Question

Can these two expressions both be true at one time?
holder.getSurface().isValid() == true;
and
canvas = holder.lockCanvas()
canvas == null;

Answer 1

From references:

isValid() – Does this object hold a valid surface? Returns true if it
holds a physical surface, so lockCanvas() will succeed.

So, if lockCanvas() succeeds it returns a non null canvas.

But you shouldn’t access Surface directly (you need synchronization), in surfaceChanged you can just call canvas = holder.lockCanvas() and draw only if canvas is not null.