MessageDialog ShowAsync在第二个对话框上抛出accessdenied异常

Question

我试图在Windows 8中再次尝试/取消对话框.对话框第一次显示正常,但是再次单击尝试再次失败,我在调用ShowAsync时得到访问被拒绝的异常.我不知道为什么,但奇怪的是,有时代码工作正常,当我放置断点时我没有得到异常.这里真的很无能为力

这是代码.

    async void DismissedEventHandler(SplashScreen sender, object e)
    {
        dismissed = true;
        loadFeeds();
    }
    private async void loadFeeds()
    {
        await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
        {
            try
            {
                RSSDataSource rssDataSource = (RSSDataSource)App.Current.Resources["RSSDataSource"];
                if (rssDataSource != null)
                {
                    await rssDataSource.DownloadFeeds();
                    await rssDataSource.GetFeedsAsync();
                }

                AdDataSource ads = (AdDataSource)App.Current.Resources["AdDataSource"];

                if (ads != null)
                {
                    await ads.DownloadAds();
                }
                rootFrame.Navigate(typeof(HomePageView));

                Window.Current.Content = rootFrame;
            }
            catch
            {
                ShowError();
            }

        });
    }
    async void ShowError()
    {
        // There was likely a problem initializing
        MessageDialog msg = new MessageDialog(CONNECTION_ERROR_MESSAGE, CONNECTION_ERROR_TITLE);

        // Add buttons and set their command handlers
        msg.Commands.Add(new UICommand(COMMAND_LABEL_RETRY, new UICommandInvokedHandler(this.CommandInvokedHandler)));
        msg.Commands.Add(new UICommand(COMMAND_LABEL_CLOSE, new UICommandInvokedHandler(this.CommandInvokedHandler)));
        // Set the command to be invoked when a user presses 'ESC'
        msg.CancelCommandIndex = 0;

        await msg.ShowAsync();
    }

    /// <summary>
    /// Callback function for the invocation of the dialog commands
    /// </summary>
    /// <param name="command">The command that was invoked</param>
    private void CommandInvokedHandler(IUICommand command)
    {
        string buttonLabel = command.Label;
        if (buttonLabel.Equals(COMMAND_LABEL_RETRY))
        {
            loadFeeds();
        }
        else
        {
            // Close app
            Application.Current.Exit();
        }
    }

Answer 1

好的,我找到了一个快速解决方案,

定义一个IAsyncOperation类varialble

IAsyncOperation<IUICommand> asyncCommand = null;

并将其设置为MessageDialog的ShowAsync方法

asyncCommand = msg.ShowAsync();

在重试的命令处理程序中/再次尝试检查asyncCommand是否为null,并在必要时取消上一次操作

if(asyncCommand != null)
{
   asyncCommand.Cancel();
}

如果有更好的方法,请告诉我.

Answer 2

我迟到了,但这里有一种方法可以随时等待对话框的结果,也不用担心连续调用太多:

首先在应用程序中定义静态变量和方法:

 private static IAsyncOperation<IUICommand> messageDialogCommand = null;
 public async static Task<bool> ShowDialog(MessageDialog dlg) {

    // Close the previous one out
    if (messageDialogCommand != null) {
       messageDialogCommand.Cancel();
       messageDialogCommand = null;
    }

    messageDialogCommand = dlg.ShowAsync();
    await messageDialogCommand;
    return true;
 }

现在,您可以传入任何对话框并始终等待执行.这就是为什么这会返回一个bool而不是void.您不必担心倍数之间的碰撞.为什么不让这个方法接受一个字符串？由于标题和是/否命令处理程序,您可以将其分配到您正在使用的特定对话框中.

调用如:

await App.ShowDialog(new MessageDialog("FOO!"));

要么

var dlg = new MessageDialog("FOO?", "BAR?");
dlg.Commands.Add(new UICommand("Yes", new UICommandInvokedHandler(YesHandler)));
dlg.Commands.Add(new UICommand("No", new UICommandInvokedHandler(NoHandler)));
await App.ShowDialog(dlg);