Android: How to invalidate multiple parts of screen

Question

I need to be able to selectively invalidate multiple (about 20) rectangles on the screen for performance reasons, so tried the following:

Vector<Rect> myRects = new Vector<Rect>();

// ... add some Rects to myRects

for (Rect r : myRects) {
    invalidate(r);
}

However this seems to invalidates a union of all the Rect's, forming one large rectangle which covers all of small ones I'm trying to invalidate.

How can one invalidate multiple areas on the screen, and only those areas?