我正在实现一个自定义的 UIGestureRecognizer 子类。

我想以与 UIPanGestureRecognizer 相同的方式实现 velocityInView:。但我不确定该怎么做。如何计算以点/秒为单位的速度？

最佳答案

首先，如果您使用的是 Swift，则需要创建桥接 header 和 #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。如果您使用的是 Objective-C，只需输入 import .h 或 .m 文件中的语句。

其次，一旦您创建了 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和 previousTime .

第三，要继续设置您需要覆盖以下方法的所有内容，我认为这一切都是不言自明的:

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，现在我们可以进入更有趣的部分 - 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
}

速度使用以下公式计算:

velocity = deltaDistance / deltaTime

在这种情况下，获取速度的每个分量(x 和 y)并使用该方程式计算每个轴上的速度。如果您想结合速度的两个分量，您可以像这样使用毕达哥拉斯定理:

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

第五，在您的 View Controller 中，您现在可以添加手势识别器并使用 action在屏幕上拖动时获得速度:

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)")
}

关于ios - 如何实现 velocityInView : for a custom gesture recognizer?，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28552408/