How can I animate show/hide of an item in an ItemsControl, but only animate if the window is already visible?

I have an ItemsControl with a number of elements, each one with its own ViewModel instance. Each item's ViewModel knows whether that item should be visible (currently each ViewModel has a Visibility property that the UI binds to). When my window first opens, some of these items are visible, others are collapsed. Later, some items' visibility might change in response to user interaction. The window sizes to its content, so the window resizes when items are shown or hidden. And the window is initially centered on the screen (which means everything has to be arranged properly right away, so the window knows its initial size and can center itself accordingly).

Now I want to add animations whenever an item is shown or hidden -- but I only want to animate if the item's visibility changes after the window is already shown. So if the window is already open, and the user does something that makes one of the ViewModels want to appear, it animates in; if the user does something to make one of the ViewModels disappear, it animates out. But when the window first opens, I want everything to start out rock-solid -- no lingering animations.

And I want the window to still set its initial size based on its initially-visible content, and I still want it to be initially centered onscreen. (Although actually, in this case, it would be acceptable if it centered itself as if all items were visible, if event ordering made it work out that way.)

I know a fair bit about WPF, but I admit I'm lost when it comes to triggers and storyboards. I haven't really done anything with WPF animations before, and I'm not sure where to begin.

I already tried using Reveal from the Bag of Tricks, but I had several problems with it, the biggest being that it doesn't have the "only use animations after the window is shown" behavior that I want -- my window would appear and the initially-visible elements would still be animating in. It also didn't play well with my layout (it was centering the elements horizontally, instead of stretching them to the ItemsControl's width), and a few other problems that might or might not be fixable.

I'm not too picky about whether I animate by stretching (e.g. by animating a LayoutTransform from SizeX=1 SizeY=0 to SizeX=1 SizeY=1, thus starting with squished text and expanding to normal size) or by just changing the Height (thus starting with only part of the content visible and revealing more as the animation progresses) -- I'm fine with either.

I'm open to writing my own Panel de开发者_C百科scendant (I've done it before) if that's the best way to solve this, and I can always steal code from Reveal and hack until I get it working -- but it seems like there should already be an easier way to do this, if I just knew what it was. I'm open to learning about triggers and storyboards, or whatever, if someone can point me in the right direction.