Applying a Gradient to CAShapeLayer

Question

Does anyone have any experience in applying a Gradient to a CAShapeLayer? CAShapeLayer is a fantastic layer class, but it appears to only support solid fill coloring, wherea

Answer 1

Many thanks to @Palimondo for a great answer!

In case someone is looking for Swift 4 + filling animation code of this solution:

    let myView = UIView(frame: .init(x: 0, y: 0, width: 200, height: 150))
    view.addSubview(myView)
    myView.center = view.center

    // Start and finish point
    let startPoint = CGPoint(x: myView.bounds.minX, y: myView.bounds.midY)
    let finishPoint = CGPoint(x: myView.bounds.maxX, y: myView.bounds.midY)

    // Path
    let path = UIBezierPath()
    path.move(to: startPoint)
    path.addLine(to: finishPoint)

    // Gradient Mask
    let gradientMask = CAShapeLayer()
    let lineHeight = myView.frame.height
    gradientMask.fillColor = UIColor.clear.cgColor
    gradientMask.strokeColor = UIColor.black.cgColor
    gradientMask.lineWidth = lineHeight
    gradientMask.frame = myView.bounds
    gradientMask.path = path.cgPath

    // Gradient Layer
    let gradientLayer = CAGradientLayer()
    gradientLayer.startPoint = CGPoint(x: 0.0, y: 0.5)
    gradientLayer.endPoint = CGPoint(x: 1.0, y: 0.5)

    // make sure to use .cgColor
    gradientLayer.colors = [UIColor.red.cgColor, UIColor.green.cgColor]
    gradientLayer.frame = myView.bounds
    gradientLayer.mask = gradientMask

    myView.layer.addSublayer(gradientLayer)

    // Corner radius
    myView.layer.cornerRadius = 10
    myView.clipsToBounds = true

Extra. In case you also need a "filling animation", add this lines:

    // Filling animation
    let animation = CABasicAnimation(keyPath: "strokeEnd")
    animation.fromValue = 0
    animation.duration = 10
    gradientMask.add(animation, forKey: "LineAnimation")

Answer 2

This is a great solution, but you might encounter unexpected problems if you're creating a category on CAShapeLayer where you don't immediately have the view.

See Setting correct frame of a newly created CAShapeLayer

Bottom line, get the bounds of the path then set the gradient mask's frame using the path bounds and translate as necessary. Good thing here is that by using the path's bounds rather than any other frame, the gradient will only fit within the path bounds (assuming that's what you want).

// Category on CAShapeLayer

CGRect pathBounds = CGPathGetBoundingBox(self.path);

CAShapeLayer *gradientMask = [CAShapeLayer layer];
gradientMask.fillColor = [[UIColor blackColor] CGColor];
gradientMask.frame = CGRectMake(0, 0, pathBounds.size.width, pathBounds.size.height);
gradientMask.path = self.path;

CAGradientLayer *gradientLayer = [CAGradientLayer layer];
gradientLayer.startPoint = CGPointMake(0.5,1.0);
gradientLayer.endPoint = CGPointMake(0.5,0.0);
gradientLayer.frame = CGRectMake(0, 0, pathBounds.size.width, pathBounds.size.height);

NSMutableArray *colors = [NSMutableArray array];
for (int i = 0; i < 10; i++) {
    [colors addObject:(id)[[UIColor colorWithHue:(0.1 * i) saturation:1 brightness:.8 alpha:1] CGColor]];
}
gradientLayer.colors = colors;

[gradientLayer setMask:gradientMask];
[self addSublayer:gradientLayer];

Answer 3

You could use the path of your shape to create a masking layer and apply that on the gradient layer, like this:

UIView *v = [[UIView alloc] initWithFrame:self.window.frame];

CAShapeLayer *gradientMask = [CAShapeLayer layer];   
gradientMask.fillColor = [[UIColor clearColor] CGColor];
gradientMask.strokeColor = [[UIColor blackColor] CGColor];
gradientMask.lineWidth = 4;
gradientMask.frame = CGRectMake(0, 0, v.bounds.size.width, v.bounds.size.height);

CGMutablePathRef t = CGPathCreateMutable();    
CGPathMoveToPoint(t, NULL, 0, 0);
CGPathAddLineToPoint(t, NULL, v.bounds.size.width, v.bounds.size.height);

gradientMask.path = t;


CAGradientLayer *gradientLayer = [CAGradientLayer layer];
gradientLayer.startPoint = CGPointMake(0.5,1.0);
gradientLayer.endPoint = CGPointMake(0.5,0.0);
gradientLayer.frame = CGRectMake(0, 0, v.bounds.size.width, v.bounds.size.height);
NSMutableArray *colors = [NSMutableArray array];
for (int i = 0; i < 10; i++) {
    [colors addObject:(id)[[UIColor colorWithHue:(0.1 * i) saturation:1 brightness:.8 alpha:1] CGColor]];
}
gradientLayer.colors = colors;

[gradientLayer setMask:gradientMask];
[v.layer addSublayer:gradientLayer];

If you want to also use the shadows, you would have to place a "duplicate" of the shape layer under the gradient layer, recycling the same path reference.