// this is where the magic happens
private void Thumbnail_LayoutUpdated(object sender, EventArgs e)
{
if (IntPtr.Zero == thumb)
{
InitialiseThumbnail(this.Source);
}
if (IntPtr.Zero != thumb)
{
if (!target.RootVisual.IsAncestorOf(this))
{
//we are no longer in the visual tree
ReleaseThumbnail();
return;
}
DwmQueryThumbnailSourceSize(this.thumb, out WPF.SysSize size);
Rect visib = WPF.GetVisibleArea(this);
ThumbnailProperties props = new ThumbnailProperties();
if (visib == Rect.Empty)
{
props.Visible = false;
props.Flags = ThumbnailFlags.Visible;
}
else
{
double ratioSrc = (double)size.Width / (double)size.Height;
double ratioDest = visib.Width / visib.Height;
// Center the thumbnail in the visible area
double newSize;
if (ratioSrc > ratioDest)
{
newSize = visib.Height * ratioDest / ratioSrc;
visib.Y += (visib.Height - newSize) / 2.0;
visib.Height = newSize;
}
else
{
newSize = visib.Width * ratioSrc / ratioDest;
visib.X += (visib.Width - newSize) / 2.0;
visib.Width = newSize;
}
// Translate to screen pixels
Point a = target.RootVisual.PointToScreen(visib.TopLeft);
Point b = target.RootVisual.PointToScreen(visib.BottomRight);
Point w = Application.Current.MainWindow.PointToScreen(new Point(0, 0));
props.Destination = new WPF.SysRect(
(int)Math.Ceiling(a.X - w.X), (int)Math.Ceiling(a.Y - w.Y),
(int)Math.Ceiling(b.X - w.X), (int)Math.Ceiling(b.Y - w.Y));
props.Visible = this.Visibility == Visibility.Visible;
props.Flags = ThumbnailFlags.Visible | ThumbnailFlags.RectDestination;
}
DwmUpdateThumbnailProperties(thumb, ref props);
}
}