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POOverriding WP8 Navigation - crash in PhoneApplicationPage
 

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  1. POOverriding WP8 Navigation - crash in PhoneApplicationPage
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    <p>I'm trying to do something that's arguably a bad idea, but I think it's still possible. I'm trying to override how WP8 handles the Back Button and implement it myself. I theorize that if I:</p> <h3>The Plan</h3> <ol> <li>Only ever create one "Frame" and "Page" in the entire application</li> <li>Always handle <code>PhoneApplicationPage.BackKeyPress</code> myself unless they were about to back out of the application.</li> </ol> <h3>The Repro</h3> <p><a href="http://cl.ly/3R3F3L1X1Y0q" rel="nofollow">Here's a sample project that has the crash</a></p> <h3>The code</h3> <p>..then it <em>should</em> work. However, my attempts are being thwarted by Windows Phone. <a href="https://github.com/reactiveui/ReactiveUI/blob/master/ReactiveUI.Mobile/WP8AutoSuspendApplication.cs" rel="nofollow">Here's the code</a>:</p> <pre><code>// This basically happens on PhoneApplicationService.OnLaunched _viewModelChanged.StartWith(ViewModel).Where(x =&gt; x != null).Subscribe(vm =&gt; { var page = default(IViewFor); var frame = RootVisual as PhoneApplicationFrame; // Find the initial PhoneApplicationPage for the app page = RxApp.GetService&lt;IViewFor&gt;("InitialPage"); // Depending on how we're being signalled (i.e. if this is cold start // vs. resume), we need to create the PhoneApplicationFrame ourselves if (frame == null) { frame = new PhoneApplicationFrame() { Content = page, }; } page.ViewModel = vm; var pg = page as PhoneApplicationPage; if (pg != null) { pg.BackKeyPress += (o, e) =&gt; { if (ViewModel.Router.NavigationStack.Count &lt;= 1 || ViewModel.Router.NavigateBack.CanExecute(null)) { return; } e.Cancel = true; ViewModel.Router.NavigateBack.Execute(null); }; } // Finally, set Application.RootVisual RootVisual = frame; }); </code></pre> <h3>Sadness</h3> <p>This works great, until right <em>after</em> this code executes, where a DispatcherItem queued by the framework crashes the app:</p> <pre><code>System.NullReferenceException occurred Message: A first chance exception of type 'System.NullReferenceException' occurred in Microsoft.Phone.ni.dll Microsoft.Phone.ni.dll!Microsoft.Phone.Controls.PhoneApplicationPage.InternalOnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e) Unknown Microsoft.Phone.ni.dll!Microsoft.Phone.Controls.PhoneApplicationPage.Microsoft.Phone.Controls.IPhoneApplicationPage.InternalOnNavigatedFromX(System.Windows.Navigation.NavigationEventArgs e) Unknown Microsoft.Phone.ni.dll!System.Windows.Navigation.NavigationService.RaiseNavigated(object content, System.Uri uri, System.Windows.Navigation.NavigationMode mode, bool isNavigationInitiator, Microsoft.Phone.Controls.IPhoneApplicationPage existingContentPage, Microsoft.Phone.Controls.IPhoneApplicationPage newContentPage) Unknown Microsoft.Phone.ni.dll!System.Windows.Navigation.NavigationService.CompleteNavigation(System.Windows.DependencyObject content, System.Windows.Navigation.NavigationMode mode) Unknown Microsoft.Phone.ni.dll!System.Windows.Navigation.NavigationService.ContentLoader_BeginLoad_Callback(System.IAsyncResult result) Unknown Microsoft.Phone.ni.dll!System.Windows.Navigation.PageResourceContentLoader.BeginLoad_OnUIThread(System.AsyncCallback userCallback, System.Windows.Navigation.PageResourceContentLoader.PageResourceContentLoaderAsyncResult result) Unknown Microsoft.Phone.ni.dll!System.Windows.Navigation.PageResourceContentLoader.BeginLoad.AnonymousMethod__0(object args) Unknown [Native to Managed Transition] mscorlib.ni.dll!System.Delegate.DynamicInvokeImpl(object[] args) Unknown System.Windows.ni.dll!System.Windows.Threading.DispatcherOperation.Invoke() Unknown System.Windows.ni.dll!System.Windows.Threading.Dispatcher.Dispatch(System.Windows.Threading.DispatcherPriority priority) Unknown System.Windows.ni.dll!System.Windows.Threading.Dispatcher.OnInvoke(object context) Unknown System.Windows.ni.dll!System.Windows.Hosting.CallbackCookie.Invoke(object[] args) Unknown System.Windows.RuntimeHost.ni.dll!System.Windows.RuntimeHost.ManagedHost.InvokeDelegate(System.IntPtr pHandle, int nParamCount, System.Windows.Hosting.NativeMethods.ScriptParam* pParams, System.Windows.Hosting.NativeMethods.ScriptParam* pResult) Unknown </code></pre>
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    1. COcould this be related to the fact this is a 'hardware' button and the event is transitioning from the phone OS to the application and therefore the phone OS is going to do what ever it wants before you get notified...
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      1. USAwkwardCoder
    2. CO@AwkwardCoder That's probably true - I'm screwing something up in initialization but I'm having trouble figuring out what it is
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      1. USPaul Betts
 

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