Key capture in android

I am working on an application thats captures all the keys tha开发者_Python百科t are being pressed. As of now I can capture the keys in application. But how can I capture the keys while the application is running in android. I am aware of services concept, but am unable to capture the keys while application in running in background.

It would make sense that you aren't able to do so as it would allow you to steal users passwords that they type into other applications.