Swift 3 update (playground oriented programming)

touch.swift

import Cocoa
import XCPlayground
import PlaygroundSupport

class TouchView: NSView {
    var (path, currentPath) = (NSBezierPath(), NSBezierPath())
    
    override func draw(_ dirtyRect: NSRect) {
        guard let contextPtr = NSGraphicsContext.current()?.graphicsPort else {return}
        let context = unsafeBitCast(contextPtr, to: CGContext.self)
        context.clear(dirtyRect)
        path.stroke()
        currentPath.lineWidth = 2.0
        currentPath.stroke()
    }
    override func mouseDown(with event: NSEvent) {
        currentPath = NSBezierPath()
        currentPath.move(to: event.locationInWindow)
    }
    override func mouseDragged(with event: NSEvent) {
        currentPath.line(to: event.locationInWindow)
        needsDisplay = true
    }
    
    override func mouseUp(with event: NSEvent) {
        path.append(currentPath)
        currentPath = NSBezierPath()
        needsDisplay = true
    }
}

let touchView: TouchView = {
    $0.wantsLayer = true
    $0.layer?.backgroundColor = NSColor.white.cgColor
    return $0
}(TouchView(frame: NSRect(x: 0, y: 0, width: 300, height: 200)))

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = touchView

playground oriented programming is definitely the future.