buttons ontop of everything in android

I am currently working on a calculator application for android. I am finding it difficult to conveniently take a screen shot of the app. So i have put my calculator on pause so I can write a simple screen shot app. It runs in the notification bar and when you click the notification the notification bar slides away and a few seconds later a snapshot is taken. This works perfectly. Now I would like to add functionality to take snapshots even for full screen apps. I have seen apps that put a sliding drawer on the screen onto of whatever app is running. That is there is always a button on the screen no matter what you are doing that when you click/drag it that sliding drawer came out. How did they do that? It should be a simple process to use a button instead of a drawer and when it's clicked hide it, snapshot and unhide it.

So my question bsically is

How can开发者_开发知识库 i put a usable button on the screen that stays above whatever app is running, even the homescreen

I realize I'm not answering your question directly, but if you simply want to get a screen capture of your application running, I would use ddms in your android tools directory. It has a menu option for getting a screen grab. I use it frequently.

Just go to Device > Screen Capture

More details here : http://developer.android.com/guide/developing/debugging/ddms.html