swift - iOS: Why does the key of animation matter? -

i'm trying implement animation move view position tap. each tap cancel previous animation , start moving again current position.

class moveanimationviewcontroller: uiviewcontroller {     lazy var block: uiview = {         let block = uiview()         block.frame = cgrect(x: 100, y: 100, width: 100, height: 100)         block.backgroundcolor = uicolor.greencolor()         return block     }()      override func viewdidload() {         super.viewdidload()         view.addsubview(block)         view.addgesturerecognizer(uitapgesturerecognizer(target: self, action: "tap:"))     }      func tap(gesture: uitapgesturerecognizer) {         let fromposition = (block.layer.presentationlayer() ?? block.layer).position         let topostion = gesture.locationinview(view)          block.layer.removeallanimations()          let animation = cabasicanimation(keypath: "position")         animation.duration = 2         animation.fromvalue = nsvalue(cgpoint: fromposition)         animation.tovalue = nsvalue(cgpoint: topostion)         block.layer.addanimation(animation, forkey: nil)          block.layer.position = topostion     } } 

however, block view jumps destination directly without animation.

replace following code

block.layer.addanimation(animation, forkey: nil) 

with

block.layer.addanimation(animation, forkey: "move") 

will fix problem, why?

---

by way, following statement looks illegal, since presentationlayer method returns anyobject?, has no position property. right?

let fromposition = (block.layer.presentationlayer() ?? block.layer).position 

it should replaced this, guess. compiler doesn't warn me. bug?

let fromposition = (block.layer.presentationlayer() as? calayer ?? block.layer).position 

if name animation, i.e. associate key it, replace other animations same key. if give no name automatic cancel , replace doesn't happen.