gloss-ios/VisibilityTracker.swift

// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
//  Created by Sam Deane on 27/07/22.
//  All code (c) 2022 - present day, Elegant Chaos Limited.
// -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

import SwiftUI

public enum VisibilityChange {
    case hidden
    case shown
}

public class VisibilityTracker<ID: Hashable>: ObservableObject {
    /// The global bounds of the container view.
    public var containerBounds: CGRect
    
    /// Dictionary containing the offset of every visible view.
    public var visibleViews: [ID:CGFloat]
    
    /// Ids of the visible views, sorted by offset.
    /// The first item is the top view, the last one, the bottom view.
    public var sortedViewIDs: [ID]
    
    /// Action to perform when a view becomes visible or is hidden.
    public var action: Action
    
    /// The id of the top visible view.
    public var topVisibleView: ID? { sortedViewIDs.first }
    
    /// The id of the bottom visible view.
    public var bottomVisibleView: ID? { sortedViewIDs.last }

    /// Action callback signature.
    public typealias Action = (ID, VisibilityChange, VisibilityTracker<ID>) -> ()

    public init(action: @escaping Action) {
        self.containerBounds = .zero
        self.visibleViews = [:]
        self.sortedViewIDs = []
        self.action = action
    }
    
    public func reportContainerBounds(_ bounds: CGRect) {
        containerBounds = bounds
    }
    
    public func reportContentBounds(_ bounds: CGRect, id: ID)  {
        let topLeft = bounds.origin
        let size = bounds.size
        let bottomRight = CGPoint(x: topLeft.x + size.width, y: topLeft.y + size.height)
        let isVisible = containerBounds.contains(topLeft) || containerBounds.contains(bottomRight)
        let wasVisible = visibleViews[id] != nil


        visibleViews[id] = -1 * (bounds.origin.y - containerBounds.origin.y)
        if (abs(visibleViews[id]! ) > 500) {
            visibleViews.removeValue(forKey: id)
        }
        sortViews()
        action(id, .shown, self)

        // if isVisible {
        //     visibleViews[id] = bounds.origin.y - containerBounds.origin.y
        //     sortViews()
        //     if !wasVisible {
        //         action(id, .shown, self)
        //     }
        // } else {
        //     if wasVisible {
        //         visibleViews.removeValue(forKey: id)
        //         sortViews()
        //         action(id, .hidden, self)
        //     }
        // }
    }
    
    func sortViews() {
        let sortedPairs = visibleViews.sorted(by: { $0.1 < $1.1 })
        sortedViewIDs = sortedPairs.map { $0.0 }
    }
}