Question

我正在尝试做一些可能是坏主意的事情，但我认为这仍然是可能的。我试图覆盖WP8如何处理后退按钮并自己实现它。我认为如果我：

计划

只在整个应用程序中创建一个“框架”和“页面”
除非他们即将退出应用程序，否则请始终自行处理PhoneApplicationPage.BackKeyPress。

Repro

Here's a sample project that has the crash

代码

..然后它应该工作。但是，我的尝试被Windows Phone挫败了。 Here's the code：

// This basically happens on PhoneApplicationService.OnLaunched
_viewModelChanged.StartWith(ViewModel).Where(x => x != null).Subscribe(vm => {
    var page = default(IViewFor);
    var frame = RootVisual as PhoneApplicationFrame;

    // Find the initial PhoneApplicationPage for the app
    page = RxApp.GetService<IViewFor>("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) => {
            if (ViewModel.Router.NavigationStack.Count <= 1 ||
                ViewModel.Router.NavigateBack.CanExecute(null)) {
                return;
            }

            e.Cancel = true;
            ViewModel.Router.NavigateBack.Execute(null);
        };
    }

    // Finally, set Application.RootVisual
    RootVisual = frame;
});

伤心

这很有效，直到在执行此代码之后，框架排队的DispatcherItem会崩溃应用程序：

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

Answer 1

所以，我已经解决了这个问题 - 我的代码存在问题，因为我没有弄清楚WP8是如何工作的:)这就是我现在所理解的，这可能也是错的，但我还是会写的

您的WP8应用程序如何初始化：

操作系统通过补充App.xaml.cs
这意味着，您的构造函数会被运行，并且作为其中一部分，您创建了PhoneApplicationFrame
创建PhoneApplicationFrame似乎还设置了一个全局静态变量（在App.xaml中创建PhoneApplicationService时也会发生相同的事情，它会设置PhoneApplicationService.Current）。
NavigationService然后尝试通过资源字符串（即'/MainPage.xaml'）重新创建XAML视图。它会重新创建以前被逻辑删除的那个，或者如果没有，它默认为WMAppManifest中的那个（这是我不理解的部分）。
PhoneApplicationFrame.Navigated被NavigationService调用 - 这是你可以实际开始做的事情，包括最重要的是，设置Application.RootVisual ，这将发送Loading ...屏幕< / LI>
PhoneApplicationService.Launched或PhoneApplicationService.Activated一旦基本上全部设置完毕，最终会触发，具体取决于您的应用被唤醒的方式。

Answer 2

发现了这个问题。好吧，冰山一角。

InternalOnNavigatedFrom方法的代码是：

internal override void InternalOnNavigatedFrom(NavigationEventArgs e)
{
    PhoneApplicationPage content = e.Content as PhoneApplicationPage;
    string str = ((content == null) || (content.Title == null)) ? string.Empty : content.Title;
    PerfUtil.BeginLogMarker(MarkerEvents.TH_ONNAVIGATEDFROM_PAGE, string.Format("{0},{1},{2}", (base.Title == null) ? "" : base.Title, e.NavigationMode, str));
    this.OnNavigatedFrom(e);
    PerfUtil.EndLogMarker(MarkerEvents.TH_ONNAVIGATEDFROM_PAGE, string.Format("{0},{1},{2}", (base.Title == null) ? "" : base.Title, e.NavigationMode, str));
    DeviceStatus.KeyboardDeployedChanged -= new EventHandler(this.OnKeyboardDeployedChanged);
    Task rootTask = ApplicationHost.Current.RootTask;
    rootTask.OnVisibleRegionChange = (ITask.VisibleRegionChanged) Delegate.Remove(rootTask.OnVisibleRegionChange, new ITask.VisibleRegionChanged(this.OnVisibleRegionChange));
    Task task2 = ApplicationHost.Current.RootTask;
    task2.OnSipVisibilityChange = (ITask.SipVisibilityChange) Delegate.Remove(task2.OnSipVisibilityChange, new ITask.SipVisibilityChange(this.OnSipVisibilityChange));
    this._lastSipHeight = 0.0;
    this._dictionary = null;
}

经过一些调试后，我得出结论：e或Application.Current.RootTask都不为空。在我挠头之后，我查看了KeyboardDeployedChanged事件处理程序的代码：

public static  event EventHandler KeyboardDeployedChanged
{
    [SecuritySafeCritical] add
    {
        if (KeyboardDeployedSubscription == null)
        {
            KeyboardDeployedSubscription = new SubscriptionHandler(DeviceTypes.KeyBoard);
        }
        KeyboardDeployedSubscription.Changed += value;
    }
    [SecuritySafeCritical] remove
    {
        KeyboardDeployedSubscription.Changed -= value;
    }
}

这段代码编写得很糟糕。如果在remove之前调用处理程序的add部分，KeyboardDeployedSubscription将为null并且将引发异常。为了测试我的理论，我订阅了App的构造函数中的事件：

    public App()
    {
        // Global handler for uncaught exceptions.
        UnhandledException += Application_UnhandledException;

        DeviceStatus.KeyboardDeployedChanged += (sender, e) => { };

果然，异常消失了。现在，为了理解你的代码为什么触发这个问题，我回溯到框架的哪个部分应该订阅该事件。唯一的候选者是InternalOnNavigatedTo方法。

因此，您的问题是，即使OnNavigatedFrom从未被调用，也会调用OnNavigatedTo。

Answer 3

由于您正在使用Windows Phone的内置自动导航到WMAppManifest.xml中定义的页面，我尝试删除自动导航，它基本上有效（没有例外）。

我刚刚更换了

<DefaultTask Name="_default" NavigationPage="MainPage.xaml" />

带

<DefaultTask Name="_default" />

不确定这是否能解决您的问题，但至少不会再崩溃。

覆盖WP8导航 - 在PhoneApplicationPage中崩溃

计划

Repro

代码

伤心

3 个答案:

您的WP8应用程序如何初始化：