The blog of dlaa.me

I'd almost finished patting myself on the back for managing to implement WPF's LayoutTransform on Silverlight using just the RenderTransform available on that platform. (Background reading for LayoutTransformControl: Motivation and introduction for Beta 1, Significant enhancements and update for Beta 2, Update for RTW) Yes, everything was peachy - until I was contacted by kind reader Matthew Serbinski with a report of an InvalidOperationException being thrown by Silverlight when using LayoutTransformControl with a ScaleTransform with ScaleY=0... :(

You've probably already recognized ScaleY=0 as something of an edge case for layout: a value which collapses everything into nothingness. I looked into how WPF's LayoutTransform code handled this situation and discovered that it specifically detected circumstances corresponding to a transformation matrix without an inverse - and skipped performing the usual layout computations. My LayoutTransformControl implementation didn't look for this special case, ended up violating one of the rules of the layout system, and triggered the reported exception.

So I added a little bit of code to handle such input the same way WPF does. And while I was at it, I checked to see if maybe there were other special cases that LayoutTransformControl wasn't handling properly... Sure enough, I found one other scenario: that of needing to layout within an container having no width or height. In this case, LayoutTransformControl's behavior wasn't wrong enough to cause an exception (or any visible problem I noticed), but I made a similar tweak for consistency with WPF.

Changes in place, I modified the LayoutTransformControl sample application, its Silverlight test framework, and its WPF test framework to allow setting both ScaleX and ScaleY to 0. (I'd formerly limited ScaleX/ScaleY to positive values which is why I didn't realize there was a problem myself.) Then I spent some time playing around with the test apps: trying all kinds of things and looking for any other anomalous behavior. Nothing turned up, so I updated the LayoutTransformControl source code download and started writing this post... :)

Changes to code always involve a certain amount of risk that a regression will be introduced. Fortunately, the changes here are small, self-contained, and easy to test - so I'm optimistic they won't cause problems for those of you already using LayoutTransformControl in your projects. Of course, if there are any new problems - or existing ones I don't know about yet! - please let me know and I'll look into them as quickly as I can.

Thank you for your help - happy LayoutTransform-ing!