在win 10通用应用程序中使用mvvm-light导航服务时,无法将类型为Page的对象强制转换为"Windows.UI.Xaml.Controls.Frame"

Question

我在我的新Windows 10通用应用程序C#/ XAML上遇到以下错误:

GalaSoft.MvvmLight.Platform.dll中出现类型'System.InvalidCastException'的例外,但未在用户代码中处理.附加信息:无法将类型''的对象强制转换为'Windows.UI.Xaml.Controls.Frame'.

在我的一个页面视图模型中的以下导航命令中:

 _navigationService.NavigateTo(ViewModelLocator.MedicineBoxPageKey);

我正在尝试使用汉堡包菜单样式导航(请参阅此示例).应用程序由Microsoft提供如何执行此操作的示例):

1-在我的所有页面中共享一个方便的解决方案.上面提到的示例使用AppShell Page作为应用程序的根而不是Frame,它封装了导航菜单和后退按钮的某些行为.那将是理想的.

2-使用MVVM-Light导航服务可以方便地处理视图模型中的所有导航.

以下是App.xml.Cs如何初始化shell页面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();

这是AppShell类定义:

public sealed partial class AppShell : Page
    {
        public static AppShell Current = null;

        public AppShell()
        {
            this.InitializeComponent();
         }
     }

从我到目前为止所尝试的,mvvm-light导航服务仅在使用Frame作为应用程序的根并注意页面时才起作用(否则我们会得到这个转换错误).但是,使用Frame似乎不是一个选项,因为示例应用程序将其放置:

使用Page作为应用程序的根目录可以提供设计时体验,并确保在Mobile上运行时,应用程序内容不会显示在系统的StatusBar下,默认情况下可以使用透明背景显示.如果它们出现在设备上,它还会考虑软件导航按钮的存在.应用可以通过切换到UseCoreWindow来选择退出.

我还尝试从mvvm-light导航服务覆盖navigationTo方法,但是在我能够捕获它之前,这个bug似乎已经发生了.

有没有人有解决方案使用mvvm-light导航服务和shell页面作为app root(管理汉堡包菜单等)？

非常感谢!

Answer 1

我和Laurent Bugnion谈过,他建议我实施自己的导航服务来处理导航.为此,我创建了一个实现MVVM Light的INavigationService接口的PageNavigationService.

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);
        }
    }
}

基本上它是他实施的副本.但是,不是解析为Frame,而是解析为AppShell并使用AppFrame属性进行导航.

我把它放到我的ViewModelLocator.代替:

var navigationService = new NavigationService();

我会用:

var navigationService = new PageNavigationService();

编辑:我注意到在您使用新导航服务导航后使用反向键时NavMenuListView中存在一个excpetion,因为所选项目为null.我通过调整SetSelectedItem方法并在强制转换后在for循环中添加nullcheck来修复它:

    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;
            }
        }
    }

但可能有一个比这更优雅的解决方案.