Parallax table view helper

Parallax for a table view

//
//  ParallaxController.swift
//  ParallaxTableVIew
//
//  Created by Avner on 5/2/15.
//  Copyright (c) 2015 Avner. All rights reserved.
//

import UIKit
@objc protocol ParallaxControllerDelegate {
    /**
    Percent of parallax view under only the nav bar
    
    :param: controller             the controller
    :param: intersectNavBarPercent The percent under - calculated by (max Y point of parallax) - height of nav bar iff max Y < navbar height
    */
    optional func parallaxController(controller : ParallaxController,intersectNavBarPercent : CGFloat)
}

class ParallaxController: NSObject {
    weak var tableView : UITableView?
    weak var parallaxView : UIView?
    weak var viewController : UIViewController?
    weak var delegate : ParallaxControllerDelegate?
    var navigationBarHeight : CGFloat?
    var parallaxHeight : CGFloat?
    var view : UIView? {
        get {
            return viewController?.view
        }
    }
    init(parentViewController : UIViewController, tableView : UITableView,parallaxView : UIView,parallaxHeight : CGFloat, navigationBarHeight : CGFloat) {
        super.init()
        self.viewController = parentViewController
        self.tableView = tableView
        self.parallaxView = parallaxView
        self.parallaxHeight = parallaxHeight
        self.navigationBarHeight = navigationBarHeight
        self.tableView?.addObserver(self, forKeyPath: "contentOffset", options: NSKeyValueObservingOptions.New, context: nil)
    }
    
    override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
        
        if let tableView = self.tableView, parallaxHeight = self.parallaxHeight, navigationBarHeight = self.navigationBarHeight, parallaxView = self.parallaxView , view = self.view {
            if tableView.contentOffset.y > 0 {
                var newOriginY = ceil(max(navigationBarHeight + parallaxHeight - tableView.contentOffset.y, navigationBarHeight))
                tableView.frame = CGRectMake(0, newOriginY, view.bounds.size.width, view.bounds.size.height - newOriginY)
                parallaxView.frame = CGRectMake(0, floor(newOriginY - parallaxView.frame.height)-1, parallaxView.frame.width, parallaxView.frame.height)
            } else {
                parallaxView.frame = CGRectMake(0, 0, parallaxView.frame.width, tableView.frame.origin.y + ceil(abs(tableView.contentOffset.y)))
            }
            // pixel fraction fix
            if parallaxView.frame.origin.y > 0 {
                parallaxView.frame.origin.y = 0
            }
            
            // get an indication of how much of the parallax is under only the nav bar
            var virtOriginY = CGRectGetMaxY(parallaxView.keyWindowFrame!) - navigationBarHeight
            if virtOriginY < navigationBarHeight {
                var percent =  (navigationBarHeight - virtOriginY)/navigationBarHeight
                self.delegate?.parallaxController?(self, intersectNavBarPercent: percent)
            } else {
                self.delegate?.parallaxController?(self, intersectNavBarPercent: 0)
            }

        }
    }
    deinit {
        self.tableView?.removeObserver(self, forKeyPath: "contentOffset")
    }

}

extension UIView {
    var keyWindowFrame : CGRect? {
        get {
            return UIApplication.sharedApplication().keyWindow?.convertRect(self.frame, fromView: self.superview)
        }
    }
}