VisibilityTracking.swift

//
//  VisibilityTracker.swift
//  Babylist
//
//  Created by Aaron Smith on 8/16/24.
//  Copyright © 2024 Babylist. All rights reserved.
//

import Foundation

protocol VisibilityTrackingView : UIView {
    var visibilityTrackingId:AnyHashable { get }
    func performVisibilityTracking() -> Bool
}

/// This is optional and provides a way to specify the exact views
/// we know are visibilty trackable
protocol VisibilityTrackingViewSource {
    var visibilityTrackingViewsToTrack:[UIView] { get }
}

class VisibilityTracker {
    
    /// The required height visible
    var amountVisible:CGFloat = 0.9
    
    /// Optional source for views to track.
    var viewSource:VisibilityTrackingViewSource?
    
    /// Views we've tracked already
    var tracked:[AnyHashable:Bool] = [:]
    
    func scrollViewDidScroll(scrollView:UIScrollView) {
        let views:[UIView] = if let viewSource {
            viewSource.visibilityTrackingViewsToTrack
        } else {
            scrollView.subviews
        }
        track(views: views, scrollView: scrollView)
    }
    
    func scrollViewDidScroll(collectionView:UICollectionView) {
        let views:[UIView] = if let viewSource {
            viewSource.visibilityTrackingViewsToTrack
        } else {
            collectionView.visibleCells
        }
        track(views: views, scrollView: collectionView)
    }
    
    func scrollViewDidScroll(tableView:UITableView) {
        let views:[UIView] = if let viewSource {
            viewSource.visibilityTrackingViewsToTrack
        } else {
            tableView.visibleCells
        }
        track(views: views, scrollView: tableView)
    }
    
    private func track(views:[UIView], scrollView:UIScrollView) {
        for view in views {
            // if we encounter a view that is also a source, track those.
            // this allows cells to be sources that provide explicit views (any subviews) to track.
            if let view = view as? VisibilityTrackingViewSource {
                track(views: view.visibilityTrackingViewsToTrack, scrollView: scrollView)
            }
            guard let visibilityView = view as? (any VisibilityTrackingView) else { continue }
            let intersection = CGRectIntersection(scrollView.bounds, view.frame)
            if intersection.size.height >= (view.bounds.size.height * amountVisible) {
                let id = visibilityView.visibilityTrackingId
                if tracked[id] == nil {
                    if visibilityView.performVisibilityTracking() {
                        tracked[id] = true
                    }
                }
            }
        }
    }
    
}