How SurfaceHolder callbacks are related to Activity lifecycle?

Edit: if the targetSDK is greater than 10, putting the app to sleep calls onPause and onStop. Source

I looked at the lifecycle of both the Activity and the SurfaceView in a tiny camera app on my gingerbread phone. You are entirely correct; the surface is not destroyed when the power button is pressed to put the phone to sleep. When the phone goes to sleep, the Activity does onPause. (And does not do onStop.) It does onResume when the phone wakes up, and, as you point out, it does this while the lock screen is still visible and accepting input, which is a bit odd. When I make the Activity invisible by pressing the Home button, the Activity does both onPause and onStop. Something causes a callback to surfaceDestroyed in this case between the end of onPause and the start of onStop. It’s not very obvious, but it does seem very consistent.

When the power button is pressed to sleep the phone, unless something is explicitly done to stop it, the camera keeps running! If I have the camera do a per-image callback for each preview frame, with a Log.d() in there, the log statements keep coming while the phone is pretending to sleep. I think that is Very Sneaky.

As another confusion, the callbacks to surfaceCreated and surfaceChanged happen after onResume in the activity, if the surface is being created.

As a rule, I manage the camera in the class that implements the SurfaceHolder callbacks.

class Preview extends SurfaceView implements SurfaceHolder.Callback {
    private boolean previewIsRunning;
    private Camera camera;

    public void surfaceCreated(SurfaceHolder holder) {
        camera = Camera.open();
        // ...
        // but do not start the preview here!
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        // set preview size etc here ... then
        myStartPreview();
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        myStopPreview();
        camera.release();
        camera = null;
    }

   // safe call to start the preview
   // if this is called in onResume, the surface might not have been created yet
   // so check that the camera has been set up too.
   public void myStartPreview() {
       if (!previewIsRunning && (camera != null)) {
           camera.startPreview();
           previewIsRunning = true;
       }
   }

   // same for stopping the preview
   public void myStopPreview() {
       if (previewIsRunning && (camera != null)) {
           camera.stopPreview();
           previewIsRunning = false;
       }
   }
}

and then in the Activity:

@Override public void onResume() {
    preview.myStartPreview();  // restart preview after awake from phone sleeping
    super.onResume();
}
@Override public void onPause() {
    preview.myStopPreview();  // stop preview in case phone is going to sleep
    super.onPause();
}

and that seems to work OK for me. Rotation events cause the Activity to be destroyed and recreated, which causes the SurfaceView to be destroyed and recreated too.