问题
I m hitting the following error on my new windows 10 universal app C#/XAML:
An exception of type 'System.InvalidCastException' occurred in GalaSoft.MvvmLight.Platform.dll but was not handled in user code Additional information: Unable to cast object of type '' to type 'Windows.UI.Xaml.Controls.Frame'.
on the following navigating command in one of my page's view model:
_navigationService.NavigateTo(ViewModelLocator.MedicineBoxPageKey);
I am trying to have a hamburger menu style navigation (see this sample). app by Microsoft on an example of how to do this) to:
1- have a convenient solution shared across all my pages. The sample mentioned above uses an AppShell Page as the root of the app instead of a Frame, that encapsulates the navigation menu and some behavior of the back button. That would be ideal.
2- Use the MVVM-Light navigation service to handle all the navigation from my view model conveniently.
Here is how the App.xml.Cs initializes the shell page onLaunched:
AppShell shell = Window.Current.Content as AppShell;
// Do not repeat app initialization when the Window already has content,
// just ensure that the window is active
if (shell == null)
{
// Create a a AppShell to act as the navigation context and navigate to the first page
shell = new AppShell();
// Set the default language
shell.Language = Windows.Globalization.ApplicationLanguages.Languages[0];
shell.AppFrame.NavigationFailed += OnNavigationFailed;
if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)
{
//TODO: Load state from previously suspended application
}
}
// Place our app shell in the current Window
Window.Current.Content = shell;
if (shell.AppFrame.Content == null)
{
// When the navigation stack isn't restored, navigate to the first page
// suppressing the initial entrance animation.
shell.AppFrame.Navigate(typeof(MedicinesStorePage), e.Arguments, new Windows.UI.Xaml.Media.Animation.SuppressNavigationTransitionInfo());
}
// Ensure the current window is active
Window.Current.Activate();
And here is the AppShell class definition:
public sealed partial class AppShell : Page
{
public static AppShell Current = null;
public AppShell()
{
this.InitializeComponent();
}
}
From what I have tried so far, the mvvm-light navigation service only works when a Frame is used a root of the app and note a Page (otherwise we get this casting bug). However using a Frame does not seem to be a option either since as the sample app puts it:
Using a Page as the root for the app provides a design time experience as well as ensures that when it runs on Mobile the app content won't appear under the system's StatusBar which is visible by default with a transparent background. It will also take into account the presence of software navigation buttons if they appear on a device. An app can opt-out by switching to UseCoreWindow.
I also tried to overide the navigationTo method from the mvvm-light navigation service but the bug seems to occur before I could catch it.
Does anyone has a solution to use the mvvm-light navigation service and a shell page as the app root (that manages the hamburger menu, etc.)?
Thanks a lot!
回答1:
I talked to Laurent Bugnion and he recommended me to implemented my own navigation service who handles the navigation. For this I made a PageNavigationService who implements the INavigationService Interface of MVVM Light.
public class PageNavigationService : INavigationService
{
/// <summary>
/// The key that is returned by the <see cref="CurrentPageKey" /> property
/// when the current Page is the root page.
/// </summary>
public const string RootPageKey = "-- ROOT --";
/// <summary>
/// The key that is returned by the <see cref="CurrentPageKey" /> property
/// when the current Page is not found.
/// This can be the case when the navigation wasn't managed by this NavigationService,
/// for example when it is directly triggered in the code behind, and the
/// NavigationService was not configured for this page type.
/// </summary>
public const string UnknownPageKey = "-- UNKNOWN --";
private readonly Dictionary<string, Type> _pagesByKey = new Dictionary<string, Type>();
/// <summary>
/// The key corresponding to the currently displayed page.
/// </summary>
public string CurrentPageKey
{
get
{
lock (_pagesByKey)
{
var frame = ((AppShell) Window.Current.Content).AppFrame;
if (frame.BackStackDepth == 0)
{
return RootPageKey;
}
if (frame.Content == null)
{
return UnknownPageKey;
}
var currentType = frame.Content.GetType();
if (_pagesByKey.All(p => p.Value != currentType))
{
return UnknownPageKey;
}
var item = _pagesByKey.FirstOrDefault(
i => i.Value == currentType);
return item.Key;
}
}
}
/// <summary>
/// If possible, discards the current page and displays the previous page
/// on the navigation stack.
/// </summary>
public void GoBack()
{
var frame = ((Frame) Window.Current.Content);
if (frame.CanGoBack)
{
frame.GoBack();
}
}
/// <summary>
/// Displays a new page corresponding to the given key.
/// Make sure to call the <see cref="Configure" />
/// method first.
/// </summary>
/// <param name="pageKey">
/// The key corresponding to the page
/// that should be displayed.
/// </param>
/// <exception cref="ArgumentException">
/// When this method is called for
/// a key that has not been configured earlier.
/// </exception>
public void NavigateTo(string pageKey)
{
NavigateTo(pageKey, null);
}
/// <summary>
/// Displays a new page corresponding to the given key,
/// and passes a parameter to the new page.
/// Make sure to call the <see cref="Configure" />
/// method first.
/// </summary>
/// <param name="pageKey">
/// The key corresponding to the page
/// that should be displayed.
/// </param>
/// <param name="parameter">
/// The parameter that should be passed
/// to the new page.
/// </param>
/// <exception cref="ArgumentException">
/// When this method is called for
/// a key that has not been configured earlier.
/// </exception>
public void NavigateTo(string pageKey, object parameter)
{
lock (_pagesByKey)
{
if (!_pagesByKey.ContainsKey(pageKey))
{
throw new ArgumentException(
string.Format(
"No such page: {0}. Did you forget to call NavigationService.Configure?",
pageKey),
"pageKey");
}
var shell = ((AppShell) Window.Current.Content);
shell.AppFrame.Navigate(_pagesByKey[pageKey], parameter);
}
}
/// <summary>
/// Adds a key/page pair to the navigation service.
/// </summary>
/// <param name="key">
/// The key that will be used later
/// in the <see cref="NavigateTo(string)" /> or <see cref="NavigateTo(string, object)" /> methods.
/// </param>
/// <param name="pageType">The type of the page corresponding to the key.</param>
public void Configure(string key, Type pageType)
{
lock (_pagesByKey)
{
if (_pagesByKey.ContainsKey(key))
{
throw new ArgumentException("This key is already used: " + key);
}
if (_pagesByKey.Any(p => p.Value == pageType))
{
throw new ArgumentException(
"This type is already configured with key " + _pagesByKey.First(p => p.Value == pageType).Key);
}
_pagesByKey.Add(
key,
pageType);
}
}
}
Basicly it's a copy of his implementation. But instead of parsing to a Frame I parse to an AppShell and use the AppFrame Property to navigate.
I put this to my ViewModelLocator. Instead of:
var navigationService = new NavigationService();
I will just use:
var navigationService = new PageNavigationService();
EDIT: I Noticed that there is an excpetion in the NavMenuListView when you use the backkey after you navigated with the new navigationservice since the selected item is null. I fixed it with adjusting the SetSelectedItem Method and adding a nullcheck in the for loop after the cast:
public void SetSelectedItem(ListViewItem item)
{
var index = -1;
if (item != null)
{
index = IndexFromContainer(item);
}
for (var i = 0; i < Items.Count; i++)
{
var lvi = (ListViewItem) ContainerFromIndex(i);
if(lvi == null) continue;
if (i != index)
{
lvi.IsSelected = false;
}
else if (i == index)
{
lvi.IsSelected = true;
}
}
}
But there might be a more elegant solution than this.
来源：https://stackoverflow.com/questions/32132933/unable-to-cast-object-of-type-page-to-type-windows-ui-xaml-controls-frame-when