How to draw radial gradients in a CALayer?

Question

I know CAGradientLayer doesn't support radial gradient at the moment and only has option of kCAGradientLayerAxial.

I want something like below:

I have looked around for this issue and found that there is a way around this. But the explanations were not clear to me. So I want to know if I can draw radial gradients using CAGradientLayer and if so, then how to do it?

Answer 1

From what I understood, you just need a layer that draws a gradient, and CGContextDrawRadialGradient works perfectly for that need. And to reiterate on what you said, CAGradientLayer doesn't support radial gradients, and nothing we can do about that, except unnecessary swizzling that can be done cleanly with a CALayer subclass.

(note: the gradient drawing code was taken from here. It isn't what this answer is about.)

---

viewDidLoad:

GradientLayer *gradientLayer = [[GradientLayer alloc] init];
gradientLayer.frame = self.view.bounds;

[self.view.layer addSublayer:gradientLayer];

---

CALayer subclass:

- (instancetype)init
{
    self = [super init];
    if (self) {
        [self setNeedsDisplay];
    }
    return self;
}

- (void)drawInContext:(CGContextRef)ctx
{

    size_t gradLocationsNum = 2;
    CGFloat gradLocations[2] = {0.0f, 1.0f};
    CGFloat gradColors[8] = {0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.0f,0.5f};
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, gradColors, gradLocations, gradLocationsNum);
    CGColorSpaceRelease(colorSpace);

    CGPoint gradCenter= CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
    CGFloat gradRadius = MIN(self.bounds.size.width , self.bounds.size.height) ;

    CGContextDrawRadialGradient (ctx, gradient, gradCenter, 0, gradCenter, gradRadius, kCGGradientDrawsAfterEndLocation);


    CGGradientRelease(gradient);
}

Answer 2

Swift 5

import UIKit

final class RadialGradientLayer: CALayer {

    let centerColor: CGColor
    let edgeColor: CGColor

    init(centerColor: UIColor, edgeColor: UIColor) {
        self.centerColor = centerColor.cgColor
        self.edgeColor = edgeColor.cgColor
        super.init()
        needsDisplayOnBoundsChange = true
    }

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

    override func draw(in ctx: CGContext) {
        ctx.saveGState()
        let gradient = CGGradient(colorsSpace: CGColorSpaceCreateDeviceRGB(),
                                  colors: [centerColor, edgeColor] as CFArray,
                                  locations: [0.0, 1.0])

        let gradCenter = CGPoint(x: bounds.midX, y: bounds.midY)
        let gradRadius = min(bounds.width, bounds.height)

        ctx.drawRadialGradient(gradient!,
                               startCenter: gradCenter,
                               startRadius: 0.0,
                               endCenter: gradCenter,
                               endRadius: gradRadius,
                               options: .drawsAfterEndLocation)
    }
}

Answer 3

Here is the Swift3 code I use :

import UIKit

class RadialGradientLayer: 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.cgColor, UIColor.blue.cgColor]

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

        let colorSpace = CGColorSpaceCreateDeviceRGB()
        var locations = [CGFloat]()
        for i in 0...colors.count-1 {
            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 4

I don't know when but now you can change CAGradientLayer 's type to kCAGradientLayerRadialand it works.

Theoretically the performance of the Core Animation way is better than Core Graphics.

Some example code:

class View : UIView {

    let gradientLayer = CAGradientLayer()
    override init(frame: CGRect) {
        super.init(frame: frame)

        gradientLayer.startPoint = CGPoint(x: 0.5, y: 0.5)
        gradientLayer.endPoint = CGPoint(x: 1, y: 1)
        gradientLayer.locations = [
            NSNumber(value: 0.6),
            NSNumber(value: 0.8)
        ]
        gradientLayer.type = .radial
        gradientLayer.colors = [
            UIColor.green.cgColor,
            UIColor.purple.cgColor,
            UIColor.orange.cgColor,
            UIColor.red.cgColor
        ]
        self.layer.addSublayer(gradientLayer)
    }
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func layoutSublayers(of layer: CALayer) {
        assert(layer === self.layer)
        gradientLayer.frame = layer.bounds
    }
}