使用Swift 3.0实时绘制直线

import UIKit

class DrawingView: UIView {
    var startPoint: CGPoint?
    var endPoint: CGPoint?
    
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        
        guard let startPoint = startPoint, let endPoint = endPoint else {
            return
        }
        
        let path = UIBezierPath()
        path.move(to: startPoint)
        path.addLine(to: endPoint)
        
        UIColor.black.setStroke()
        path.lineWidth = 2.0
        path.stroke()
    }
}

// 在ViewController中使用DrawingView来实时绘制直线
class ViewController: UIViewController {
    var drawingView: DrawingView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        drawingView = DrawingView(frame: view.bounds)
        view.addSubview(drawingView)
    }
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        if let touch = touches.first {
            let startPoint = touch.location(in: drawingView)
            drawingView.startPoint = startPoint
        }
    }
    
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        if let touch = touches.first {
            let currentPoint = touch.location(in: drawingView)
            drawingView.endPoint = currentPoint
            drawingView.setNeedsDisplay()
        }
    }
    
    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
        if let touch = touches.first {
            let endPoint = touch.location(in: drawingView)
            drawingView.endPoint = endPoint
            drawingView.setNeedsDisplay()
        }
    }
}