如何为自定义手势识别器实现velocityInView:？

我正在实施一个自定义UIGestureRecognizer子类。

我想实施velocityInView:同样的方式UIPanGestureRecognizer已经做到了。但我不确定如何去做。如何计算以点/秒为单位的速度？

Firstly,如果您使用 Swift，则需要创建一个桥接标头并#import <UIKit/UIGestureRecognizerSubclass.h>所以你可以覆盖UIGestureRegoniser的触摸开始/移动/取消/结束方法。如果你使用的是 Objective-C，只需这样写import.h 或 .m 文件中的语句。

第二，一旦你创建了你的UIGestureRecognizer子类中你需要这些变量：

private lazy var displayLink: CADisplayLink = {
    let link = CADisplayLink(target: self, selector: Selector("timerCalled"))
    link.addToRunLoop(NSRunLoop.currentRunLoop(), forMode: NSRunLoopCommonModes)
    return link
}()

private var previousTouchLocation: CGPoint?
private var currentTouchLocation : CGPoint?

private var previousTime: Double?
private var currentTime : Double?

显示链接用于更新currentTime and previousTime.

Thirdly,要继续设置所有内容，您需要覆盖以下方法，我认为这都是不言自明的：

override func touchesBegan(touches: Set<NSObject>!, withEvent event: UIEvent!) {
    self.state = .Began
    displayLink.paused = false
}

override func touchesMoved(touches: Set<NSObject>!, withEvent event: UIEvent!) {
    if let view = self.view,
        touch = touches.first as? UITouch {
        self.state = .Changed

        let newLocation = touch.locationInView(view)
        self.previousTouchLocation = self.currentTouchLocation
        self.currentTouchLocation = newLocation
    }
}

override func touchesEnded(touches: Set<NSObject>!, withEvent event: UIEvent!) {
    self.state = .Ended
    displayLink.paused = true
}

override func touchesCancelled(touches: Set<NSObject>!, withEvent event: UIEvent!) {
    self.state = .Cancelled
    displayLink.paused = true
}

第四，现在我们可以进行更有趣的部分 - 计算速度：

func velocityInView(targetView: UIView) -> CGPoint {
    var velocity = CGPoint.zeroPoint

    if let view = self.view,
        prevTouchLocation = self.previousTouchLocation,
        currTouchLocation = self.currentTouchLocation,
        previousTime = self.previousTime,
        currentTime  = self.currentTime {

        let targetPrevLocation = view.convertPoint(prevTouchLocation, toView: targetView)
        let targetCurrLocation = view.convertPoint(currTouchLocation, toView: targetView)

        let deltaTime = CGFloat(currentTime - previousTime)

        let velocityX = (targetCurrLocation.x - targetPrevLocation.x) / deltaTime
        let velocityY = (targetCurrLocation.y - targetPrevLocation.y) / deltaTime

        velocity = CGPoint(x: velocityX, y: velocityY)
    }

    return velocity
}

使用以下公式计算速度：

velocity = deltaDistance / deltaTime

在这种情况下，采用速度的每个分量（x 和 y）并使用该方程计算每个轴上的速度。如果你想结合速度的两个分量，你可以使用毕达哥拉斯定理，如下所示：

let distance = hypot(targetCurrLocation.x - targetPrevLocation.x, 
                     targetCurrLocation.y - targetPrevLocation.y)
let velocity = distance / deltaTime

Fifthly,在您的视图控制器中，您现在可以添加手势识别器并使用action获取在屏幕上拖动时的速度：

override func viewDidLoad() {
    super.viewDidLoad()

    let recognizer = VelocityGestureRecognizer(target: self, action: Selector("movedGesture:"))
    self.view.addGestureRecognizer(recognizer)
}

func movedGesture(recognizer: VelocityGestureRecognizer) {
    let v = recognizer.velocityInView(self.view)
    println("velocity = \(v)")
}