How to disable the WPF ScrollViewer to refresh the view - wpf

Question

I am using a stack panel for displaying a slideshow. The stack panel contains two images.
Each image is bound to the View Model containing among other properties the path to the image on the disk. In the general case there are more than 2 images in the list.
There is an endless while loop used for changing the current and next pointer to the image and in this way after the slide transition is finished the the current image becomes previous and the next becomes current.
The slide transition is made by changing the scroll viewer from 0 to 100% of scrollable extended area.
This works and even very fast. It is the easiest way to implement a slide transition for an endless list of images using MVVM pattern.
My problem is that from time to time the slide transition slips and doesn't finishes at exact 100% visually but the storyboard animation completed event is triggered.
In the ViewModel I am moving the Current and Next poitners and the image blinks. Some flickering is visible from time to time.
I need to find a way on how to temporarily disable the ScrollViewer to display any visual change when I am repositioning the scoller from 100% to 0%.
I need to invalidate the WPF ScrollViewer only temporarily until I change the scroll position from 100% to 0% but this must not be visible on screen.
I mean the values must be changed but the screen must be frozen.
Is it possible somehow ?
XAML
<UserControl.Resources>
<Storyboard x:Key="sbVerticalSlideEasingTransition"
AutoReverse="True" RepeatBehavior="Forever">
<Storyboard x:Key="sbVerticalSlideTransition"
FillBehavior="HoldEnd">
<!--SCROLL TO THE MULTIPLIER OF THE HEIGHT (BASICALLY AS THE PERCENTAGE)-->
<DoubleAnimationUsingKeyFrames
Storyboard.TargetName="Mediator"
Storyboard.TargetProperty="ScrollableHeightMultiplier">
<LinearDoubleKeyFrame KeyTime="00:0:0" Value="0" />
<LinearDoubleKeyFrame KeyTime="00:0:5" Value="1" />
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</UserControl.Resources>
<Grid x:Name="gdContainer" Background="Black">
<ScrollViewer x:Name="Scroller" Visibility="{Binding Path=IsSlider, Mode=TwoWay, NotifyOnSourceUpdated=True, ElementName=slidePlayer,
Converter='{StaticResource BoolToVisibility}'}"
VerticalScrollBarVisibility="Hidden"
HorizontalScrollBarVisibility="Disabled">
<StackPanel x:Name="ScrollPanel">
<ctrl:MediaElementEx x:Name="slide1" Height="{Binding Path=ActualHeight, ElementName=gdContainer}" Width="{Binding Path=ActualWidth, ElementName=gdContainer}" />
<ctrl:MediaElementEx x:Name="slide2" Height="{Binding Path=ActualHeight, ElementName=gdContainer}" Width="{Binding Path=ActualWidth, ElementName=gdContainer}" />
</StackPanel>
</ScrollViewer>
<!-- Mediator that forwards the property changes -->
<cmn:ScrollViewerOffsetMediator x:Name="Mediator" ScrollViewer="{Binding ElementName=Scroller}"/>
</Grid>
ScrollViewer Mediator
This is used to bind the ScrollOffset of the scrollviewer that is not bindable directly
/// <summary>
/// Mediator that forwards Offset property changes on to a ScrollViewer
/// instance to enable the animation of Horizontal/VerticalOffset.
/// </summary>
public class ScrollViewerOffsetMediator : FrameworkElement
{
/// <summary>
/// ScrollViewer instance to forward Offset changes on to.
/// </summary>
public ScrollViewer ScrollViewer
{
get { return (ScrollViewer)GetValue(ScrollViewerProperty); }
set { SetValue(ScrollViewerProperty, value); }
}
public static readonly DependencyProperty ScrollViewerProperty =
DependencyProperty.Register(
"ScrollViewer",
typeof(ScrollViewer),
typeof(ScrollViewerOffsetMediator),
new PropertyMetadata(OnScrollViewerChanged));
private static void OnScrollViewerChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
var mediator = (ScrollViewerOffsetMediator)o;
var scrollViewer = (ScrollViewer)(e.NewValue);
if (null != scrollViewer)
{
scrollViewer.ScrollToVerticalOffset(mediator.VerticalOffset);
}
}
/// <summary>
/// VerticalOffset property to forward to the ScrollViewer.
/// </summary>
public double VerticalOffset
{
get { return (double)GetValue(VerticalOffsetProperty); }
set { SetValue(VerticalOffsetProperty, value); }
}
public static readonly DependencyProperty VerticalOffsetProperty =
DependencyProperty.Register(
"VerticalOffset",
typeof(double),
typeof(ScrollViewerOffsetMediator),
new PropertyMetadata(OnVerticalOffsetChanged));
public static void OnVerticalOffsetChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
var mediator = (ScrollViewerOffsetMediator)o;
if (null != mediator.ScrollViewer)
{
mediator.ScrollViewer.ScrollToVerticalOffset((double)(e.NewValue));
}
}
/// <summary>
/// HorizontalOffset property to forward to the ScrollViewer.
/// </summary>
public double HorizontalOffset
{
get { return (double)GetValue(HorizontalOffsetProperty); }
set { SetValue(VerticalOffsetProperty, value); }
}
public static readonly DependencyProperty HorizontalOffsetProperty =
DependencyProperty.Register(
"HorizontalOffset",
typeof(double),
typeof(ScrollViewerOffsetMediator),
new PropertyMetadata(OnHorizontalOffsetChanged));
public static void OnHorizontalOffsetChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
var mediator = (ScrollViewerOffsetMediator)o;
if (null != mediator.ScrollViewer)
{
mediator.ScrollViewer.ScrollToHorizontalOffset((double)(e.NewValue));
}
}
/// <summary>
/// Multiplier for ScrollableHeight property to forward to the ScrollViewer.
/// </summary>
/// <remarks>
/// 0.0 means "scrolled to top"; 1.0 means "scrolled to bottom".
/// </remarks>
public double ScrollableHeightMultiplier
{
get { return (double)GetValue(ScrollableHeightMultiplierProperty); }
set { SetValue(ScrollableHeightMultiplierProperty, value); }
}
public static readonly DependencyProperty ScrollableHeightMultiplierProperty =
DependencyProperty.Register(
"ScrollableHeightMultiplier",
typeof(double),
typeof(ScrollViewerOffsetMediator),
new PropertyMetadata(OnScrollableHeightMultiplierChanged));
public static void OnScrollableHeightMultiplierChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
var mediator = (ScrollViewerOffsetMediator)o;
var scrollViewer = mediator.ScrollViewer;
if (null != scrollViewer)
{
scrollViewer.ScrollToVerticalOffset((double)(e.NewValue) * scrollViewer.ScrollableHeight);
if (((double)e.NewValue == 0) && ((double)e.OldValue == 1))
{
Debug.WriteLine("[1] {0:HH:mm:ss.fff} ScrollableHeightMultiplier 1 => 0 ", DateTime.Now);
}
}
}
/// <summary>
/// Multiplier for ScrollableWidth property to forward to the ScrollViewer.
/// </summary>
/// <remarks>
/// 0.0 means "scrolled to top"; 1.0 means "scrolled to bottom".
/// </remarks>
public double ScrollableWidthMultiplier
{
get { return (double)GetValue(ScrollableWidthMultiplierProperty); }
set { SetValue(ScrollableWidthMultiplierProperty, value); }
}
public static readonly DependencyProperty ScrollableWidthMultiplierProperty =
DependencyProperty.Register(
"ScrollableWidthMultiplier",
typeof(double),
typeof(ScrollViewerOffsetMediator),
new PropertyMetadata(OnScrollableWidthMultiplierChanged));
public static void OnScrollableWidthMultiplierChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
{
var mediator = (ScrollViewerOffsetMediator)o;
var scrollViewer = mediator.ScrollViewer;
if (null != scrollViewer)
{
scrollViewer.ScrollToHorizontalOffset((double)(e.NewValue) * scrollViewer.ScrollableWidth);
}
}
}