Question

I’m reading up on SurfaceView and how to use it, and I’ve come across some information that states that a SurfaceView has View#willNotDraw() set to false by default, and that it’s up to you to call SurfaceView#onDraw(). I also read that RomainGuy said that this is done by default because it is more efficient. My question now is, when should you handle calling SurfaceView#onDraw() in a separate thread, and when should you just set View#willNotDraw() to true, and just call SurfaceView#invalidate(). Is there a difference between the two, and does one improve performance more than the other?

Answer 1

See:

http://developer.android.com/reference/android/view/View.html#setWillNotDraw(boolean)

I’m not sure where you got your information, but at least the javadoc says that most users will set this to false to get Android to send it onDraw events itself. As for your question about when you should do this, I would say it comes down to why you’re using a SurfaceView.

If your view is displaying something dynamic (e.g. for a game or something that has a tight event loop), you’ll want to be controlling exactly when updates happen, especially if you’ll have the information to use one of the more detailed forms of invalidate to save redrawing the entire View. You won’t want Android to call invalidate for you, and that’s why the flag is there.

If, on the other hand, you are simply drawing something static, it makes sense to let Android’s UI stack control the invalidations.

By the way, invalidate only posts a request to re-draw the View, so be aware of this if you intend to use the event-loop style (onDraw will be called sometime after you call it).

Edit: some clarifications.