Custom UIView's drawRect never called if initial frame is CGRectZero

A quick look at the Apple docs says this

Changing the frame rectangle automatically redisplays the view without calling its drawRect: method. If you want UIKit to call the drawRect: method when the frame rectangle changes, set the contentMode property to UIViewContentModeRedraw.

This is in the documentation for the frame property:
https://developer.apple.com/documentation/uikit/uiview/1622621-frame?language=objc

If you want the view to redraw, just call setNeedsDisplay on it and you should be fine (or, as the docs say, you can set it to UIViewContentModeRedraw, but it is up to you)