AccessibilitySerivce

http://developer.android.com/reference/android/accessibilityservice/AccessibilityService.html
http://www.androidperformance.com/android-build-your-own-android-notification-service-app.html

An accessibility service runs in the background and receives callbacks by the system when AccessibilityEvents are fired. Such events denote some state transition in the user interface, for example, the focus has changed, a button has been clicked, etc. Such a service can optionally request the capability for querying the content of the active window. Development of an accessibility service requires extending this class and implementing its abstract methods.