How to create a gradient perimeter using CALayer - ios

Question

I've been trying for some time now, using CAGradientLayer to produce this .
Initially I tried having a gradient background using the .colors property, however this is only a background fill. Trying this approach, I tried to add another CALayer inside that had a black background, however i could never get the radius correct, and it would create a line of various thickness at the rounded corners.
Is there a better way to create this rounded rect border with a gradient to it? Will CALayer not achieve this and should I move over to UIBezierPath or CGContextRef?
Code for failed attempt:
UIView *view = [[UIView alloc]initWithFrame:CGRectMake(12*twelthWidth - squareCentre.x - squareSize.width, squareCentre.y, squareSize.width, squareSize.height)];
// Create the rounded layer, and mask it using the rounded mask layer
CAGradientLayer *roundedLayer = [CAGradientLayer layer];
[roundedLayer setFrame:view.bounds];
roundedLayer.cornerRadius = view.bounds.size.width/5;
roundedLayer.masksToBounds = YES;
roundedLayer.colors = [NSArray arrayWithObjects:(id)[[UIColor redColor] CGColor], (id)[[UIColor blueColor] CGColor], nil];
roundedLayer.borderWidth = 4;
roundedLayer.borderColor = [UIColor clearColor].CGColor;
// Add these two layers as sublayers to the view
int BorderWidth = 5;
CALayer *solidColour = [CALayer layer];
solidColour.cornerRadius = view.bounds.size.width/5;
solidColour.masksToBounds = YES;
solidColour.backgroundColor = [UIColor blackColor].CGColor;
[solidColour setFrame:CGRectMake(BorderWidth, BorderWidth, roundedLayer.bounds.size.width - 2*BorderWidth, roundedLayer.bounds.size.height - 2*BorderWidth)];
[view.layer insertSublayer:roundedLayer atIndex:0];
[view.layer insertSublayer:solidColour above:roundedLayer];
[self.view addSubview:view];
Which produces:
Whereby the corners aren't right. Could it be that I need to calculate a different radius for the second layer?
Edit
After setting the radius of the solid colour to solidColour.bounds.size.width/5, it still isn't right - It goes too thin at the corners.

Answer 1

The problem you are seeing is because the inner and outer corner radius are the same. That is what causes the line thickness to vary. This illustration from CSS-Tricks highlights the issue (even thought you aren't using CSS, the problem is still the same):
The solution is to calculate the inner radius as:
innerRadis = outerRadius - lineThickness
As shown in this illustration by Joshua Hibbert:

Answer 2

Instead of using the view bounds to set the cornerRadius of the solid layer, use the layer's frame value after you've set it to the correct inset value:
solidColour.masksToBounds = YES;
solidColour.backgroundColor = [UIColor blackColor].CGColor;
[solidColour setFrame:CGRectMake(BorderWidth, BorderWidth, roundedLayer.bounds.size.width - 2*BorderWidth, roundedLayer.bounds.size.height - 2*BorderWidth)];
solidColour.cornerRadius = solidColour.frame.size.width/5;
[view.layer insertSublayer:roundedLayer atIndex:0];
[view.layer insertSublayer:solidColour above:roundedLayer];
With this I get the following image

Answer 3

Just for your convenience if you use swift
let v = yourUIVIEW
let outerRadius:CGFloat = 60
let borderWidth:CGFloat = 8;
let roundedLayer = CAGradientLayer()
roundedLayer.frame = v.bounds
roundedLayer.cornerRadius = outerRadius
roundedLayer.masksToBounds = true
roundedLayer.endPoint = CGPoint(x: 1 , y: 0.5)
roundedLayer.colors = [ UIColor.redColor().CGColor, UIColor.blueColor()!.CGColor]
let innerRadius = outerRadius - borderWidth
//Solid layer
let solidLayer = CALayer()
solidLayer.masksToBounds = true
solidLayer.backgroundColor = UIColor.whiteColor().CGColor
solidLayer.cornerRadius = innerRadius
solidLayer.frame = CGRect(x: borderWidth, y: borderWidth, width: roundedLayer.bounds.width - 2*borderWidth, height:roundedLayer.bounds.height - 2*borderWidth )
v.layer.insertSublayer(roundedLayer, atIndex: 0)
v.layer.insertSublayer(solidLayer, above: roundedLayer)