Disappointments with DWM

DWM is a great new Vista technology that allows you to create live thumbnails of application windows (among other things). If you use Vista, you've probably noticed DWM in action with the task bar:

These thumbnails are a live view of what's happening in the window, and are quite a neat way to visually see what's happening "behind the scenes" so to speak. However, they have some very unfortunate drawbacks which are really annoying.

For starters -- you can only display a thumbnail for a top-level window. This totally destroys them for the use I had in mind, as for other great places to use this. The task bar is nothing more than a fancy name for a tab panel, and so it stands to reason that most places you'd use a tab panel would be a great place to use DWM thumbnails. For instance, in the REALbasic IDE, I was going to have a thumbnail appear when you hover the mouse over a tab in the tabs bar. This way you get a more visual understanding of what's being edited in a particular tab. But since it requires a top-level window to work... that's just not going to happen.

Now... I do not understand why this limitation is required. It seems like a fairly contrived limitation. However, I also have only a cursory understanding of how the thumbnails work internally -- so it could be that there's a valid reason for the limitation. It's just unfortunate, because it totally killed a really fun feature for the IDE.

And, just in case you want to use thumbnails in your own applications -- here's another tidbit of information. DWM will only work with a *visible* window. If the window is minimized, then DWM will cache the previous contents of the window and display that. It's interesting to note though, that if DWM doesn't have the opportunity to cache the window's image, you get a stock "blank" window thumbnail. This is the same thumbnail you will get if the window the window is outright hidden. However, if the window is simply off-screen, then it will display just fine (but it will also show up in the Alt+Tab list and accept focus/input, so it's not very hidden!).

Regardless, I've got some rudimentary DWM support in REALbasic code. It's nothing too fancy, but it gets the job done.

Structure DwmThumbnailProperties

  flags as Integer

  destination as Rect

  source as Rect

  opacity as UInt8

  visible as Integer

  sourceClientAreaOnly as Integer

End Structure

 

Structure Rect

  left as Integer

  top as Integer

  right as Integer

  bottom as Integer

End Structure

 

Soft Declare Function DwmRegisterThumbnail Lib "DwmApi" ( dest as Integer, source as Integer, ByRef id as Integer ) as Integer

Soft Declare Function DwmUpdateThumbnailProperties Lib "DwmApi" ( id as Integer, ByRef properties as DwmThumbnailProperties ) as Integer

Soft Declare Sub DwmUnregisterThumbnail Lib "DwmApi" ( id as Integer )


dim id as Integer

 

if DwmRegisterThumbnail( self.Handle, sourceWindow.Handle, id ) = 0 then

    dim properties as DwmThumbnailProperties

    

    properties.flags = &h8 + &h1 + &h4 + &h10

    properties.destination.top = 22

    properties.destination.right = self.Width

    properties.destination.bottom = 22 + self.Height

    properties.opacity = 255

    properties.visible = 1

    properties.sourceClientAreaOnly = 0

    call DwmUpdateThumbnailProperties( id, properties )

end if

Of course, this code doesn't clean up the thumbnail ID (once you unregister the thumbnail, the live view is destroyed), nor does it contain much of any error checking, comments, etc. But it gives you a reasonable idea of how things work -- the basic idea is that you register a source window handle, and a target window destination. The source window's contents will be automatically drawn into the destination window once you call DwmUpdateThumbnailProperties (which tells the window manager where and how you want the thumbnail drawn).

Enjoy!