[Combine][UIKit][UIScrollView] Horizontal Swipe Direction Detector

UIScrollViewHorizontalSwipeEmittingDelegate.swift

import UIKit
import Combine

enum ScrollSwipeDirection {
    case leading, trailing, trailingEnd
}

class UIScrollViewHorizontalSwipeEmittingDelegate: NSObject, UIScrollViewDelegate {
    public let swipeDirection = PassthroughSubject<ScrollSwipeDirection, Never>()
    
    private var startDraggingOffset: CGFloat = 0
    
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        self.startDraggingOffset = scrollView.contentOffset.x
    }
    
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let contentOffsetDelta = startDraggingOffset - targetContentOffset.pointee.x
        
        if contentOffsetDelta > 0 {
            swipeDirection.send(.leading)
        } else {
            let distanceToEndOffset = scrollView.contentSize.width - scrollView.frame.width - targetContentOffset.pointee.x
            swipeDirection.send(distanceToEndOffset <= 0 ? .trailingEnd : .trailing)
        }
        // Update startDraggingOffset in case Cx swipes in the middle of a momentum scroll
        // (which won't trigger scrollViewWillBeginDragging and thus will not set startDraggingOffset)
        self.startDraggingOffset = targetContentOffset.pointee.x
    }
}