iOS绘制弧度有两个函数，一个设置中心点另外一个是设置点到点，
<pre><code>` public func addArc(center: CGPoint, radius: CGFloat, startAngle: CGFloat, endAngle: CGFloat, clockwise: Bool)

public func addArc(tangent1End: CGPoint, tangent2End: CGPoint, radius: CGFloat)`</code></pre>

我们设置起点弧度为0，终点弧度为1.5 * PI（等于270角度),最后的clockwise的设置为true.
<pre><code>` UIGraphicsBeginImageContext(self.view.bounds.size);

    let context:CGContext = UIGraphicsGetCurrentContext()!
    
    let path:CGMutablePath = CGMutablePath()
  
    context.addArc(center:CGPoint(x: 150, y: 150), radius: 50, startAngle: 0, endAngle: 1.5*CGFloat(M_PI), clockwise: true)
    
    context.move(to: CGPoint(x: 150, y: 100))
    context.addLine(to: CGPoint(x: 150, y: 150))
    context.addLine(to: CGPoint(x: 200, y: 150))
    
    context.addPath(path)
    
    UIColor.gray.setFill()
    UIColor.red.setStroke()
    
    context.drawPath(using: .fillStroke)
    
    let image:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    
    let imgView:UIImageView = UIImageView.init(image: image)
    self.view.addSubview(imgView)`</code></pre>

效果图:

FlyElephant.png

顺时针设为false:
<pre><code>context.addArc(center:CGPoint(x: 150, y: 150), radius: 50, startAngle: 0, endAngle: 1.5*CGFloat(M_PI), clockwise: false)</code></pre>
效果图:

FlyElephant.png

圆角矩形

通过线和线之间的相切，我们也可以获取圆角，代码如下:
<pre><code>` UIGraphicsBeginImageContext(self.view.bounds.size);

    let context:CGContext = UIGraphicsGetCurrentContext()!
    
    let path:CGMutablePath = CGMutablePath()
    
    context.move(to: CGPoint(x: 100, y: 300))
    
    context.addArc(tangent1End: CGPoint(x: 200, y: 300), tangent2End: CGPoint(x: 200, y: 400), radius: 5.0)
    context.addArc(tangent1End: CGPoint(x: 200, y: 400), tangent2End: CGPoint(x: 100, y: 400), radius: 5.0)
    
    context.addArc(tangent1End: CGPoint(x: 100, y: 400), tangent2End: CGPoint(x: 100, y: 300), radius: 5.0)
    context.addArc(tangent1End: CGPoint(x: 100, y: 300), tangent2End: CGPoint(x: 200, y: 300), radius: 5.0)
    
    context.addPath(path)
    
    UIColor.gray.setFill()
    UIColor.red.setStroke()
    
    context.drawPath(using: .fillStroke)
    
    let image:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    
    let imgView:UIImageView = UIImageView.init(image: image)
    self.view.addSubview(imgView)`</code></pre>

效果图如下:

FlyElephant.png