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：