Tabbed Page Behavior causes app crash in Xamarin.Forms

Question

I am trying to implement an event to command behavior(as demonstrated by Xamarin:https://github.com/xamarin/xamarin-forms-samples/tree/master/Behaviors/EventToCommandBehavior)

As soon as I try to add a TabbedPage.Behaviors section to my TabbedPage.Xaml the app will crash on launch in iOS with the following exception:

    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

On Android the app simply hangs on launch and never displays the tabbed page.

MyTabbedPage.Xaml looks like this:

<?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>

Wiring up the behavior in the code behind leads to the exact same problem. If I remove the behavior entry in the TabbedPage.Behaviors section the app runs fine on both platforms.

Can anyone shed some light on what I might be doing wrong?

Answer 1

I believe you might need to change the type that EventToCommandBehavior inherits from. Open the EventToCommandBehavior.cs file and change:

public class EventToCommandBehavior : BehaviorBase<View>

to:

public class EventToCommandBehavior : BehaviorBase<VisualElement>

You will also need to modify the signature of two methods:

protected override void OnAttachedTo (View bindable)

protected override void OnDetachingFrom (View bindable)

to:

protected override void OnAttachedTo(VisualElement bindable)

protected override void OnDetachingFrom(VisualElement bindable)

Answer 2

If you're following the straight up example for the EventToCommandBehavior, my guess would be that there is an exception being thrown because your command expects a different type than what is being passed in. Without seeing your project, or knowing what you're trying to accomplish, it's hard to give you a clearer direction.

You might try taking a look at this Gist that provides an example of how to add a behavior to a TabbedPage on CurrentPageChanged: https://gist.github.com/dansiegel/cdc81671f3610d8992d70c65c202f0a4