WPF Custom ScrollViewer - ItemHeight when Virtualizing

Question

I have a ListBox which uses a custom ScrollViewer (to provide fake "Touch" scrolling experience on a windows xp embedded touchpanel)

<ControlTemplate TargetType="{x:Type auc:DragSortableListView}">
    <auc:DragScrollViewer ...>
        <ItemsPresenter .../>
    </auc:DragScrollViewer>
</ControlTemplate>

In that "DragScrollViewer" I use the IScrollInfo-Interface to perform scrolling which works well.

Plus I use UI virtualization because we have large amounts of data bound to the list view and the scrolling (when virtualization turned on) happens not pixel based, but index based as I figured out. That means that if I scroll to vertical offset 5 via IScrollInfo, it scrolls to the 5th item.

My problem is, that I don't know how to convert the pixel-based-mouse-offset (when the user moved the "mouse" 50pixel) to the item-count-based offset the IScrollInfo.SetVerticalOffset() expects (offset has been 3, an item is 10pixel => set offset to 8). It'd be easy if I knew the Item-Height, but I'm inside the ScrollViewer. How can the ScrollViewer possibly know if there's a ItemsPresenter down the visual tree, right? And what if the items have different height (which is not the case, but hypothetically)?

Any suggestions on how to solve that issue?

Mike Fuchs Mike Fuchs · Accepted Answer · 2013-02-19T15:52:34

Have you had a look at this article?

The way I see it, he is storing the mouse offset in _Offset, then calling InvalidateArrange(), which might (just guessing here) in turn query the VerticalOffset property and handle the appropriate scrolling.

private Vector _Offset;
public double VerticalOffset  { get { return _Offset.Y; } }

public void SetVerticalOffset(double offset)
{
  offset = Math.Max(0, Math.Min(offset, ExtentHeight - ViewportHeight));
  if (offset != _Offset.Y)
  {
    _Offset.Y = offset;
    InvalidateArrange();
  }
}

I suppose this would then never require you to actually transform your pixel offset into item count offset. If this is incorrect, please provide your IScrollInfo implementation.

WPF Custom ScrollViewer - ItemHeight when Virtualizing

1 Answers