How to make an activity that can appear above other activities?

I've seen some android apps do this, so I know it can be done, I'm just baffled as to how. I can't think of how it would be done. Think of what I'm talking about as similar to the recently open applications view. When you hold down the home key the Recently viewe开发者_JAVA百科d applications pops up. It doesn't exit your current activity, it creates something that runs on top of it. This can be accessed in any activity no matter what. I want to know how to make something similar but through touching a certain part of the screen, or making a gesture on the screen. Can anyone point me in the right direction?

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I'm guessing that you're talking about an activity that has transparent view areas, thus permitting view to whatever is underneath it.

How do I create a transparent Activity on Android?

Activity should be transparent, but has black background