Possible to InvalidateVisual() on a given region instead of entire WPF control?

Question

I have a complex WPF control that draws a lot of primitives in its OnRender (it's sort of like a map). When a small portion of it changes, I'd only like to re-issue render commands for the affected elements, instead of running the entire OnRender over. While I'm fine with my OnRender function's performance on a resize or whatever, it's not fast enough for mouse hover-based highlighting of primitives.

Currently the only way I know how to force a screen update is to call InvalidateVisual(). No way to send in a dirty rect region to invalidate.

Is the lowest granularity of WPF screen composition the UI element? Will I need to do my renders of primitives into an intermediate target and then have that use InvalidateVisual() to update to the screen?

Answer 1

WPF doesn't work quite like that, so you can't invalidate regions. However, there are some optimizations that can be made. There is a Measure, Arrange, and then Render pass. If a control moves but what actually renders doesn't change then you can tell WPF to do only the arrange pass. You can trigger these invalidations off of dependency property value changes with FrameworkPropertyMetadata and FrameworkPropertyMetadataOptions (http://msdn.microsoft.com/en-us/library/system.windows.frameworkpropertymetadataoptions.aspx).