android: activity's onResume vs onWindowFocusChanged
From activity.java onResume's comment:
Called after onRestoreInstanceState, onRestart, or onPause, for your activity to start interacting with the user. This is a good place to begin animations, open exclusive-access devices (such as the camera), etc.
Keep in mind that onResume is not the best indicator that your activity is visible to the user; a system window such as the keyguard may be in front. Use onWindowFocusChanged to know for certain that your activity is visible to the user (for example, to resume a game).
Derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown.