回到顶部按钮

 void MainPage_Loaded(object sender, RoutedEventArgs e)
        {


            #region 回到顶部按钮

            // Visual States are always on the first child of the control template
            FrameworkElement element = VisualTreeHelper.GetChild(viewer_hot, 0) as FrameworkElement;
            if (element != null)
            {
                VisualStateGroup group = FindVisualState(element, "ScrollStates");
                if (group != null)
                {
                    //group.CurrentStateChanging += (s, args) => pivot.Title = args.NewState.Name;
                    group.CurrentStateChanging += (s, args) =>
                    {
                        if (args.NewState.Name == "Scrolling")
                        {
                            imgTopBack.Visibility = System.Windows.Visibility.Collapsed;
                        }
                        else if (viewer_hot.VerticalOffset > 1)
                        {
                            imgTopBack.Visibility = System.Windows.Visibility.Visible;
                        }
                    };
                }
            }

            #endregion
}

  VisualStateGroup FindVisualState(FrameworkElement element, string name)
        {
            if (element == null) return null;
            IList groups = VisualStateManager.GetVisualStateGroups(element);
            foreach (VisualStateGroup group in groups)
                if (group.Name == name) return group;
            return null;
        }

 private void imgTopBack_Tap(object sender, System.Windows.Input.GestureEventArgs e)
        {
            viewer_hot.ScrollToVerticalOffset(0.0);
            imgTopBack.Visibility = Visibility.Collapsed;

            e.Handled = true;
        }

xaml 中：

 <Image Height="56" Width="56" HorizontalAlignment="Left" VerticalAlignment="Bottom" Margin="12,0,0,80" Name="imgTopBack" Source="/images/topBack.png" Stretch="Fill" Visibility="Collapsed" Tap="imgTopBack_Tap" />