Property observers for UIView bounds and frame rea

问题:

While I am exploring the option to observe a UIView's bounds or frame change (mentioned here and here), I have encountered a very strange discrepancy: didSet and willSet will be triggered differently based on where you put your UIView in the view hierarchy:

• If I use property observer for UIView at the root of a view controller, I will only get didSet and willSet events from frame changes.
• If I use property observer for UIView that is a subview inside a view controller, I will only get didSet and willSet events from bounds changes.

I’d like to note first that I’m explicitly avoiding KVO approach mentioned here since it’s not officially supported. I’m also not looking to use viewDidLayoutSubviews() mentioned here since that won’t work for observing changes of subviews (see the doc). This question assumes my preference to use didSet and willSet to observe a UIView’s bounds / frame changes.

The closest question I have come across is this question but it covers only the initialization phrase, and also doesn’t mention the case of observing a subview.

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