Why doesn't my snow fall to the ground? - ios

Question

I am trying to make a application (in swift) where snow falls in the background. The only problem is I have added the gravity animation, however the snow just stays where it is.
Here is my code:
ViewController.swift
import UIKit
#IBDesignable
class ViewController: UIViewController {
#IBInspectable var BgColor:UIColor = UIColor.whiteColor()
override func viewDidLoad() {
super.viewDidLoad()
self.view.backgroundColor = BgColor
/*listSubviewsOfView(self.view)*/ /*Not needed to answer this*/
var snow = Snow(frame: CGRect(x: 0, y: 0, width: 5, height: 5))
snow.opaque = false
self.view.addSubview(snow)
let animator = UIDynamicAnimator(referenceView: self.view)
let gravity = UIGravityBehavior(items: [snow])
let direction = CGVectorMake(0.0, 1.0)
gravity.gravityDirection = direction
animator.addBehavior(gravity)
/*
var snow = Snow(frame: CGRect(x: 0, y: 0, width: 5, height: 5))
snow.opaque = false
snow.viewHeight = UIScreen.mainScreen().bounds.height
snow.addSubview(snow)
let animator = UIDynamicAnimator(referenceView: view)
let gravity = UIGravityBehavior(items: [snow])
animator.addBehavior(gravity)
*/
}
/*Not needed to solve this*/
/*
func listSubviewsOfView(views: UIView) {
var index = 0
let randomNumbers = [Int](1...24).shuffle()
for view in views.subviews
{
if let _ = view.restorationIdentifier
{
view.setValue(String(Int(randomNumbers[index])), forKey: "updateText")
index++
}
if index == randomNumbers.count {
break
}
}
}
*/
}
Snow.swift:
import Foundation
import UIKit
#IBDesignable
class Snow:UIView
{
var viewHeight = CGFloat(0)
/*
required init(coder aDecoder: NSCoder) {
//Initilse UIView
super.init(coder: aDecoder)!
}
*/
override func drawRect(rect: CGRect) {
let path = UIBezierPath(ovalInRect: rect)
UIColor.whiteColor().setFill()
path.fill()
}
}
The question is, why does my snow stay on top of the screen at (0,0) and not fall down even though I have told it to have the gravity affect?

Answer 1

Make animator and gravity properties of your view controller.
import UIKit
#IBDesignable
class ViewController: UIViewController {
#IBInspectable var BgColor:UIColor = UIColor.whiteColor()
var animator: UIDynamicAnimator? = nil;
let gravity = UIGravityBehavior()
override func viewDidLoad() {
super.viewDidLoad()
self.view.backgroundColor = UIColor.blackColor()
let snow = Snow(frame: CGRect(x: 0, y: 0, width: 5, height: 5))
snow.opaque = false
self.view.addSubview(snow)
animator = UIDynamicAnimator(referenceView:self.view);
animator?.addBehavior(gravity)
gravity.addItem(snow)
let direction = CGVectorMake(0.0, 1.0)
gravity.gravityDirection = direction
}
}