/*

    Copyright (C) 2017 Apple Inc. All Rights Reserved.

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    A `UIView` subclass that is used to mask the contents of a `CollectionViewController`.

*/

import UIKit

class GradientMaskView: UIView {

    // MARK: Properties

    /// The position in points between which the gradient is drawn.

    var maskPosition: (start: CGFloat, end: CGFloat) {

        didSet {

            updateGradientLayer()

        }

    }

    /// The `CAGradientLayer` responsible for drawing the alpha gradient.

    private let gradientLayer: CAGradientLayer = {

        let layer = CAGradientLayer()

        layer.colors = [UIColor(white: 0.0, alpha: 0.0).cgColor, UIColor(white: 0.0, alpha: 1.0).cgColor]

        return layer

    }()

    // MARK: Initialization

    override init(frame: CGRect) {

        maskPosition = (start: 0, end: 0)

        super.init(frame: frame)

        layer.addSublayer(gradientLayer)

        updateGradientLayer()

    }

    required init?(coder aDecoder: NSCoder) {

        maskPosition = (start: 0, end: 0)

        super.init(coder: aDecoder)

        layer.addSublayer(gradientLayer)

        updateGradientLayer()

    }

    // MARK: UIView

    override func layoutSubviews() {

        super.layoutSubviews()

        updateGradientLayer()

    }

    // MARK: Convenience

    private func updateGradientLayer() {

        // Update the `gradientLayer` frame to match the view's bounds.

        gradientLayer.frame = CGRect(origin: CGPoint.zero, size: bounds.size)

        /*

            Update the `gradientLayer`'s gradient locations with the `maskPosition`

            converted to the `gradientLayer`'s unit coordinate space.

        */

        gradientLayer.locations = [maskPosition.end / bounds.size.height as NSNumber, maskPosition.start / bounds.size.height as NSNumber]

    }

}