如何取消任务？

Question

我在我的应用程序中重写BackButton并显示MessageDialog。

public Scenario_3() 
    { 
     this.InitializeComponent(); 
     Windows.Phone.UI.Input.HardwareButtons.BackPressed += HardwareButtons_BackPressed; 
    } 


private async void HardwareButtons_BackPresseed(object sender, BackPressedEventArgs e) 
    { 
     e.Handled = true; 
     var dialog = new MessageDialog("Do you want exit?"); 

     dialog.Commands.Add(new Windows.UI.Popups.UICommand("yes") { Id = 0 }); 
     dialog.Commands.Add(new Windows.UI.Popups.UICommand("no") { Id = 1 }); 

     dialog.DefaultCommandIndex = 0; 
     dialog.CancelCommandIndex = 1; 

     var result = await dialog.ShowAsync(); 

     if (result.Label == "yes") 
     { 
      this.Frame.Navigate(typeof(BlankPage1)); 

      ((Frame)Window.Current.Content).BackStack.Clear(); 
     } 

导航到BlankPage1后，我再次看到MessageDialog如果按后退按钮。 如何在导航后取消此重写？

Answer 1

根据您对我评论的回复，您似乎忘记了在离开页面时退订BackPressed活动。这样做将确保事件处理程序仅在需要时才被调用。

例如：

private async void HardwareButtons_BackPresseed(object sender, BackPressedEventArgs e) 
{ 
    e.Handled = true; 
    var dialog = new MessageDialog("Do you want exit?"); 

    dialog.Commands.Add(new Windows.UI.Popups.UICommand("yes") { Id = 0 }); 
    dialog.Commands.Add(new Windows.UI.Popups.UICommand("no") { Id = 1 }); 

    dialog.DefaultCommandIndex = 0; 
    dialog.CancelCommandIndex = 1; 

    var result = await dialog.ShowAsync(); 

    if (result.Label == "yes") 
    { 
     // Leaving this page. Stop listening for Back button presses. 
     Windows.Phone.UI.Input.HardwareButtons.BackPressed -= HardwareButtons_BackPressed; 

     this.Frame.Navigate(typeof(BlankPage1)); 

     ((Frame)Window.Current.Content).BackStack.Clear(); 
    }