WPF ScrollViewer set ScrollableHeight

Question

I have a user control which paints content in OnRender. The Height of that control grows by and by.

I added this control to a ScrollViewer. The control does only repaint the currently visible area (viewport) (+/- a view lines for smoother scrolling). Everything works fine so far...

But since the control usualy grows up to a few hundred thousands of pixels I want to keep the Height of my control as small as possible and provide a different Height value to bind to the ScrollableHeight of the ScrollViewer (same goes for VerticalOffset). But there is no setter for ScrollableHeight. It binds automatically to the Height of my Control. Neither can I override Height.

How can I customize my ScrollViewer (or VerticalScrollbar) to keep the real Height of my control small?

Answer 1

I did something like this in the past. What you need to do is to write your own layout Panel and implement the IScrollInfo in it. The interface looks big, but most of it is just calling one of the main set methods. The layouter needs to set some of the IScrollInfo properties, like ExtentHeight, Offset etc. and these are your way to customize how the ScrollViewer will calculate the scroll position and the scrollable area for your "virtual" canvas size. For implementing the IScrollInfo i used this tutorial as a guidance.