What is invalid region in Android?

Under "How Android Draws Views" topic, there is such a sentence :

Drawing begins with the root node of the layout. It is requested to measure and draw the layout tree. Drawing is handled by walking the tree and rendering each View that intersects the invalid region.

And I didn't quite understan开发者_开发问答d the term "invalid region".

Here is the source article of the quotation.

Can someone elaborate it?

Thanks.

The "invalid region" is essentially the portion of the screen that requires repainting. It's used to make painting more efficient by only repainting the invalid region and leaving the rest alone.