Property observers for UIView bounds and frame react differently

Question

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.

Details

To see this in action, check out 🔨 my sample project.

I am really puzzled why bounds observers are sometimes not called, so I added frame observers as well, and even frame observers are sometimes not called. Eventually, I was able to find the key setting where they work differently: the view’s placement in view hierarchy, as described above.

How I test: in both cases, rotating the device to change the view’s frame / bounds.

Here’s my UIView subclass:

public class BoundsObservableView: UIView {

    public weak var boundsDelegate: ViewBoundsObserving?

    public override var bounds: CGRect {
        willSet {
            print("BOUNDS willSet bounds: \(bounds), frame: \(frame)")
            boundsDelegate?.boundsWillChange(self)
        }
        didSet {
            print("BOUNDS didSet bounds: \(bounds), frame: \(frame)")
            boundsDelegate?.boundsDidChange(self)
        }
    }

    public override var frame: CGRect {
        willSet {
            print("FRAME willSet frame: \(frame), bounds: \(bounds)")
            boundsDelegate?.boundsWillChange(self)
        }
        didSet {
            print("FRAME didSet frame: \(frame), bounds: \(bounds)")
            boundsDelegate?.boundsDidChange(self)
        }
    }
}

In my sample code, if you rotate the device, you will see that in one case where I’m observing the root view (ViewController’s self.view -- shown in blue), I would never get notified of bounds change, despite it having actually changed. The opposite goes for a subview -- I never got notified of frame change, despite it having changed.

Environment

I'm testing this project on Xcode 9.3 with iOS 11.4 SDK on devices like iPad Air and iPad Pro. I have not tried in on iOS 12 beta.

My Questions

• Why do didSet and willSet get triggered differently when UIView is placed differently in the view hierarchy?
• When didSet for bounds is triggered, why won’t didSet for frame triggered as well (for subviews)? Vice versa (for root view)?
• What is a way, if any, to ensure that I can always observe bounds change of a UIView no matter where I put it in the view hierarchy?

Answer 1

From my repost in Apple Developer Forum, QuinceyMorris helped me clarifying issues with this approach as well as an approach that would work no matter where I put the view in the view hierarchy.

... an Obj-C property can change value without having its setter called. Changing the instance variable (of simple properties) is a very common Obj-C pattern. It is of course not KVO compliant without additional work, but that's why KVO compliance is not found universally.

... Your willSet/didSet accessors will only trigger when the change goes through their own property. There is nothing you can predict or assume about which property will be used. Even if you see a regularity now, there may be edge cases that are different, and the behavior may change in the future.

Based on his recommendation that I override layoutSubviews, here's my updated subclass (just like this answer):

public protocol ViewBoundsObserving: class {
    // Notifies the delegate that view's `bounds` has changed.
    // Use `view.bounds` to access current bounds
    func boundsDidChange(_ view: BoundsObservableView, from previousBounds: CGRect);
}

/// You can observe bounds change with this view subclass via `ViewBoundsObserving` delegate.
public class BoundsObservableView: UIView {
    
    public weak var boundsDelegate: ViewBoundsObserving?
    
    private var previousBounds: CGRect = .zero
    
    public override func layoutSubviews() {
        if (bounds != previousBounds) {
            print("Bounds changed from \(previousBounds) to \(bounds)")
            boundsDelegate?.boundsDidChange(self, from: previousBounds)
            previousBounds = bounds
        }
        
        // UIView's implementation will layout subviews for me using Auto Resizing mask or Auto Layout constraints.
        super.layoutSubviews()
    }
}