TabDemo.iOS/Main.cs:17 

在Android應用程序只是掛在發射和永不顯示標籤頁。

MyTabbedPage.Xaml看起來是這樣的：

<?xml version="1.0" encoding="UTF-8"?> 
<TabbedPage 
    xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms" 
    prism:ViewModelLocator.AutowireViewModel="True" 
    xmlns:local="clr-namespace:TabDemo.Views" 
    xmlns:behaviors="clr-namespace:TabDemo.Views.Behaviors" 
    x:Class="TabDemo.Views.MyTabbedPage"> 
    <TabbedPage.Behaviors> 
     <behaviors:EventToCommandBehavior 
      EventName="CurrentPageChanged" 
      Command="{Binding OnCurrentPageChangedCommand}" /> 
    </TabbedPage.Behaviors> 
    <NavigationPage 
     Title="Main"> 
     <x:Arguments> 
      <local:MainPage /> 
     </x:Arguments> 
    </NavigationPage> 
    <NavigationPage 
     Title="Second"> 
     <x:Arguments> 
      <local:SecondPage /> 
     </x:Arguments> 
    </NavigationPage> 
</TabbedPage> 

連線了在代碼的行爲背後導致完全相同的問題。如果我刪除TabbedPage.Behaviors部分中的行爲條目，則該應用程序在兩個平臺上均可正常運行。

任何人都可以闡明我可能做錯了什麼？

Answer 1

我相信你可能需要改變EventToCommandBehavior繼承的類型。打開EventToCommandBehavior.cs文件並更改：

public class EventToCommandBehavior : BehaviorBase<View> 

到：

public class EventToCommandBehavior : BehaviorBase<VisualElement> 

您還需要修改的兩種方法的簽名：

protected override void OnAttachedTo (View bindable) 

protected override void OnDetachingFrom (View bindable) 

到：

protected override void OnAttachedTo(VisualElement bindable) 

protected override void OnDetachingFrom(VisualElement bindable) 

Answer 2

如果你正在關注str對於EventToCommandBehavior來說是一個例子，我的猜測是有一個異常被拋出，因爲你的命令需要的類型不同於傳入的類型。沒有看到你的項目，或者知道你想要完成什麼，很難給你一個更清晰的方向。

，你可以嘗試採取看看這個要點，提供瞭如何將行爲添加到TabbedPage上CurrentPageChanged一個例子：https://gist.github.com/dansiegel/cdc81671f3610d8992d70c65c202f0a4