6.swift

class CircleView : UIView {
    
    override func draw(_ rect: CGRect) {
        let 畫筆 = UIBezierPath()
        畫筆.move(to: .zero)
//        畫筆.addLine(to: CGPoint(x: 100, y: 100))
        
        
        // x^2 + y^2 = r^2
        
        // cos(θ) = x/r ==> x = r * cos(θ)
        // sin(θ) = y/r == y = r * sin(θ)
        
        let radius:Double = Double(rect.width) / 2 - 20 //直徑
        let 圓點 = CGPoint(x: rect.width / 2, y: rect.height / 2)
        畫筆.move(to: CGPoint(x: 圓點.x + CGFloat(radius), y: 圓點.y))
        for i in stride(from: 0, to: 361, by: 1) {
            
            // 弧度 = 度 * PI / 180
            let 弧度 = Double(i) * Double.pi / 180
            let x = Double(圓點.x) + radius * cos(弧度)
            let y = Double(圓點.y) + radius * sin(弧度)
            畫筆.addLine(to: CGPoint(x: x, y: y))
        }
        UIColor.blue.setStroke()
        畫筆.lineWidth = 5
        畫筆.stroke()
    }

}

let view = CircleView(frame: CGRect(x: 0, y: 0, width: 300, height: 300))
view.backgroundColor = UIColor.yellow