GradientView.swift

//
// Copyright 2018 Signal Messenger, LLC
// SPDX-License-Identifier: AGPL-3.0-only
//

import SignalServiceKit

public class GradientView: UIView {

    override public class var layerClass: AnyClass { CAGradientLayer.self }

    public var gradientLayer: CAGradientLayer { layer as! CAGradientLayer }

    public var colors: [UIColor] {
        didSet {
            updateGradientColors()
        }
    }

    public var locations: [CGFloat]? {
        didSet {
            updateGradientLocations()
        }
    }

    public convenience init(from fromColor: UIColor, to toColor: UIColor) {
        self.init(colors: [fromColor, toColor])
    }

    public init(colors: [UIColor], locations: [CGFloat]? = nil) {
        self.colors = colors
        self.locations = locations
        super.init(frame: .zero)
        updateGradientColors()
        updateGradientLocations()
    }

    public required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    private func updateGradientColors() {
        guard gradientWithinPerformanceLimits() else { return }
        gradientLayer.colors = colors.map { $0.cgColor }
    }

    private func updateGradientLocations() {
        guard gradientWithinPerformanceLimits() else { return }
        if let locations, !locations.isEmpty {
            gradientLayer.locations = locations.map { NSNumber(value: $0) }
        } else {
            gradientLayer.locations = nil
        }
    }

    private func gradientWithinPerformanceLimits() -> Bool {
        if (colors.count * 4 + (locations?.count ?? 0)) > 512 {
            owsFailDebug("Too many gradient points")
            return false
        }
        return true
    }

    /// Sets the `startPoint` and `endPoint` of the layer to reflect an angle in degrees
    /// where 0° starts at 12 o'clock and proceeds in a clockwise direction.
    public func setAngle(_ angle: UInt32) {
        // While design provides gradients with 0° at 12 o'clock, core animation's
        // coordinate system works with 0° at 3 o'clock moving in a counter clockwise
        // direction. We need to convert the provided angle accordingly before
        // calculating the gradient's start and end points.

        let caAngle =
            (360 - angle) // Invert to counter clockwise direction
                + 90 // Rotate 90° counter clockwise to shift the start from 3 o'clock to 12 o'clock

        let radians = CGFloat(caAngle) * .pi / 180.0

        // (x,y) in terms of the signed unit circle
        var endPoint = CGPoint(x: cos(radians), y: sin(radians))

        // extrapolate to signed unit square
        if abs(endPoint.x) > abs(endPoint.y) {
            endPoint.x = endPoint.x > 0 ? 1 : -1
            endPoint.y = endPoint.x * tan(radians)
        } else {
            endPoint.y = endPoint.y > 0 ? 1 : -1
            endPoint.x = endPoint.y / tan(radians)
        }

        // The signed unit square is a coordinate space from:
        // (-1,-1) to (1,1), but the gradient coordinate space
        // ranges from (0,0) to (1,1) with 0 being the top
        // left. Convert each point accordingly to calculate
        // the final points.
        func convertPointToGradientSpace(_ point: CGPoint) -> CGPoint {
            return CGPoint(
                x: (point.x + 1) * 0.5,
                y: 1.0 - (point.y + 1) * 0.5,
            )
        }

        // The start point will always be at the opposite side of the signed unit square.
        gradientLayer.startPoint = convertPointToGradientSpace(CGPoint(x: -endPoint.x, y: -endPoint.y))
        gradientLayer.endPoint = convertPointToGradientSpace(endPoint)
    }

}