CODEWITHSUNDEEP
wpfscrollviewer
Jens
Jens

Reputation: 505

WPF Custom ScrollViewer - ItemHeight when Virtualizing

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?

Upvotes: 3

Views: 738

Answers (1)

Mike Fuchs
Mike Fuchs

Reputation: 12319

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.

Upvotes: 1

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