Draw a semi-circle button iOS

The problem is that your shape layer is positioned way outside the buttons bounds. If you just add the shape layer as a subview of your button's layer you'll see the problem:

button.layer.addSublayer(circleShape)

You have to adjust the arc center point, so that the arc lies within the button's bounds:

let circlePath = UIBezierPath.init(arcCenter: CGPointMake(button.bounds.size.width / 2, 0), radius: button.bounds.size.height, startAngle: 0.0, endAngle: CGFloat(M_PI), clockwise: true)
let circleShape = CAShapeLayer()
circleShape.path = circlePath.CGPath
button.layer.mask = circleShape

And you'll get this: