ViewController.swift

import UIKit

class ViewController: UIViewController, UICollisionBehaviorDelegate {
    
    
    let data = ["Getting sstartw with Swift","SiriKit Basic","SprieKit for Apple Watch"]
    
    
    var views = [UIView]()
    var animator:UIDynamicAnimator!
    var gravity:UIGravityBehavior!
    var snap:UISnapBehavior!
    var previousTouchPoint:CGPoint!
    var viewDragging = false
    var viewPinned = false
    
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        
        animator = UIDynamicAnimator(referenceView: self.view)
        gravity = UIGravityBehavior()
        
        
        animator.addBehavior(gravity)
        gravity.magnitude = 4
        
        var offset:CGFloat = 250
        
        for i in 0 ... data.count - 1 {
            if let view = addViewController(atOffset: offset, dataForVC: data[i]){
                views.append(view)
                offset -= 50
            }
        }
        
    }
    
    //optional return
    func addViewController (atOffset offset:CGFloat, dataForVC data:AnyObject?) -> UIView? {
        
        
        let frameForView = self.view.bounds.offsetBy(dx: 0, dy: self.view.bounds.size.height - offset)
        let sb = UIStoryboard(name: "Main", bundle: nil)
        let stackElementVC = sb.instantiateViewControllerWithIdentifier("StackElement") as! SEVC
        
        
        if let view = stackElementVC.view {
            view.frame = frameForView
            
            view.layer.cornerRadius = 5
            view.layer.shadowOffset = CGSizeMake(2, 2)
            view.layer.shadowColor = UIColor.blackColor().CGColor
            view.layer.shadowRadius = 3
            view.layer.shadowOpacity = 0.5
            
            
            //adding some data to our controller
            
            if let headerStr = data as? String {
                stackElementVC.headerString = headerStr
            }
            
            
            
            self.addChildViewController(stackElementVC)
            self.view.addSubview(view)
            
            
            stackElementVC.didMoveToParentViewController(self)
            
            
            //pan gesture recognizer
            
            
            //let pGr = UIPanGestureRecognizer(target: self, action: #selector(ViewController.handle)
            
            
            let panGestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(ViewController.handlePan(_:)))
            
            view.addGestureRecognizer(panGestureRecognizer)
            
            
            let collision = UICollisionBehavior(items: [view])
            //the following line is used to create the snappy smooth animation effect
            collision.collisionDelegate = self
            animator.addBehavior(collision)
            
            //defining a boundary
            let boundary = view.frame.origin.y + view.frame.size.height
            
            
            // lower boundary
            var boundaryStart = CGPoint(x: 0, y: boundary)
            var boundaryEnd = CGPoint(x: self.view.bounds.size.width, y: boundary)
            collision.addBoundaryWithIdentifier(1, fromPoint: boundaryStart, toPoint: boundaryEnd)
            
            
            // upper boundary
            boundaryStart = CGPoint(x: 0, y: 0)
            boundaryEnd = CGPoint(x: self.view.bounds.size.width, y: 0)
            collision.addBoundaryWithIdentifier(2, fromPoint: boundaryStart, toPoint: boundaryEnd)
            
            
            gravity.addItem(view)
            
            
            let itemBehavior = UIDynamicItemBehavior(items: [view])
            animator.addBehavior(itemBehavior)
            
            return view
            
            
            
            
            
            
            
        }
        
        return nil
        
    }
    
    func handlePan (gestureRecognizer:UIPanGestureRecognizer){
        
        //getting the toucpoint -> where did our user click in hte screen
        
       // let touchPoint = gestureRecognizer.locationOfTouch(in:, inView: self.view)
        //let draggedView = gestureRecognizer.view!
        
        let touchPoint = gestureRecognizer.locationInView(self.view)
        let draggedView = gestureRecognizer.view!
        
        if gestureRecognizer.state == .Began {
            let dragStartPoint = gestureRecognizer.locationInView(draggedView)
            
            // the y value here is fully depended on your scaling, meaning you can readjust this to your desire
            if dragStartPoint.y < 200 {
                viewDragging = true
                previousTouchPoint = touchPoint
            }
        } else if gestureRecognizer.state == .Changed && viewDragging {
            let yOffset = previousTouchPoint.y - touchPoint.y
            
            draggedView.center = CGPoint(x: draggedView.center.x, y:draggedView.center.y - yOffset)
            previousTouchPoint = touchPoint
            
        } else if gestureRecognizer.state == .Ended && viewDragging {
            
            

            pin(draggedView)
            addVelocity(toView: draggedView, fromGestureRecgonizer: gestureRecognizer)
            animator.updateItemUsingCurrentState(draggedView)
            viewDragging = false
        }
        
    }
    
        
        //determinor
        func pin (view:UIView){
        let viewHasReachedPinLocation = view.frame.origin.y < 100
            
            if viewHasReachedPinLocation {
                if !viewPinned {
                    //creating the snappy behavior
                    var snapPosition = self.view.center
                    snapPosition.y += 30
                    
                    snap = UISnapBehavior(item: view, snapToPoint: snapPosition)
                    animator.addBehavior(snap)
                  
                    
                    setVisibility(view, alpha: 0)
                      viewPinned = true
                    
                }
            } else {
                if viewPinned {
                    animator.removeBehavior(snap)
                    setVisibility(view, alpha: 1)
                    viewPinned = false
                }
            }
        }
        
        
        
        
        func setVisibility(view:UIView, alpha:CGFloat){
            for aView in views {
                if aView != view {
                    aView.alpha = alpha
                }
            }
        }
        
        //detects if one of the items is thrown, and attempt to apply semi-realistic physics to it
        func addVelocity(toView view:UIView, fromGestureRecgonizer panGesture:UIPanGestureRecognizer){
            var velocity = panGesture.velocityInView(self.view)
            velocity.x = 0
            
            if let behavior = itemBehavior(forview: view){
                behavior.addLinearVelocity(velocity, forItem: view)
            }
            
            
            
            
        }
        // this function will iterate until it finds the perfect UIDYNBHV, then apply the linaer velocity
        func itemBehavior (forview view:UIView) ->UIDynamicItemBehavior? {
            
            
            
            for behavoir in animator.behaviors {
                
                
                
                
                if let itemBehavior = behavoir as? UIDynamicItemBehavior {
                    
                    
                    
                    if let possibleView = itemBehavior.items.first as? UIView where possibleView == view {
                        return itemBehavior
                    }
                    
                
                }
            }
            
            
            return nil
            
            
        }
        
//        
//      func  collisionBehavior(UICollisionBehavior, beganContactForItem: UIDynamicItem, withBoundaryIdentifier: NSCopying?, atPoint: CGPoint){
//        
//        if NSNumber(IntegerLiteralType: 2).isEqual(identifer){
//            
//        }
//        
//        }
    
    
    func collisionBehavior(behavior: UICollisionBehavior, beganContactForItem item: UIDynamicItem, withBoundaryIdentifier identifier: NSCopying?, atPoint p: CGPoint) {
        if NSNumber(integerLiteral: 2).isEqual(identifier){
            let view = item as! UIView
            pin(view)
        }
    }
    
    }