CALayer: How do I call the draw function?

Question

I have subclassed CALayer to create a radial gradient (I started with this code here). The problem is that I need to change the colors of the gradient, which is in the draw function. I want to be able to redraw the layer every time I change colors. I can't directly call draw(in:) because I don't know what CGContext to use. Any ideas?

import Foundation
import UIKit

class CARadialGradientLayer: CALayer {

    required override init() {
        super.init()
        needsDisplayOnBoundsChange = true
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    required override init(layer: Any) {
        super.init(layer: layer)
    }

    public var colors = [UIColor(red: 0.2, green: 0.2, blue: 0.2, alpha: 0.5).cgColor, UIColor(red: 0.2, green: 0.2, blue: 0.2, alpha: 0.00).cgColor] {
        didSet {


            /* Redraw the layer to update colors */


        }
    }

    override func draw(in ctx: CGContext) {
        ctx.saveGState()

        let colorSpace = CGColorSpaceCreateDeviceRGB()
        var locations = [CGFloat]()

        for i in 0...colors.endIndex {
            locations.append(CGFloat(i) / CGFloat(colors.count))
        }

        let gradient = CGGradient(colorsSpace: colorSpace, colors: colors as CFArray, locations: locations)
        let center = CGPoint(x: bounds.width / 2.0, y: bounds.height / 2.0)
        let radius = min(bounds.width / 2.0, bounds.height / 2.0)
        ctx.drawRadialGradient(gradient!, startCenter: center, startRadius: 0.0, endCenter: center, endRadius: radius, options: CGGradientDrawingOptions(rawValue: 0))
    }
}

Answer 1

In didSet, call setNeedsDisplay().

It's not up to you when drawing happens. The system handles that. But it's up to you to tell the system when drawing is needed.