如何在将UIKit Dynamics的重力应用于UIView时更新UIView的框架？

Question

I'm experiencing UIKit Dynamics right now and here is what I would like to do :

1. "Drop" multiple UIView instances from the top of the screen to the bottom using gravity => OK
2. Trigger an event when each view's distance from the bottom is like 100px. => NOT OK :(

Here is what I tried :

1. Add on observer on the "frame" property for each view => I don't know why but observeValueForKeyPath is never called (see code below)...
2. Add a transparent collision boundary to each view and listen to UICollisionBehaviorDelegate => This kinda works but it is not the behavior I want. Because of course the view is "stopped" by the boundary and I want it to go down

This is my test code :

var animator: UIDynamicAnimator!
var gravity: UIGravityBehavior!
override func viewDidAppear(animated: Bool) {
    super.viewDidAppear(animated)

    self.animator = UIDynamicAnimator(referenceView: view)
    self.gravity = UIGravityBehavior()
    self.animator.addBehavior(gravity)

    let timer = NSTimer.scheduledTimerWithTimeInterval(2.0, target: self, selector: Selector("createView"), userInfo: nil, repeats: true)
}

func createView() {
    var newView = UIView(frame: CGRectMake((UIScreen.mainScreen().bounds.size.width/2) - 20, -40, 40, 40))
    self.view.addSubview(newView)

    self.gravity.addItem(newView)

    newView.addObserver(self, forKeyPath: "frame", options: NSKeyValueObservingOptions.New, context: nil)
}

override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
    if keyPath == "frame" {
        if let newView = object as? UIView {
            println(newView.frame.origin.y)
            if (newView.frame.origin.y > UIScreen.mainScreen().bounds.size.height - 100) {
                println("Trigger event !")
            }
        }
    }
}

With this code, the views will get down correctly but no event will be triggered when it the views are 100px far from the bottom

I would greatly appreciate any help

Answer 1

A way to do it is by adding an UICollisionBehavior, creating a view position 100px above the bottom and define an action which would be called when your items collide with this.

I can't create a sample for you now, but http://www.raywenderlich.com/50197/uikit-dynamics-tutorial will be a great help, look for a section called "Invisible boundaries and collisions". It's objective-C, though, but that shouldn't be a problem as this is pretty easy.

Answer 2

I've been trying to make this work too, really there should be a delegate callback from UIDynamicAnimator after it updates frames.

I've managed to solve it like this:

class FrameReportingView: UIView {

    override var center: CGPoint {
        didSet{
            print("center: \(center)")
        }
    }
}

I'm using a UIPushBehavior and the center is set each frame. You can implement a delegate on FrameReportingView to create a callback after the center is changed.