无限旋转 UIView 并暂停/停止动画，保持状态，并从当前状态继续旋转

Question

我很难尝试暂停并从当前的旋转状态继续任何 UIView 旋转技术。

试过了：

CGAffineTransformRotate
CGAffineTransformMakeRotation
CATransform3DMakeRotation
CABasicAnimation

基本上，如果我调用view.layer.removeAllAnimations() 或layer.speed = 0，它们都会重置当前的旋转度数。

还尝试对视图进行快照以使用图像而不是真正的旋转，但没有运气，因为快照忽略了旋转。

Answer 1

终于明白了，关于 SO 的答案不止一个，很多在 Objective-c 中，将它们全部放在一个 UIView 扩展中，甚至记录在案：

extension UIView {

        /**
         Will rotate `self` for ever.

         - Parameter duration: The duration in seconds of a complete rotation (360º).
         - Parameter clockwise: If false, will rotate counter-clockwise.
         */
        func startRotating(duration duration: Double, clockwise: Bool) {
            let kAnimationKey = "rotation"
            var currentState = CGFloat(0)

            // Get current state
            if let presentationLayer = layer.presentationLayer(), zValue = presentationLayer.valueForKeyPath("transform.rotation.z"){
                currentState = CGFloat(zValue.floatValue)
            }

            if self.layer.animationForKey(kAnimationKey) == nil {
                let animate = CABasicAnimation(keyPath: "transform.rotation")
                animate.duration = duration
                animate.repeatCount = Float.infinity
                animate.fromValue = currentState //Should the value be nil, will start from 0 a.k.a. "the beginning".
                animate.byValue = clockwise ? Float(M_PI * 2.0) : -Float(M_PI * 2.0)
                self.layer.addAnimation(animate, forKey: kAnimationKey)
            }
        }

        /// Will stop a `startRotating(duration: _, clockwise: _)` instance.
        func stopRotating() {
            let kAnimationKey = "rotation"
            var currentState = CGFloat(0)

            // Get current state
            if let presentationLayer = layer.presentationLayer(), zValue = presentationLayer.valueForKeyPath("transform.rotation.z"){
                currentState = CGFloat(zValue.floatValue)
            }

            if self.layer.animationForKey(kAnimationKey) != nil {
                self.layer.removeAnimationForKey(kAnimationKey)
            }

            // Leave self as it was when stopped.
            layer.transform = CATransform3DMakeRotation(currentState, 0, 0, 1)
        }

    }

像yourView.startRotating(duration: 1, clockwise: true)一样使用它，稍后停止使用yourView.stopRotating()。