private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e) {
   var rootFrame = Window.Current.Content as Frame;
   if (rootFrame != null && rootFrame.CanGoBack) {
     rootFrame.GoBack();
     e.Handled = true;
   }
 }

        async void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
        {
            if (this.Frame.SourcePageType.FullName == "StatusUp.MainPage")
            {
                e.Handled = true;
                // Create the message dialog and set its content
                var messageDialog = new MessageDialog("Close Application?");

                // Add commands and set their callbacks; both buttons use the same callback function instead of inline event handlers
                messageDialog.Commands.Add(new UICommand(
                    "Yes",
                    new UICommandInvokedHandler(this.CommandInvokedHandler)));
                messageDialog.Commands.Add(new UICommand(
                    "No",
                    new UICommandInvokedHandler(this.CommandInvokedHandler)));

                // Set the command that will be invoked by default
                messageDialog.DefaultCommandIndex = 0;

                // Set the command to be invoked when escape is pressed
                messageDialog.CancelCommandIndex = 1;

                // Show the message dialog
                try
                {
                    messageDialog.ShowAsync();
                }
                catch { }
            }
        }

 protected void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     e.Handled = true;
     //We need this to handle backpress in ViewModels
     if (this.DataContext is BaseViewModel)
         (this.DataContext as BaseViewModel).GoBack();
 }

 private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     e.Handled = true;
     if (CurrentPage == VLCPage.MainPageHome)
         e.Handled = false;
     GoBack_Specific();
 }

 private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (Frame.CanGoBack) {
         e.Handled = true;
         Frame.GoBack();
     }
 }

 void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e) {
     Frame rootFrame = Window.Current.Content as Frame;
     if (rootFrame != null && rootFrame.CanGoBack) {
         rootFrame.Navigate(typeof(PollutionReportPage));
         e.Handled = true;
     }
 }

        async void HardwareButtons_BackPressed_DetailPage(object sender, BackPressedEventArgs e)
        {
            e.Handled = true;
            if (string.IsNullOrEmpty(ViewModel.Tasks.Title))
            {
                NavigateHelper.NavigateToPage(typeof(MainPage));
                return;
            }
            //此处最好进行一个判断，是更新还是新增操作，然后进行相关的数据库操作
            Task taskNeedAwait = null;
            FrameTransitionParam param = new FrameTransitionParam();
            if (this.IsNavigateForUpdate)//true表示更新
            {
                // 修改的数据
                param.UpdateTasks = ViewModel.Tasks;
                taskNeedAwait = ViewModel.UpdateTasks();
            }
            else//false 表示新增
            {
                param.NewTasks = ViewModel.Tasks;
                taskNeedAwait = ViewModel.AddTasks();
            }
            // 在跳转前取消注册
            HardwareButtons.BackPressed -= HardwareButtons_BackPressed_DetailPage;
            NavigateHelper.NavigateToPageWithParam(typeof(MainPage), param);

            await taskNeedAwait;
        }

        private void HardwareButtonsOnBackPressed(object sender, BackPressedEventArgs backPressedEventArgs)
        {
            if (ViewModel.IsSignInPageVisible)
            {
                var storyboard = (Storyboard)Resources["SignInModalHideStoryboard"];
                if (storyboard == null) return;
                storyboard.Begin();
                ViewModel.GoBackToStartCommand.Execute(null);
                backPressedEventArgs.Handled = true;
            }

            if (ViewModel.IsRegisterPageVisible)
            {
                var storyboard = (Storyboard)Resources["RegistrationModalHideStoryboard"];
                if (storyboard == null) return;
                storyboard.Begin();
                ViewModel.GoBackToStartCommand.Execute(null);
                backPressedEventArgs.Handled = true;
            }

            if (ViewModel.IsCaptchaPageVisible)
            {
                var storyboard = (Storyboard)Resources["CaptchaModalHideStoryboard"];
                if (storyboard == null) return;
                storyboard.Begin();
                ViewModel.GoBackToStartCommand.Execute(null);
                backPressedEventArgs.Handled = true;
            }
        }

        private void OnBackPress(object sender, BackPressedEventArgs e)
        {
            e.Handled = true;

             if (Frame.CanGoBack)
            Frame.GoBack();
        }

 private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (Frame.BackStackDepth == 0)
     {
         Application.Current.Exit();
     }
 }

 private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (!e.Handled)
     {
         e.Handled = true;
         Task t = BackButtonPressed();
     }
 }

 void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (appBarToggleButton.IsChecked == true)
     {
         appBarToggleButton.IsChecked = false;
         e.Handled = true;
     }
 }

 private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (InstanceMessageService.RootFrame != null && InstanceMessageService.RootFrame.CanGoBack)
     {
         e.Handled = true;
         InstanceMessageService.RootFrame.GoBack();
     }
 }

 public void OnBackPressed(object sender, BackPressedEventArgs e)
 {
     if (Frame.CanGoBack)
     {
         e.Handled = true;
         Frame.GoBack();
     }
 }

 void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (VideoViewComboBox.IsDropDownOpen)
     {
         VideoViewComboBox.IsDropDownOpen = false;
         e.Handled = true;
     }
 }

 void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (IsPaneOpen)
     {
         IsPaneOpen = false;
         e.Handled = true;
     }
 }

 private void OnBackPressed(object sender, BackPressedEventArgs e)
 {
     if (this.RootFrame != null && this.RootFrame.CanGoBack)
     {
         e.Handled = true;
         this.RootFrame.GoBack();
     }
 }

 // handle hardware back button press
 void OnBackPressed(object sender, BackPressedEventArgs e)
 {
     var shell = (Shell)Window.Current.Content;
     if (shell.RootFrame.CanGoBack) {
         e.Handled = true;
         shell.RootFrame.GoBack();
     }
 }

 private void BackPressed(object s, BackPressedEventArgs args)
 {
     if (ConfirmationWeb.Visibility == Visibility.Visible && ConfirmationWeb.CanGoBack && confWebUrl != confUrl)
     {
         ConfirmationWeb.GoBack();
         args.Handled = true;
     }
 }

 private void MainPage_BackPressed(object sender, BackPressedEventArgs e)
 {
     if (WebViewControl.CanGoBack)
     {
         WebViewControl.GoBack();
         e.Handled = true;
     }
 }