Android - Need Calculator to pop up when editing a EditText

I'm trying to figure out a way to have a simple calculator pop up when I start to edit a EditText. Once I add the numbers together I want the result to go to the EditText i'm working on. I've seen this in other app's. Is this something that needs to be write from scratch or is the开发者_Python百科re a better way to accomplish it?

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I have never seen this widget before, but I imagine it would be pretty fun to write.

I think it would be pretty easy to write as a sort of custom dialog.