ObservableCollectionViewCell

ObservableCollectionViewCell.swift

//
//  ObservableCollectionViewCell.swift
//
//  Created by Justin D'Arcangelo on 7/3/25.
//

import UIKit

class ObservableCollectionViewCell: UICollectionViewCell {
    var isMostlyVisible: Bool {
        guard !isHidden, alpha > 0, !bounds.isEmpty, let window, window.hitTest(window.convert(center, from: superview), with: nil) == self else {
            return false
        }
        return true
    }

    private var scrollViews: [UIScrollView] {
        let superviews = Array(sequence(first: superview, next: { $0?.superview }))
        return superviews.filter({ $0 is UIScrollView }) as? [UIScrollView] ?? []
    }

    @IBOutlet var label: UILabel!

    override func prepareForReuse() {
        for observedScrollView in observedScrollViews {
            observedScrollView.removeObserver(self, forKeyPath: "contentOffset")
        }

        observedScrollViews.removeAll()

        super.prepareForReuse()
    }

    override func layoutSubviews() {
        for scrollView in scrollViews {
            if !observedScrollViews.contains(scrollView) {
                scrollView.addObserver(self, forKeyPath: "contentOffset", context: nil)
                observedScrollViews.insert(scrollView)
            }
        }
        
        checkVisibility()

        super.layoutSubviews()
    }

    private var observedScrollViews: Set<UIScrollView> = []
    private var wasPreviouslyMostlyVisible: Bool = false
    
    private func checkVisibility() {
        if wasPreviouslyMostlyVisible != isMostlyVisible {
            print("isMostlyVisible", label.text!, isMostlyVisible)
            wasPreviouslyMostlyVisible = isMostlyVisible
        }
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        checkVisibility()
    }
}

1. layoutSubviews is invoked by the collection view when the component is "rendered" (not necessarily on-screen)
2. when rendered, we walk up the superview chain and find all of the UIScrollView containers and observe the contentOffset property on them
3. when any parent scroll views' contentOffset prop changes, we check if our "mostly visible" status (>50%) has changed and if so, we log it
4. prepareForReuse is invoked by the collection view when the component is about to be recycled/un-rendered and at that point we stop observing the parent scroll views