认识CoreAnimation

public class CALayer : NSObject, NSCoding, CAMediaTiming {

    public func presentationLayer() -> AnyObject?
    public func modelLayer() -> AnyObject

    public var bounds: CGRect
    public var position: CGPoint
    public var anchorPoint: CGPoint
    public var transform: CATransform3D
    public var frame: CGRect
    public var hidden: Bool

    public var superlayer: CALayer? { get }
    public func removeFromSuperlayer()
    public func addSublayer(layer: CALayer)
    public func insertSublayer(layer: CALayer, below sibling: CALayer?)
    public func insertSublayer(layer: CALayer, above sibling: CALayer?)
    public func replaceSublayer(layer: CALayer, with layer2: CALayer)
    public var sublayerTransform: CATransform3D

    public var mask: CALayer?
    public var masksToBounds: Bool

    public func hitTest(p: CGPoint) -> CALayer?
    public func containsPoint(p: CGPoint) -> Bool

    public var shadowColor: CGColor?
    public var shadowOpacity: Float
    public var shadowOffset: CGSize
    public var shadowRadius: CGFloat

    public var contents: AnyObject?
    public var contentsRect: CGRect

    public var cornerRadius: CGFloat
    public var borderWidth: CGFloat
    public var borderColor: CGColor?
    public var opacity: Float
}

class ViewController: UIViewController {

    let layer = CAShapeLayer()

    override func viewDidLoad() {
        super.viewDidLoad()
        layer.strokeEnd = 0
        layer.lineWidth = 6
        layer.fillColor = UIColor.clearColor().CGColor
        layer.strokeColor = UIColor.redColor().CGColor
        self.view.layer.addSublayer(layer)
    }

    @IBAction func actionToAnimate() {
        layer.path = UIBezierPath(arcCenter: self.view.center, radius: 100, startAngle: 0, endAngle: 2*CGFloat(M_PI), clockwise: true).CGPath
        layer.strokeEnd = 1
    }
}

@IBAction func actionToAnimate() {
    timer = NSTimer(timeInterval: 0.05, target: self, selector: selector(timerCallback), userInfo: nil, repeats: true)
    NSRunLoop.currentRunLoop().addTimer(timer!, forMode: NSRunLoopCommonModes)
    layer.path = UIBezierPath(arcCenter: self.view.center, radius: 100, startAngle: 0, endAngle: 2*CGFloat(M_PI), clockwise: true).CGPath
    layer.strokeEnd = 1
}

@objc private func timerCallback() {
    print("========================\nmodelLayer: \t\(layer.modelLayer().strokeEnd)\ntpresentationLayer: \t\(layer.presentationLayer()!.strokeEnd)")
    if fabs((layer.presentationLayer()?.strokeEnd)! - 1) < 0.01 {
        if let _ = timer {
            timer?.invalidate()
            timer = nil
        }
    }
}

========================
modelLayer: 	1.0
presentationLayer: 	0.294064253568649
========================
modelLayer: 	1.0
presentationLayer: 	0.676515340805054
========================
modelLayer: 	1.0
presentationLayer: 	0.883405208587646
========================
modelLayer: 	1.0
presentationLayer: 	0.974191427230835
========================
modelLayer: 	1.0
presentationLayer: 	0.999998211860657

public class CAAnimation : NSObject, NSCoding, NSCopying, CAMediaTiming, CAAction {

    public class func defaultValueForKey(key: String) -> AnyObject?
    public func shouldArchiveValueForKey(key: String) -> Bool

    public var timingFunction: CAMediaTimingFunction?

    public var delegate: AnyObject?

    public var removedOnCompletion: Bool
}

public protocol CAMediaTiming {
    public var beginTime: CFTimeInterval { get set }
    public var duration: CFTimeInterval { get set }
    public var speed: Float { get set }
    public var timeOffset: CFTimeInterval { get set }
    public var repeatCount: Float { get set }
    public var repeatDuration: CFTimeInterval { get set }
    public var autoreverses: Bool { get set }
    public var fillMode: String { get set }
}

public protocol CAAction {

    /* Called to trigger the event named 'path' on the receiver. The object
     * (e.g. the layer) on which the event happened is 'anObject'. The
     * arguments dictionary may be nil, if non-nil it carries parameters
     * associated with the event. */

    @available(iOS 2.0, *)
    public func runActionForKey(event: String, object anObject: AnyObject, arguments dict: [NSObject : AnyObject]?)
}

let animation = CABasicAnimation(keyPath: "position.y")
animation.toValue = NSNumber(float: 160)
layer.position.y = 160
layer.addAnimation(animation, forKey: nil)

func startAnimation() {
    let displayLink = CADisplayLink(target: self, selector: selector(fluctAnimation(_:)))
    displayLink.addToRunLoop(NSRunLoop.currentRunLoop(), forMode: NSRunLoopCommonModes)
    let move = CABasicAnimation(keyPath: "position.y")
    move.toValue = NSNumber(float: 160)
    leftLayer.position.y = 160
    rightLayer.position.y = 160
    leftLayer.addAnimation(move, forKey: nil)
    rightLayer.addAnimation(move, forKey: nil)

    let spring = CASpringAnimation(keyPath: "position.y")
    spring.damping = 15
    spring.initialVelocity = 40
    spring.toValue = NSNumber(float: 160)
    centerLayer.position.y = 160
    centerLayer.addAnimation(spring, forKey: "spring")
}

@objc private func fluctAnimation(link: CADisplayLink) {
    let path = UIBezierPath()
    path.moveToPoint(CGPointZero)

    guard let _ = centerLayer.animationForKey("spring") else {
        return
    }

    let offset = leftLayer.presentationLayer()!.position.y - centerLayer.presentationLayer()!.position.y
    var controlY: CGFloat = 160
    if offset < 0 {
        controlY = centerLayer.presentationLayer()!.position.y + 30
    } else if offset > 0 {
        controlY = centerLayer.presentationLayer()!.position.y - 30
    }

    path.addLineToPoint(leftLayer.presentationLayer()!.position)
    path.addQuadCurveToPoint(rightLayer.presentationLayer()!.position, controlPoint: CGPoint(x: centerLayer.position.x, y: controlY))
    path.addLineToPoint(CGPoint(x: UIScreen.mainScreen().bounds.width, y: 0))
    path.closePath()
    fluctLayer.path = path.CGPath
}

print("===========normal call===========")
print("\(self.view.actionForLayer(self.view.layer, forKey: "opacity"))")
UIView.animateWithDuration(0.25) {
    print("===========animate block call===========")
    print("\(self.view.actionForLayer(self.view.layer, forKey: "opacity"))")
}

===========normal call===========
Optional(<null>)
===========animate block call===========
Optional(<CABasicAnimation:0x7f8712d30c90; delegate = <UIViewAnimationState: 0x7f8712d304d0>; fillMode = both; timingFunction = easeInEaseOut; duration = 0.25; fromValue = 1; keyPath = opacity>)

class LXDActionLayer: CALayer {
    override func addAnimation(anim: CAAnimation, forKey key: String?) {
        print("***********************************************")
        print("Layer will add an animation: \(anim)")
        super.addAnimation(anim, forKey: key)
    }
}

class LXDActionView: UIView {

    override class func layerClass() -> AnyClass {
        return LXDActionLayer.classForCoder()
    }
}

override func viewDidLoad() {
    super.viewDidLoad()

    let actionView = LXDActionView()
    view.addSubview(actionView)
    print("===========normal call===========")
    print("\(self.view.actionForLayer(self.view.layer, forKey: "opacity"))")
    actionView.layer.opacity = 0.5
    UIView.animateWithDuration(0.25) {
        print("===========animate block call===========")
        print("\(self.view.actionForLayer(self.view.layer, forKey: "opacity"))")
    actionView.layer.opacity = 0
}

===========normal call===========
Optional(<null>)
===========animate block call===========
Optional(<CABasicAnimation:0x7f8f00eb1850; delegate = <UIViewAnimationState: 0x7f8f00eb0e90>; fillMode = both; timingFunction = easeInEaseOut; duration = 0.25; fromValue = 1; keyPath = opacity>)
***********************************************
Layer will add an animation: <CABasicAnimation: 0x7f8f00eb2400>