ScrollIndicatorBehavior.1.swift

import UIKit

final class ScrollIndicatorBehavior {
    // MARK: Properties

    var leftInset: CGFloat {
        didSet { updateShadowFrame() }
    }

    var rightInset: CGFloat {
        didSet { updateShadowFrame() }
    }

    var shadowWidth: CGFloat {
        didSet { updateShadowFrame() }
    }

    private let leftShadow: GradientView = {
        let view = GradientView()
        view.locations = [0, 1]
        view.colors = [UIColor.black, UIColor.clear]
        view.mode = .linear(.horizontal)
        return view
    }()

    private let rightShadow: GradientView = {
        let view = GradientView()
        view.locations = [0, 1]
        view.colors = [UIColor.clear, UIColor.black]
        view.mode = .linear(.horizontal)
        return view
    }()

    // MARK: Initialization

    init(containerView: UIView, leftInset: CGFloat = 0, rightInset: CGFloat = 0, shadowWidth: CGFloat = 20) {
        self.leftInset = leftInset
        self.rightInset = rightInset
        self.shadowWidth = shadowWidth
        containerView.addSubview(leftShadow)
        containerView.addSubview(rightShadow)
    }

    // MARK: Shadow Management

    private func updateShadowFrame() {
        guard let containerView = leftShadow.superview else { return }

        let size = CGSize(width: shadowWidth, height: containerView.frame.height)
        let leftOriginX: CGFloat
        let rightOriginX: CGFloat

        if let containerView = containerView as? UIScrollView {
            leftOriginX = containerView.contentOffset.x + leftInset
            rightOriginX = containerView.contentOffset.x + containerView.frame.width - shadowWidth - rightInset
        } else {
            leftOriginX = leftInset
            rightOriginX = containerView.frame.width - shadowWidth - rightInset
        }

        leftShadow.frame = CGRect(origin: CGPoint(x: leftOriginX, y: 0), size: size)
        rightShadow.frame = CGRect(origin: CGPoint(x: rightOriginX, y: 0), size: size)
    }

    private func setScrollPercentage(_ percentage: CGFloat) {
        if percentage.isNaN {
            leftShadow.alpha = 0
            rightShadow.alpha = 0
        } else {
            let alpha = max(min(percentage, 1), 0)
            leftShadow.alpha = alpha
            rightShadow.alpha = 1 - alpha
        }
    }

    // MARK: ScrollViewBehavior

    func afterScrolling(_ scrollView: UIScrollView) {
        let frameSize = scrollView.frame.size
        let contentSize = scrollView.contentSize

        guard frameSize.width < contentSize.width else {
            setScrollPercentage(.nan)
            return
        }

        let minLeftOffset = -scrollView.contentInset.left
        let contentOffset = scrollView.contentOffset
        let maxLeftOffset = contentSize.width - frameSize.width
        let maxMinDifference = maxLeftOffset - minLeftOffset

        guard maxMinDifference != 0 else {
            setScrollPercentage(.nan)
            return
        }

        let scrolledPercentage = (contentOffset.x - minLeftOffset) / maxMinDifference
        setScrollPercentage(scrolledPercentage)
        updateShadowFrame()
    }
}