Custom UIScrollView with fade effect

FadeScrollView.swift

//
//  FadeScrollView.swift
//
//  Created by Luís Machado on 23/06/2017.
//  Copyright © 2017 Luis Machado. All rights reserved.
//

import UIKit

class FadeScrollView: UIScrollView, UIScrollViewDelegate {

    let fadePercentage: Double = 0.2
    let gradientLayer = CAGradientLayer()
    let transparentColor = UIColor.clear.cgColor
    let opaqueColor = UIColor.black.cgColor
    
    var topOpacity: CGColor {
        let scrollViewHeight = frame.size.height
        let scrollContentSizeHeight = contentSize.height
        let scrollOffset = contentOffset.y
        
        let alpha:CGFloat = (scrollViewHeight >= scrollContentSizeHeight || scrollOffset <= 0) ? 1 : 0
        
        let color = UIColor(white: 0, alpha: alpha)
        return color.cgColor
    }
    
    var bottomOpacity: CGColor {
        let scrollViewHeight = frame.size.height
        let scrollContentSizeHeight = contentSize.height
        let scrollOffset = contentOffset.y
        
        let alpha:CGFloat = (scrollViewHeight >= scrollContentSizeHeight || scrollOffset + scrollViewHeight >= scrollContentSizeHeight) ? 1 : 0

        let color = UIColor(white: 0, alpha: alpha)
        return color.cgColor
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        self.delegate = self
        let maskLayer = CALayer()
        maskLayer.frame = self.bounds
        
        gradientLayer.frame = CGRect(x: self.bounds.origin.x, y: 0, width: self.bounds.size.width, height: self.bounds.size.height)
        gradientLayer.colors = [topOpacity, opaqueColor, opaqueColor, bottomOpacity]
        gradientLayer.locations = [0, NSNumber(floatLiteral: fadePercentage), NSNumber(floatLiteral: 1 - fadePercentage), 1]
        maskLayer.addSublayer(gradientLayer)
        
        self.layer.mask = maskLayer
    }
 
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        gradientLayer.colors = [topOpacity, opaqueColor, opaqueColor, bottomOpacity]
    }
    
}

Two questions: why conform to UIScrollViewDelegate? The scroll view will call layoutSubviews whenever it is scrolled. Second, Why in line 46 did you use self.bounds.origin.x and not 0? Otherwise this is great! Saved me a ton of time!