bottomView.backgroundColor = UIColor.white
let topLayer = CAShapeLayer()
let rd = 400.0 as CGFloat;
let pathRadius = UIBezierPath(arcCenter: CGPoint(x:UIScreen.main.bounds.width / 2,y:rd),
radius: rd,
startAngle: 0,
endAngle: CGFloat.pi,
clockwise: false)
topLayer.path = pathRadius.cgPath
bottomView.layer.mask = topLayer
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