4.swift

import UIKit

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 = 100 //直徑
        
        for i in stride(from: 0, to: 180, by: 10){
            
            // 弧度 = 度 * PI / 18-
            let 弧度 = Double(i) * Double.pi / 180
            
            let x = radius * cos(弧度)
            let y = radius * sin(弧度)
            畫筆.addLine(to: CGPoint(x: x, y: y))
        }
        
        畫筆.stroke()
    }

}

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

view.backgroundColor = UIColor.white