struct Point {
        var x:Double = 0
        var y:Double = 0
    }

func angle(a:Point, b:Point) -> Double {
        var r = atan2(b.y - a.y, b.x - a.x)
        if r < 0 {
            r = r + 2 * M_PI
        }
        return floor(r * 360 / (2 * M_PI))
    }

    //　ドラッグ時に呼ばれる
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        //タッチイベントを感知
        let touchEvent = touches.first!
        //ドラッグ前の座標
        let preDx = touchEvent.previousLocation(in: self.view).x
        let preDy = touchEvent.previousLocation(in: self.view).y
        //ドラッグ後の座標
        let newDx = touchEvent.location(in: self.view).x
        let newDy = touchEvent.location(in: self.view).y

        //ドラッグ後の座標からドラッグ前の座標を引けば現在の座標の位置が割り出せる
        let dx = newDx - preDx
        let dy = newDy - preDy

        //引数に現在の座標を入れてさっきの関数を呼ぶ
        print(angle(a: Point(x: 0, y: 0), b: Point(x: Double(dx), y: Double(dy))))
    }

import UIKit

class ViewController: UIViewController {


    override func viewDidLoad() {
        super.viewDidLoad()

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    struct Point {
        var x:Double = 0
        var y:Double = 0
    }

    func angle(a:Point, b:Point) -> Double {
        var r = atan2(b.y - a.y, b.x - a.x)
        if r < 0 {
            r = r + 2 * M_PI
        }
        return floor(r * 360 / (2 * M_PI))
    }

    //　ドラッグ時に呼ばれる
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {

        let touchEvent = touches.first!
        let preDx = touchEvent.previousLocation(in: self.view).x
        let preDy = touchEvent.previousLocation(in: self.view).y

        let newDx = touchEvent.location(in: self.view).x
        let newDy = touchEvent.location(in: self.view).y

        let dx = newDx - preDx
        let dy = newDy - preDy

        print(angle(a: Point(x: 0, y: 0), b: Point(x: Double(dx), y: Double(dy))))


    }

}