CALayer Drawing API

CALayerDrawingAPI.md

Call Chain:

CALayer.display() --- if delegate implements displayLayer ---> CALayerDelegate.displayLayer(:)
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else                                                          /
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v                                                           v
CALayer.drawInContext(:) ---> CALayerDelegate.drawLayer(:, inContext:) ---> UIView.drawRect(:)

Quirks, demoed in Pie:

1. Overriding CALayer.drawInContext(:) or UIView.drawRect(:) causes CALayer.backgroundColor or UIView.backgroundColor to be fill into CGContext, even when backgroundColor is nil(default) in which case black is used. It is different from the default behavior of nil backgroundColor being rendered transparently. Even if superview.backgroundColor is black there will be visual difference. See point 2.
2. Black background will make a big visual difference from clear blackground in the results of dynamic drawing of animation. Animation with black background results in relatively coarse edges — not only during the animation but also the final drawing.
3. contentsScale is not always correct: http://www.openradar.me/22595655.
4. Even though from the call chain it seems UIView.drawRect(:) should be no different from CALayer.drawInContext(:) as the overriding point, it is not true: UIView.drawRect(:) does not support animations while CALayer.drawInContext(:) does.

The third problem still remains. Just tested on iOS 15.4. Thank you!