UIKit Dynamics

label.frame = CGRect(x: 0.0, y: 0.0, width: label.intrinsicContentSize.width, height: label.intrinsicContentSize.height)
dynamicAnimator.updateItem(usingCurrentState: label)

self.label.frame = CGRectMake(0.0, 0.0, self.label.intrinsicContentSize.width, self.label.intrinsicContentSize.height);
[self.dynamicAnimator updateItemUsingCurrentState: self.label];

@IBOutlet var animationView: UIView!
var squareView:UIView!
var collision: UICollisionBehavior!
var animator: UIDynamicAnimator!
var gravity: UIGravityBehavior!

override func viewDidLoad() {
    super.viewDidLoad()
    let squareSize = CGSize(width: 30.0, height: 30.0)
    let centerPoint = CGPoint(x: self.animationView.bounds.midX - (squareSize.width/2), y: self.animationView.bounds.midY - (squareSize.height/2))
    let frame = CGRect(origin: centerPoint, size: squareSize)
    squareView = UIView(frame: frame)
    squareView.backgroundColor = UIColor.orangeColor()
    animationView.addSubview(squareView)
    animator = UIDynamicAnimator(referenceView: view)
    gravity = UIGravityBehavior(items: [squareView])
    animator.addBehavior(gravity)
    collision = UICollisionBehavior(items: [square])
    collision.translatesReferenceBoundsIntoBoundary = true
    animator.addBehavior(collision)
}

class ViewController: UIViewController
{
    // Adjust to change speed of view from flick
    let magnitudeMultiplier: CGFloat = 0.0008
    
    lazy var dynamicAnimator: UIDynamicAnimator =
    {
        let dynamicAnimator = UIDynamicAnimator(referenceView: self.view)
        return dynamicAnimator
    }()

    lazy var gravity: UIGravityBehavior =
    {
        let gravity = UIGravityBehavior(items: [self.orangeView])
        return gravity
    }()
    
    lazy var collision: UICollisionBehavior =
    {
        let collision = UICollisionBehavior(items: [self.orangeView])
        collision.translatesReferenceBoundsIntoBoundary = true
        return collision
    }()
    
    lazy var orangeView: UIView =
    {
        let widthHeight: CGFloat = 40.0
        let orangeView = UIView(frame: CGRect(x: 0.0, y: 0.0, width: widthHeight, height: widthHeight))
        orangeView.backgroundColor = UIColor.orange
        self.view.addSubview(orangeView)
        return orangeView
    }()
    
    lazy var panGesture: UIPanGestureRecognizer =
    {
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(self.handlePan(sender:)))
        return panGesture
    }()
    
    lazy var attachment: UIAttachmentBehavior =
    {
        let attachment = UIAttachmentBehavior(item: self.orangeView, attachedToAnchor: .zero)
        return attachment
    }()

    override func viewDidLoad()
    {
        super.viewDidLoad()
        dynamicAnimator.addBehavior(gravity)
        dynamicAnimator.addBehavior(collision)
        orangeView.addGestureRecognizer(panGesture)
    }
    
    override func viewDidLayoutSubviews()
    {
        super.viewDidLayoutSubviews()
        orangeView.center = view.center
        dynamicAnimator.updateItem(usingCurrentState: orangeView)
    }
    
    func handlePan(sender: UIPanGestureRecognizer)
    {
        let location = sender.location(in: view)
        let velocity = sender.velocity(in: view)
        let magnitude = sqrt((velocity.x * velocity.x) + (velocity.y * velocity.y))
        switch sender.state
        {
        case .began:
            attachment.anchorPoint = location
            dynamicAnimator.addBehavior(attachment)
        case .changed:
            attachment.anchorPoint = location
        case .cancelled, .ended, .failed, .possible:
            let push = UIPushBehavior(items: [self.orangeView], mode: .instantaneous)
            push.pushDirection = CGVector(dx: velocity.x, dy: velocity.y)
            push.magnitude = magnitude * magnitudeMultiplier
            dynamicAnimator.removeBehavior(attachment)
            dynamicAnimator.addBehavior(push)
        }
    }
}

@interface ViewController ()

@property (nonatomic, assign) CGFloat magnitudeMultiplier;
@property (nonatomic, strong) UIDynamicAnimator *dynamicAnimator;
@property (nonatomic, strong) UIGravityBehavior *gravity;
@property (nonatomic, strong) UICollisionBehavior *collision;
@property (nonatomic, strong) UIView *orangeView;
@property (nonatomic, strong) UIPanGestureRecognizer *panGesture;
@property (nonatomic, strong) UIAttachmentBehavior *attachment;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self.dynamicAnimator addBehavior:self.gravity];
    [self.dynamicAnimator addBehavior:self.collision];
    [self.orangeView addGestureRecognizer:self.panGesture];
    // Adjust to change speed of view from flick
    self.magnitudeMultiplier = 0.0008f;
}

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    self.orangeView.center = self.view.center;
    [self.dynamicAnimator updateItemUsingCurrentState:self.orangeView];
}

- (void)handlePan:(UIPanGestureRecognizer *)sender
{
    CGPoint location = [sender locationInView:self.view];
    CGPoint velocity = [sender velocityInView:self.view];
    CGFloat magnitude = sqrt((velocity.x * velocity.x) + (velocity.y * velocity.y));
    if (sender.state == UIGestureRecognizerStateBegan)
    {
        self.attachment.anchorPoint = location;
        [self.dynamicAnimator addBehavior:self.attachment];
    }
    else if (sender.state == UIGestureRecognizerStateChanged)
    {
        self.attachment.anchorPoint = location;
    }
    else if (sender.state == UIGestureRecognizerStateCancelled ||
             sender.state == UIGestureRecognizerStateEnded ||
             sender.state == UIGestureRecognizerStateFailed ||
             sender.state == UIGestureRecognizerStatePossible)
    {
        UIPushBehavior *push = [[UIPushBehavior alloc] initWithItems:@[self.orangeView] mode:UIPushBehaviorModeInstantaneous];
        push.pushDirection = CGVectorMake(velocity.x, velocity.y);
        push.magnitude = magnitude * self.magnitudeMultiplier;
        [self.dynamicAnimator removeBehavior:self.attachment];
        [self.dynamicAnimator addBehavior:push];
    }
}

#pragma mark - Lazy Init
- (UIDynamicAnimator *)dynamicAnimator
{
    if (!_dynamicAnimator)
    {
        _dynamicAnimator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];
    }
    return _dynamicAnimator;
}

- (UIGravityBehavior *)gravity
{
    if (!_gravity)
    {
        _gravity = [[UIGravityBehavior alloc]initWithItems:@[self.orangeView]];
    }
    return _gravity;
}

- (UICollisionBehavior *)collision
{
    if (!_collision)
    {
        _collision = [[UICollisionBehavior alloc]initWithItems:@[self.orangeView]];
        _collision.translatesReferenceBoundsIntoBoundary = YES;
    }
    return _collision;
}

- (UIView *)orangeView
{
    if (!_orangeView)
    {
        CGFloat widthHeight = 40.0f;
        _orangeView = [[UIView alloc]initWithFrame:CGRectMake(0.0, 0.0, widthHeight, widthHeight)];
        _orangeView.backgroundColor = [UIColor orangeColor];
        [self.view addSubview:_orangeView];
    }
    return _orangeView;
}

- (UIPanGestureRecognizer *)panGesture
{
    if (!_panGesture)
    {
        _panGesture = [[UIPanGestureRecognizer alloc]initWithTarget:self action:@selector(handlePan:)];
    }
    return _panGesture;
}

- (UIAttachmentBehavior *)attachment
{
    if (!_attachment)
    {
        _attachment = [[UIAttachmentBehavior alloc]initWithItem:self.orangeView attachedToAnchor:CGPointZero];
    }
    return _attachment;
}

@end

class ViewController: UIViewController
{
    lazy var dynamicAnimator: UIDynamicAnimator =
    {
        let dynamicAnimator = UIDynamicAnimator(referenceView: self.view)
        return dynamicAnimator
    }()
    
    lazy var collision: UICollisionBehavior =
    {
        let collision = UICollisionBehavior(items: [self.orangeView])
        collision.translatesReferenceBoundsIntoBoundary = true
        return collision
    }()
    
    lazy var fieldBehaviors: [UIFieldBehavior] =
    {
        var fieldBehaviors = [UIFieldBehavior]()
        for _ in 0 ..< 2
        {
            let field = UIFieldBehavior.springField()
            field.addItem(self.orangeView)
            fieldBehaviors.append(field)
        }
        return fieldBehaviors
    }()
    
    lazy var itemBehavior: UIDynamicItemBehavior =
    {
        let itemBehavior = UIDynamicItemBehavior(items: [self.orangeView])
        // Adjust these values to change the "stickiness" of the view
        itemBehavior.density = 0.01
        itemBehavior.resistance = 10
        itemBehavior.friction = 0.0
        itemBehavior.allowsRotation = false
        return itemBehavior
    }()
    
    lazy var orangeView: UIView =
    {
        let widthHeight: CGFloat = 40.0
        let orangeView = UIView(frame: CGRect(x: 0.0, y: 0.0, width: widthHeight, height: widthHeight))
        orangeView.backgroundColor = UIColor.orange
        self.view.addSubview(orangeView)
        return orangeView
    }()
    
    lazy var panGesture: UIPanGestureRecognizer =
    {
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(self.handlePan(sender:)))
        return panGesture
    }()
    
    lazy var attachment: UIAttachmentBehavior =
    {
        let attachment = UIAttachmentBehavior(item: self.orangeView, attachedToAnchor: .zero)
        return attachment
    }()

    override func viewDidLoad()
    {
        super.viewDidLoad()
        dynamicAnimator.addBehavior(collision)
        dynamicAnimator.addBehavior(itemBehavior)
        for field in fieldBehaviors
        {
            dynamicAnimator.addBehavior(field)
        }
        
        orangeView.addGestureRecognizer(panGesture)
    }
    
    override func viewDidLayoutSubviews()
    {
        super.viewDidLayoutSubviews()
        
        orangeView.center = view.center
        dynamicAnimator.updateItem(usingCurrentState: orangeView)
        
        for (index, field) in fieldBehaviors.enumerated()
        {
            field.position = CGPoint(x: view.bounds
                .midX, y:  view.bounds.height * (0.25 + 0.5 * CGFloat(index)))
            field.region = UIRegion(size: CGSize(width: view.bounds.width, height: view.bounds.height * 0.5))
        }
    }
    
    func handlePan(sender: UIPanGestureRecognizer)
    {
        let location = sender.location(in: view)
        let velocity = sender.velocity(in: view)
        switch sender.state
        {
        case .began:
            attachment.anchorPoint = location
            dynamicAnimator.addBehavior(attachment)
        case .changed:
            attachment.anchorPoint = location
        case .cancelled, .ended, .failed, .possible:
            itemBehavior.addLinearVelocity(velocity, for: self.orangeView)
            dynamicAnimator.removeBehavior(attachment)
        }
    }
}

@interface ViewController ()

@property (nonatomic, strong) UIDynamicAnimator *dynamicAnimator;
@property (nonatomic, strong) UICollisionBehavior *collision;
@property (nonatomic, strong) UIAttachmentBehavior *attachment;
@property (nonatomic, strong) UIDynamicItemBehavior *itemBehavior;
@property (nonatomic, strong) NSArray <UIFieldBehavior *> *fieldBehaviors;
@property (nonatomic, strong) UIView *orangeView;
@property (nonatomic, strong) UIPanGestureRecognizer *panGesture;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self.dynamicAnimator addBehavior:self.collision];
    [self.dynamicAnimator addBehavior:self.itemBehavior];
    for (UIFieldBehavior *field in self.fieldBehaviors)
    {
        [self.dynamicAnimator addBehavior:field];
    }
    
    [self.orangeView addGestureRecognizer:self.panGesture];
}

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    self.orangeView.center = self.view.center;
    [self.dynamicAnimator updateItemUsingCurrentState:self.orangeView];
    
    for (NSInteger i = 0; i < self.fieldBehaviors.count; i++)
    {
        UIFieldBehavior *field = self.fieldBehaviors[i];
        field.position = CGPointMake(CGRectGetMidX(self.view.bounds), CGRectGetHeight(self.view.bounds) * (0.25f + 0.5f * i));
        field.region = [[UIRegion alloc]initWithSize:CGSizeMake(CGRectGetWidth(self.view.bounds), CGRectGetHeight(self.view.bounds) * 0.5)];
    }
}

- (void)handlePan:(UIPanGestureRecognizer *)sender
{
    CGPoint location = [sender locationInView:self.view];
    CGPoint velocity = [sender velocityInView:self.view];
    if (sender.state == UIGestureRecognizerStateBegan)
    {
        self.attachment.anchorPoint = location;
        [self.dynamicAnimator addBehavior:self.attachment];
    }
    else if (sender.state == UIGestureRecognizerStateChanged)
    {
        self.attachment.anchorPoint = location;
    }
    else if (sender.state == UIGestureRecognizerStateCancelled ||
             sender.state == UIGestureRecognizerStateEnded ||
             sender.state == UIGestureRecognizerStateFailed ||
             sender.state == UIGestureRecognizerStatePossible)
    {
        [self.itemBehavior addLinearVelocity:velocity forItem:self.orangeView];
        [self.dynamicAnimator removeBehavior:self.attachment];
    }
}

#pragma mark - Lazy Init
- (UIDynamicAnimator *)dynamicAnimator
{
    if (!_dynamicAnimator)
    {
        _dynamicAnimator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];
    }
    return _dynamicAnimator;
}

- (UICollisionBehavior *)collision
{
    if (!_collision)
    {
        _collision = [[UICollisionBehavior alloc]initWithItems:@[self.orangeView]];
        _collision.translatesReferenceBoundsIntoBoundary = YES;
    }
    return _collision;
}

- (NSArray <UIFieldBehavior *> *)fieldBehaviors
{
    if (!_fieldBehaviors)
    {
        NSMutableArray *fields = [[NSMutableArray alloc]init];
        for (NSInteger i =  0; i < 2; i++)
        {
            UIFieldBehavior *field = [UIFieldBehavior springField];
            [field addItem:self.orangeView];
            [fields addObject:field];
        }
        _fieldBehaviors = fields;
    }
    return _fieldBehaviors;
}

- (UIDynamicItemBehavior *)itemBehavior
{
    if (!_itemBehavior)
    {
        _itemBehavior = [[UIDynamicItemBehavior alloc]initWithItems:@[self.orangeView]];
        // Adjust these values to change the "stickiness" of the view
        _itemBehavior.density = 0.01;
        _itemBehavior.resistance = 10;
        _itemBehavior.friction = 0.0;
        _itemBehavior.allowsRotation = NO;
    }
    return _itemBehavior;
}

- (UIView *)orangeView
{
    if (!_orangeView)
    {
        CGFloat widthHeight = 40.0f;
        _orangeView = [[UIView alloc]initWithFrame:CGRectMake(0.0, 0.0, widthHeight, widthHeight)];
        _orangeView.backgroundColor = [UIColor orangeColor];
        [self.view addSubview:_orangeView];
    }
    return _orangeView;
}

- (UIPanGestureRecognizer *)panGesture
{
    if (!_panGesture)
    {
        _panGesture = [[UIPanGestureRecognizer alloc]initWithTarget:self action:@selector(handlePan:)];
    }
    return _panGesture;
}

- (UIAttachmentBehavior *)attachment
{
    if (!_attachment)
    {
        _attachment = [[UIAttachmentBehavior alloc]initWithItem:self.orangeView attachedToAnchor:CGPointZero];
    }
    return _attachment;
}

@end

class PresentingViewController: UIViewController
{
    lazy var button: UIButton =
    {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(button)
        button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive
            = true
        button.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
        button.setTitle("Present", for: .normal)
        button.setTextColor(UIColor.blue, for: .normal)
        
        return button
    }()
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        button.addTarget(self, action: #selector(self.didPressPresent), for: .touchUpInside)
    }
    
    func didPressPresent()
    {
        let modal = ModalViewController()
        modal.view.frame = CGRect(x: 0.0, y: 0.0, width: 200.0, height: 200.0)
        modal.modalPresentationStyle = .custom
        modal.transitioningDelegate = modal
        self.present(modal, animated: true)
    }
}

@interface PresentingViewController ()
@property (nonatomic, strong) UIButton *button;
@end

@implementation PresentingViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self.button addTarget:self action:@selector(didPressPresent) forControlEvents:UIControlEventTouchUpInside];
}

- (void)didPressPresent
{
    ModalViewController *modal = [[ModalViewController alloc] init];
    modal.view.frame = CGRectMake(0.0, 0.0, 200.0, 200.0);
    modal.modalPresentationStyle = UIModalPresentationCustom;
    modal.transitioningDelegate = modal;
    [self presentViewController:modal animated:YES completion:nil];
}

- (UIButton *)button
{
    if (!_button)
    {
        _button = [[UIButton alloc] init];
        _button.translatesAutoresizingMaskIntoConstraints = NO;
        [self.view addSubview:_button];
        [_button.centerXAnchor constraintEqualToAnchor:self.view.centerXAnchor].active = YES;
        [_button.centerYAnchor constraintEqualToAnchor:self.view.centerYAnchor].active = YES;
        [_button setTitle:@"Present" forState:UIControlStateNormal];
        [_button setTitleColor:[UIColor blueColor] forState:UIControlStateNormal];
    }
    return _button;
}

@end

class ModalViewController: UIViewController
{
    lazy var button: UIButton =
    {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(button)
        button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive
         = true
        button.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
        button.setTitle("Dismiss", for: .normal)
        button.setTitleColor(.white, for: .normal)
        
        return button
    }()
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        button.addTarget(self, action: #selector(self.didPressDismiss), for: .touchUpInside)
        view.backgroundColor = .red
        view.layer.cornerRadius = 15.0
    }
    
    func didPressDismiss()
    {
        dismiss(animated: true)
    }
}

extension ModalViewController: UIViewControllerTransitioningDelegate
{
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning?
    {
        return DropOutAnimator(duration: 1.5, isAppearing: true)
    }
    
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning?
    {
        return DropOutAnimator(duration: 4.0, isAppearing: false)
    }
}

@interface ModalViewController () <UIViewControllerTransitioningDelegate>
@property (nonatomic, strong) UIButton *button;
@end

@implementation ModalViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self.button addTarget:self action:@selector(didPressPresent) forControlEvents:UIControlEventTouchUpInside];
    self.view.backgroundColor = [UIColor redColor];
    self.view.layer.cornerRadius = 15.0f;
}

- (void)didPressPresent
{
    [self dismissViewControllerAnimated:YES completion:nil];
}

- (UIButton *)button
{
    if (!_button)
    {
        _button = [[UIButton alloc] init];
        _button.translatesAutoresizingMaskIntoConstraints = NO;
        [self.view addSubview:_button];
        [_button.centerXAnchor constraintEqualToAnchor:self.view.centerXAnchor].active = YES;
        [_button.centerYAnchor constraintEqualToAnchor:self.view.centerYAnchor].active = YES;
        [_button setTitle:@"Dismiss" forState:UIControlStateNormal];
        [_button setTitleColor:[UIColor blueColor] forState:UIControlStateNormal];
    }
    return _button;
}

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
{
    return [[DropOutAnimator alloc]initWithDuration: 1.5 appearing:YES];
}

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
    return [[DropOutAnimator alloc] initWithDuration:4.0 appearing:NO];
}

@end

class DropOutAnimator: UIDynamicBehavior
{
    let duration: TimeInterval
    let isAppearing: Bool

    var transitionContext: UIViewControllerContextTransitioning?
    var hasElapsedTimeExceededDuration = false
    var finishTime: TimeInterval = 0.0
    var collisionBehavior: UICollisionBehavior?
    var attachmentBehavior: UIAttachmentBehavior?
    var animator: UIDynamicAnimator?

    init(duration: TimeInterval = 1.0,  isAppearing: Bool)
    {
        self.duration = duration
        self.isAppearing = isAppearing
        super.init()
    }
}

extension DropOutAnimator: UIViewControllerAnimatedTransitioning
{
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning)
    {
        // Get relevant views and view controllers from transitionContext
        guard let fromVC = transitionContext.viewController(forKey: .from),
              let toVC = transitionContext.viewController(forKey: .to),
              let fromView = fromVC.view,
              let toView = toVC.view else { return }
    
        let containerView = transitionContext.containerView
        let duration = self.transitionDuration(using: transitionContext)
    
        // Hold refrence to transitionContext to notify it of completion
        self.transitionContext = transitionContext
    
        // Create dynamic animator
        let animator = UIDynamicAnimator(referenceView: containerView)
        animator.delegate = self
        self.animator = animator
    
        // Presenting Animation
        if self.isAppearing
        {
            fromView.isUserInteractionEnabled = false
        
            // Position toView  just off-screen
            let fromViewInitialFrame = transitionContext.initialFrame(for: fromVC)
            var toViewInitialFrame = toView.frame
            toViewInitialFrame.origin.y -= toViewInitialFrame.height
            toViewInitialFrame.origin.x = fromViewInitialFrame.width * 0.5 - toViewInitialFrame.width * 0.5
            toView.frame = toViewInitialFrame
        
            containerView.addSubview(toView)
        
            // Prevent rotation and adjust bounce
            let bodyBehavior = UIDynamicItemBehavior(items: [toView])
            bodyBehavior.elasticity = 0.7
            bodyBehavior.allowsRotation = false
        
            // Add gravity at exaggerated magnitude so animation doesn't seem slow
            let gravityBehavior = UIGravityBehavior(items: [toView])
            gravityBehavior.magnitude = 10.0
        
            // Set collision bounds to include off-screen view and have collision in center 
            // where our final view should come to rest
            let collisionBehavior = UICollisionBehavior(items: [toView])
            let insets = UIEdgeInsets(top: toViewInitialFrame.minY, left: 0.0, bottom: fromViewInitialFrame.height * 0.5 - toViewInitialFrame.height * 0.5, right: 0.0)
            collisionBehavior.setTranslatesReferenceBoundsIntoBoundary(with: insets)
            self.collisionBehavior = collisionBehavior
        
            // Keep track of finish time in case we need to end the animator befor the animator pauses
            self.finishTime = duration + (self.animator?.elapsedTime ?? 0.0)
        
            // Closure that is called after every "tick" of the animator
            // Check if we exceed duration
            self.action =
            { [weak self] in
                guard let strongSelf = self,
                  (strongSelf.animator?.elapsedTime ?? 0.0) >= strongSelf.finishTime else { return }
                strongSelf.hasElapsedTimeExceededDuration = true
                strongSelf.animator?.removeBehavior(strongSelf)
            }
        
            // `DropOutAnimator` is a composit behavior, so add child behaviors to self
            self.addChildBehavior(collisionBehavior)
            self.addChildBehavior(bodyBehavior)
            self.addChildBehavior(gravityBehavior)
        
            // Add self to dynamic animator
            self.animator?.addBehavior(self)
        }
        // Dismissing Animation
        else
        {
            // Create allow rotation and have a elastic item
            let bodyBehavior = UIDynamicItemBehavior(items: [fromView])
            bodyBehavior.elasticity = 0.8
            bodyBehavior.angularResistance = 5.0
            bodyBehavior.allowsRotation = true
        
            // Create gravity with exaggerated magnitude
            let gravityBehavior = UIGravityBehavior(items: [fromView])
            gravityBehavior.magnitude = 10.0
        
            // Collision boundary is set to have a floor just below the bottom of the screen
            let collisionBehavior = UICollisionBehavior(items: [fromView])
            let insets = UIEdgeInsets(top: 0.0, left: -1000, bottom: -225, right: -1000)
            collisionBehavior.setTranslatesReferenceBoundsIntoBoundary(with: insets)
            self.collisionBehavior = collisionBehavior
        
            // Attachment behavior so view will have effect of hanging from a rope
            let offset = UIOffset(horizontal: 70.0, vertical: fromView.bounds.height * 0.5)
            var anchorPoint = CGPoint(x: fromView.bounds.maxX - 40.0, y: fromView.bounds.minY)
            anchorPoint = containerView.convert(anchorPoint, from: fromView)
            let attachmentBehavior = UIAttachmentBehavior(item: fromView, offsetFromCenter: offset, attachedToAnchor: anchorPoint)
            attachmentBehavior.frequency = 3.0
            attachmentBehavior.damping = 3.0
            self.attachmentBehavior = attachmentBehavior
        
            // `DropOutAnimator` is a composit behavior, so add child behaviors to self
            self.addChildBehavior(collisionBehavior)
            self.addChildBehavior(bodyBehavior)
            self.addChildBehavior(gravityBehavior)
            self.addChildBehavior(attachmentBehavior)
        
            // Add self to dynamic animator
            self.animator?.addBehavior(self)
        
            // Animation has two parts part one is hanging from rope. 
            // Part two is bouncying off-screen
            // Divide duration in two
            self.finishTime = (2.0 / 3.0) * duration + (self.animator?.elapsedTime ?? 0.0)
        
             // After every "tick" of animator check if past time limit
            self.action =
            { [weak self] in
                guard let strongSelf = self,
                  (strongSelf.animator?.elapsedTime ?? 0.0) >= strongSelf.finishTime else { return }
                strongSelf.hasElapsedTimeExceededDuration = true
                strongSelf.animator?.removeBehavior(strongSelf)
            }
        }
    
    }

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval
    {
        // Return the duration of the animation
        return self.duration
    }
}

extension DropOutAnimator: UIDynamicAnimatorDelegate
{
    func dynamicAnimatorDidPause(_ animator: UIDynamicAnimator)
    {
      // Animator has reached stasis
      if self.isAppearing
      {
            // Check if we are out of time
            if self.hasElapsedTimeExceededDuration
            {
                // Move to final positions
                let toView = self.transitionContext?.viewController(forKey: .to)?.view
                let containerView = self.transitionContext?.containerView
                toView?.center = containerView?.center ?? .zero
                self.hasElapsedTimeExceededDuration = false
            }
            
            // Clean up and call completion
            self.transitionContext?.completeTransition(!(self.transitionContext?.transitionWasCancelled ?? false))
            self.childBehaviors.forEach { self.removeChildBehavior($0) }
            animator.removeAllBehaviors()
            self.transitionContext = nil
    }
    else
    {
        if let attachmentBehavior = self.attachmentBehavior
        {
            // If we have an attachment, we are at the end of part one and start part two.
            self.removeChildBehavior(attachmentBehavior)
            self.attachmentBehavior = nil
            animator.addBehavior(self)
            let duration = self.transitionDuration(using: self.transitionContext)
            self.finishTime = 1.0 / 3.0 * duration + animator.elapsedTime
        }
        else
        {
            // Clean up and call completion
            let fromView = self.transitionContext?.viewController(forKey: .from)?.view
            let toView = self.transitionContext?.viewController(forKey: .to)?.view
            fromView?.removeFromSuperview()
            toView?.isUserInteractionEnabled = true
            self.transitionContext?.completeTransition(!(self.transitionContext?.transitionWasCancelled ?? false))
            self.childBehaviors.forEach { self.removeChildBehavior($0) }
            animator.removeAllBehaviors()
            self.transitionContext = nil
        }
    }
}
}

@interface ObjcDropOutAnimator() <UIDynamicAnimatorDelegate, UIViewControllerAnimatedTransitioning>
@property (nonatomic, strong) id<UIViewControllerContextTransitioning> transitionContext;
@property (nonatomic, strong) UIDynamicAnimator *animator;
@property (nonatomic, assign) NSTimeInterval finishTime;
@property (nonatomic, assign) BOOL elapsedTimeExceededDuration;
@property (nonatomic, assign, getter=isAppearing) BOOL appearing;
@property (nonatomic, assign) NSTimeInterval duration;
@property (nonatomic, strong) UIAttachmentBehavior *attachBehavior;
@property (nonatomic, strong) UICollisionBehavior * collisionBehavior;

@end

@implementation ObjcDropOutAnimator

- (instancetype)initWithDuration:(NSTimeInterval)duration appearing:(BOOL)appearing
{
    self = [super init];
    if (self)
    {
        _duration = duration;
        _appearing = appearing;
    }
    return self;
}

- (void) animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    // Get relevant views and view controllers from transitionContext
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *fromView = fromVC.view;
    UIView *toView = toVC.view;
    
    UIView *containerView = transitionContext.containerView;
    NSTimeInterval duration = [self transitionDuration:transitionContext];
    
    // Hold refrence to transitionContext to notify it of completion
    self.transitionContext = transitionContext;
    
    // Create dynamic animator
    UIDynamicAnimator *animator = [[UIDynamicAnimator alloc]initWithReferenceView:containerView];
    animator.delegate = self;
    self.animator = animator;
    
    // Presenting Animation
    if (self.isAppearing)
    {
        fromView.userInteractionEnabled = NO;
        
        // Position toView  just above screen
        CGRect fromViewInitialFrame = [transitionContext initialFrameForViewController:fromVC];
        CGRect toViewInitialFrame = toView.frame;
        toViewInitialFrame.origin.y -= CGRectGetHeight(toViewInitialFrame);
        toViewInitialFrame.origin.x = CGRectGetWidth(fromViewInitialFrame) * 0.5 - CGRectGetWidth(toViewInitialFrame) * 0.5;
        toView.frame = toViewInitialFrame;
        
        [containerView addSubview:toView];
        
        // Prevent rotation and adjust bounce
        UIDynamicItemBehavior *bodyBehavior = [[UIDynamicItemBehavior alloc]initWithItems:@[toView]];
        bodyBehavior.elasticity = 0.7;
        bodyBehavior.allowsRotation = NO;
        
        // Add gravity at exaggerated magnitude so animation doesn't seem slow
        UIGravityBehavior *gravityBehavior = [[UIGravityBehavior alloc]initWithItems:@[toView]];
        gravityBehavior.magnitude = 10.0f;
        
        // Set collision bounds to include off-screen view and have collision floor in center
        // where our final view should come to rest
        UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc]initWithItems:@[toView]];
        UIEdgeInsets insets = UIEdgeInsetsMake(CGRectGetMinY(toViewInitialFrame), 0.0, CGRectGetHeight(fromViewInitialFrame) * 0.5 - CGRectGetHeight(toViewInitialFrame) * 0.5, 0.0);
        [collisionBehavior setTranslatesReferenceBoundsIntoBoundaryWithInsets:insets];
        self.collisionBehavior = collisionBehavior;
        
        // Keep track of finish time in case we need to end the animator befor the animator pauses
        self.finishTime = duration + self.animator.elapsedTime;
        
        // Closure that is called after every "tick" of the animator
        // Check if we exceed duration
        __weak ObjcDropOutAnimator *weakSelf = self;
        self.action = ^{
            __strong ObjcDropOutAnimator *strongSelf = weakSelf;
            if (strongSelf)
            {
                if (strongSelf.animator.elapsedTime >= strongSelf.finishTime)
                {
                    strongSelf.elapsedTimeExceededDuration = YES;
                    [strongSelf.animator removeBehavior:strongSelf];
                }
            }
        };
        
        // `DropOutAnimator` is a composit behavior, so add child behaviors to self
        [self addChildBehavior:collisionBehavior];
        [self addChildBehavior:bodyBehavior];
        [self addChildBehavior:gravityBehavior];
        
        // Add self to dynamic animator
        [self.animator addBehavior:self];
    }
    // Dismissing Animation
    else
    {
        // Allow rotation and have a elastic item
        UIDynamicItemBehavior *bodyBehavior = [[UIDynamicItemBehavior alloc] initWithItems:@[fromView]];
        bodyBehavior.elasticity = 0.8;
        bodyBehavior.angularResistance = 5.0;
        bodyBehavior.allowsRotation = YES;
        
        // Create gravity with exaggerated magnitude
        UIGravityBehavior *gravityBehavior = [[UIGravityBehavior alloc] initWithItems:@[fromView]];
        gravityBehavior.magnitude = 10.0f;
        
        // Collision boundary is set to have a floor just below the bottom of the screen
        UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc] initWithItems:@[fromView]];
        UIEdgeInsets insets = UIEdgeInsetsMake(0, -1000, -225, -1000);
        [collisionBehavior setTranslatesReferenceBoundsIntoBoundaryWithInsets:insets];
        self.collisionBehavior = collisionBehavior;
        
        // Attachment behavior so view will have effect of hanging from a rope
        UIOffset offset = UIOffsetMake(70, -(CGRectGetHeight(fromView.bounds) / 2.0));
        
        CGPoint anchorPoint = CGPointMake(CGRectGetMaxX(fromView.bounds) - 40,
                                          CGRectGetMinY(fromView.bounds));
        anchorPoint = [containerView convertPoint:anchorPoint fromView:fromView];
        UIAttachmentBehavior *attachBehavior = [[UIAttachmentBehavior alloc] initWithItem:fromView offsetFromCenter:offset attachedToAnchor:anchorPoint];
        attachBehavior.frequency = 3.0;
        attachBehavior.damping = 0.3;
        attachBehavior.length = 40;
        self.attachBehavior = attachBehavior;
        
        // `DropOutAnimator` is a composit behavior, so add child behaviors to self
        [self addChildBehavior:collisionBehavior];
        [self addChildBehavior:bodyBehavior];
        [self addChildBehavior:gravityBehavior];
        [self addChildBehavior:attachBehavior];
        
        // Add self to dynamic animator
        [self.animator addBehavior:self];
        
        // Animation has two parts part one is hanging from rope.
        // Part two is bouncying off-screen
        // Divide duration in two
        self.finishTime = (2./3.) * duration + [self.animator elapsedTime];
        
        // After every "tick" of animator check if past time limit
        __weak ObjcDropOutAnimator *weakSelf = self;
        self.action = ^{
            __strong ObjcDropOutAnimator *strongSelf = weakSelf;
            if (strongSelf)
            {
                if ([strongSelf.animator elapsedTime] >= strongSelf.finishTime)
                {
                    strongSelf.elapsedTimeExceededDuration = YES;
                    [strongSelf.animator removeBehavior:strongSelf];
                }
            }
        };
    }
}

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
    return self.duration;
}

- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator
{
    // Animator has reached stasis
    if (self.isAppearing)
    {
        // Check if we are out of time
        if (self.elapsedTimeExceededDuration)
        {
            // Move to final positions
            UIView *toView = [self.transitionContext viewControllerForKey:UITransitionContextToViewControllerKey].view;
            UIView *containerView = [self.transitionContext containerView];
            toView.center = containerView.center;
            self.elapsedTimeExceededDuration = NO;
        }
        
        // Clean up and call completion
        [self.transitionContext completeTransition:![self.transitionContext transitionWasCancelled]];
        for (UIDynamicBehavior *behavior in self.childBehaviors)
        {
            [self removeChildBehavior:behavior];
        }
        [animator removeAllBehaviors];
        self.transitionContext = nil;
    }
    // Dismissing
    else
    {
        if (self.attachBehavior)
        {
            // If we have an attachment, we are at the end of part one and start part two.
            [self removeChildBehavior:self.attachBehavior];
            self.attachBehavior = nil;
            [animator addBehavior:self];
            NSTimeInterval duration = [self transitionDuration:self.transitionContext];
            self.finishTime = 1./3. * duration + [animator elapsedTime];
        }
        else
        {
            // Clean up and call completion
            UIView *fromView = [self.transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey].view;
            UIView *toView = [self.transitionContext viewControllerForKey:UITransitionContextToViewControllerKey].view;
            [fromView removeFromSuperview];
            toView.userInteractionEnabled = YES;
            
            [self.transitionContext completeTransition:![self.transitionContext transitionWasCancelled]];
            for (UIDynamicBehavior *behavior in self.childBehaviors)
            {
                [self removeChildBehavior:behavior];
            }
            [animator removeAllBehaviors];
            self.transitionContext = nil;
        }
    }
}

class ViewController: UIViewController
{
    var presentingAnimator: ShadeAnimator!
    var dismissingAnimator: ShadeAnimator!
    let shadeVC = ShadeViewController()
    
    lazy var label: UILabel =
    {
        let label = UILabel()
        label.textColor = .blue
        label.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(label)
        label.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
        label.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
        return label
    }()
    
    override func viewDidLoad()
    {
        super.viewDidLoad()
        label.text = "Swipe Down From Top"
        presentingAnimator = ShadeAnimator(isAppearing: true, presentingVC: self, presentedVC: shadeVC, transitionDelegate: self)
        dismissingAnimator = ShadeAnimator(isAppearing: false, presentingVC: self, presentedVC: shadeVC, transitionDelegate: self)
    }
}
extension ViewController: UIViewControllerTransitioningDelegate
{
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning?
    {
        return EmptyAnimator()
    }
    
    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning?
    {
        return EmptyAnimator()
    }
    
    func interactionControllerForPresentation(using animator: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning?
    {
        return presentingAnimator
    }
    
    func interactionControllerForDismissal(using animator: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning?
    {
        return dismissingAnimator
    }
}

@interface ObjCViewController () <UIViewControllerTransitioningDelegate>
@property (nonatomic, strong) ShadeAnimator *presentingAnimator;
@property (nonatomic, strong) ShadeAnimator *dismissingAnimator;
@property (nonatomic, strong) UILabel *label;
@property (nonatomic, strong) ShadeViewController *shadeVC;
@end

@implementation ObjCViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.label.text = @"Swipe Down From Top";
    self.shadeVC = [[ShadeViewController alloc] init];
    self.presentingAnimator = [[ShadeAnimator alloc] initWithIsAppearing:YES presentingVC:self presentedVC:self.shadeVC transitionDelegate:self];
    self.dismissingAnimator = [[ShadeAnimator alloc] initWithIsAppearing:NO presentingVC:self presentedVC:self.shadeVC transitionDelegate:self];
}

- (UILabel *)label
{
    if (!_label)
    {
        _label = [[UILabel alloc] init];
        _label.textColor = [UIColor blueColor];
        _label.translatesAutoresizingMaskIntoConstraints = NO;
        [self.view addSubview:_label];
        [_label.centerXAnchor constraintEqualToAnchor:self.view.centerXAnchor].active = YES;
        [_label.centerYAnchor constraintEqualToAnchor:self.view.centerYAnchor].active = YES;
    }
    return _label;
}

#pragma mark - UIViewControllerTransitioningDelegate

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForPresentedController:(UIViewController *)presented presentingController:(UIViewController *)presenting sourceController:(UIViewController *)source
{
    return [[EmptyAnimator alloc] init];
}

- (id<UIViewControllerAnimatedTransitioning>)animationControllerForDismissedController:(UIViewController *)dismissed
{
    return [[EmptyAnimator alloc] init];
}

- (id<UIViewControllerInteractiveTransitioning>)interactionControllerForPresentation:(id<UIViewControllerAnimatedTransitioning>)animator
{
    return self.presentingAnimator;
}

- (id<UIViewControllerInteractiveTransitioning>)interactionControllerForDismissal:(id<UIViewControllerAnimatedTransitioning>)animator
{
    return self.dismissingAnimator;
}

@end

class EmptyAnimator: NSObject
{

}

extension EmptyAnimator: UIViewControllerAnimatedTransitioning
{
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning)
    {
        
    }
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval
    {
        return 0.0
    }
}

@implementation EmptyAnimator

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    
}

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
    return 0.0;
}

@end

class ShadeAnimator: UIDynamicBehavior
{
    // Whether we are presenting or dismissing
    let isAppearing: Bool

    // The view controller that is not the shade
    weak var presentingVC: UIViewController?

    // The view controller that is the shade
    weak var presentedVC: UIViewController?

    // The delegate will vend the animator
    weak var transitionDelegate: UIViewControllerTransitioningDelegate?
    
    // Feedback generator for haptics on collisions
    let impactFeedbackGenerator = UIImpactFeedbackGenerator(style: .light)
    
    // The context given to the animator at the start of the transition
    var transitionContext: UIViewControllerContextTransitioning?
    
    // Time limit of the dynamic part of the animation
    var finishTime: TimeInterval = 4.0
    
    // The Pan Gesture that drives the transition. Not using EdgePan because triggers Notifications screen
    lazy var pan: UIPanGestureRecognizer =
    {
        let pan = UIPanGestureRecognizer(target: self, action: #selector(self.handlePan(sender:)))
        return pan
    }()
    
    // The dynamic animator that we add `ShadeAnimator` to
    lazy var animator: UIDynamicAnimator! =
    {
        let animator = UIDynamicAnimator(referenceView: self.transitionContext!.containerView)
        return animator
    }()
    
    // init with all of our dependencies
    init(isAppearing: Bool, presentingVC: UIViewController, presentedVC: UIViewController, transitionDelegate: UIViewControllerTransitioningDelegate)
    {
        self.isAppearing = isAppearing
        self.presentingVC = presentingVC
        self.presentedVC = presentedVC
        self.transitionDelegate = transitionDelegate
        super.init()
        self.impactFeedbackGenerator.prepare()
        
        if isAppearing
        {
            self.presentingVC?.view.addGestureRecognizer(pan)
        }
        else
        {
            self.presentedVC?.view.addGestureRecognizer(pan)
        }
        
    }
    
    // Setup and moves shade view controller to just above screen if appearing
    func setupViewsForTransition(with transitionContext: UIViewControllerContextTransitioning)
    {
        // Get relevant views and view controllers from transitionContext
        guard let fromVC = transitionContext.viewController(forKey: .from),
            let toVC = transitionContext.viewController(forKey: .to),
            let toView = toVC.view else { return }
        
        let containerView = transitionContext.containerView
        
        // Hold refrence to transitionContext to notify it of completion
        self.transitionContext = transitionContext
        if isAppearing
        {
            // Position toView  just off-screen
            let fromViewInitialFrame = transitionContext.initialFrame(for: fromVC)
            var toViewInitialFrame = toView.frame
            toViewInitialFrame.origin.y -= toViewInitialFrame.height
            toViewInitialFrame.origin.x = fromViewInitialFrame.width * 0.5 - toViewInitialFrame.width * 0.5
            toView.frame = toViewInitialFrame
            
            containerView.addSubview(toView)
        }
        else
        {
            fromVC.view.addGestureRecognizer(pan)
        }
    }
    
    // Handles the entire interaction from presenting/dismissing to completion
    func handlePan(sender: UIPanGestureRecognizer)
    {
        let location = sender.location(in: transitionContext?.containerView)
        let velocity = sender.velocity(in: transitionContext?.containerView)
        let fromVC = transitionContext?.viewController(forKey: .from)
        let toVC = transitionContext?.viewController(forKey: .to)
        
        let touchStartHeight: CGFloat = 90.0
        let touchLocationFromBottom: CGFloat = 20.0
        
        switch sender.state
        {
        case .began:
            let beginLocation = sender.location(in: sender.view)
            if isAppearing
            {
                guard beginLocation.y <= touchStartHeight,
                      let presentedVC = self.presentedVC else { break }
                presentedVC.modalPresentationStyle = .custom
                presentedVC.transitioningDelegate = transitionDelegate
                presentingVC?.present(presentedVC, animated: true)
            }
            else
            {
                guard beginLocation.y >= (sender.view?.frame.height ?? 0.0) - touchStartHeight else { break }
                presentedVC?.dismiss(animated: true)
            }
        case .changed:
            guard let view = isAppearing ? toVC?.view : fromVC?.view else { return }
            UIView.animate(withDuration: 0.2)
            {
                view.frame.origin.y = location.y - view.bounds.height + touchLocationFromBottom
            }
            
            transitionContext?.updateInteractiveTransition(view.frame.maxY / view.frame.height
            )
        case .ended, .cancelled:
            guard let view = isAppearing ? toVC?.view : fromVC?.view else { return }
            let isCancelled = isAppearing ? (velocity.y < 0.5 || view.center.y < 0.0) : (velocity.y > 0.5 || view.center.y > 0.0)
            addAttachmentBehavior(with: view, isCancelled: isCancelled)
            addCollisionBehavior(with: view)
            addItemBehavior(with: view)
            
            animator.addBehavior(self)
            animator.delegate = self
            
            self.action =
            { [weak self] in
                guard let strongSelf = self else { return }
                if strongSelf.animator.elapsedTime > strongSelf.finishTime
                {
                    strongSelf.animator.removeAllBehaviors()
                }
                else
                {
                    strongSelf.transitionContext?.updateInteractiveTransition(view.frame.maxY / view.frame.height
                    )
                }
            }
        default:
            break
        }
    }
    
    // Add collision behavior that causes bounce when finished
    func addCollisionBehavior(with view: UIView)
    {
        let collisionBehavior = UICollisionBehavior(items: [view])
        let insets = UIEdgeInsets(top: -view.bounds.height, left: 0.0, bottom: 0.0, right: 0.0)
        collisionBehavior.setTranslatesReferenceBoundsIntoBoundary(with: insets)
        collisionBehavior.collisionDelegate = self
        self.addChildBehavior(collisionBehavior)
    }
    
    // Add attachment behavior that pulls shade either to top or bottom
    func addAttachmentBehavior(with view: UIView, isCancelled: Bool)
    {
        let anchor: CGPoint
        switch (isAppearing, isCancelled)
        {
        case (true, true), (false, false):
            anchor = CGPoint(x: view.center.x, y: -view.frame.height)
        case (true, false), (false, true):
            anchor = CGPoint(x: view.center.x, y: view.frame.height)
        }
        let attachmentBehavior = UIAttachmentBehavior(item: view, attachedToAnchor: anchor)
        attachmentBehavior.damping = 0.1
        attachmentBehavior.frequency = 3.0
        attachmentBehavior.length = 0.5 * view.frame.height
        self.addChildBehavior(attachmentBehavior)
    }
    
    // Makes view more bouncy 
    func addItemBehavior(with view: UIView)
    {
        let itemBehavior = UIDynamicItemBehavior(items: [view])
        itemBehavior.allowsRotation = false
        itemBehavior.elasticity = 0.6
        self.addChildBehavior(itemBehavior)
    }
    
}
extension ShadeAnimator: UIDynamicAnimatorDelegate
{
    // Determines transition has ended
    func dynamicAnimatorDidPause(_ animator: UIDynamicAnimator)
    {
        guard let transitionContext = self.transitionContext else { return }
        let fromVC = transitionContext.viewController(forKey: .from)
        let toVC = transitionContext.viewController(forKey: .to)
        guard let view = isAppearing ? toVC?.view : fromVC?.view else { return }
        switch (view.center.y < 0.0, isAppearing)
        {
        case (true, true), (true, false):
            view.removeFromSuperview()
            transitionContext.finishInteractiveTransition()
            transitionContext.completeTransition(!isAppearing)
        case (false, true):
            toVC?.view.frame = transitionContext.finalFrame(for: toVC!)
            transitionContext.finishInteractiveTransition()
            transitionContext.completeTransition(true)
        case (false, false):
            fromVC?.view.frame = transitionContext.initialFrame(for: fromVC!)
            transitionContext.cancelInteractiveTransition()
            transitionContext.completeTransition(false)
        }
        childBehaviors.forEach { removeChildBehavior($0) }
        animator.removeAllBehaviors()
        self.animator = nil
        self.transitionContext = nil
    }
}
extension ShadeAnimator: UICollisionBehaviorDelegate
{
    // Triggers haptics
    func collisionBehavior(_ behavior: UICollisionBehavior, beganContactFor item: UIDynamicItem, withBoundaryIdentifier identifier: NSCopying?, at p: CGPoint)
    {
        guard p.y > 0.0 else { return }
        impactFeedbackGenerator.impactOccurred()
    }
}
extension ShadeAnimator: UIViewControllerInteractiveTransitioning
{
    // Starts transition
    func startInteractiveTransition(_ transitionContext: UIViewControllerContextTransitioning)
    {
        setupViewsForTransition(with: transitionContext)
    }
}

@interface ShadeAnimator() <UIDynamicAnimatorDelegate, UICollisionBehaviorDelegate>
@property (nonatomic, assign) BOOL isAppearing;
@property (nonatomic, weak) UIViewController *presentingVC;
@property (nonatomic, weak) UIViewController *presentedVC;
@property (nonatomic, weak) NSObject<UIViewControllerTransitioningDelegate> *transitionDelegate;
@property (nonatomic, strong) UIImpactFeedbackGenerator *impactFeedbackGenerator;
@property (nonatomic, strong) id<UIViewControllerContextTransitioning> transitionContext;
@property (nonatomic, assign) NSTimeInterval finishTime;
@property (nonatomic, strong) UIPanGestureRecognizer *pan;
@property (nonatomic, strong) UIDynamicAnimator *animator;
@end

@implementation ShadeAnimator

- (instancetype)initWithIsAppearing:(BOOL)isAppearing presentingVC:(UIViewController *)presentingVC presentedVC:(UIViewController *)presentedVC transitionDelegate:(id<UIViewControllerTransitioningDelegate>)transitionDelegate
{
    self = [super init];
    if (self)
    {
        _isAppearing = isAppearing;
        _presentingVC = presentingVC;
        _presentedVC = presentedVC;
        _transitionDelegate = transitionDelegate;
        _impactFeedbackGenerator = [[UIImpactFeedbackGenerator alloc]initWithStyle:UIImpactFeedbackStyleLight];
        [_impactFeedbackGenerator prepare];
        if (_isAppearing)
        {
            [_presentingVC.view addGestureRecognizer:self.pan];
        }
        else
        {
            [_presentedVC.view addGestureRecognizer:self.pan];
        }
    }
    return self;
}

#pragma mark - Lazy Init
- (UIPanGestureRecognizer *)pan
{
    if (!_pan)
    {
        _pan = [[UIPanGestureRecognizer alloc]initWithTarget:self action:@selector(handlePan:)];
    }
    return _pan;
}

- (UIDynamicAnimator *)animator
{
    if (!_animator)
    {
        _animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.transitionContext.containerView];
    }
    return _animator;
}

#pragma mark - Setup
- (void)setupViewForTransitionWithContext:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *toView = toVC.view;
    UIView *containerView = transitionContext.containerView;
    self.transitionContext = transitionContext;
    if (self.isAppearing)
    {
        CGRect fromViewInitialFrame = [transitionContext initialFrameForViewController:fromVC];
        CGRect toViewInitialFrame = toView.frame;
        toViewInitialFrame.origin.y -= CGRectGetHeight(toViewInitialFrame);
        toViewInitialFrame.origin.x = CGRectGetWidth(fromViewInitialFrame) * 0.5 - CGRectGetWidth(toViewInitialFrame) * 0.5;
        
        [containerView addSubview:toView];
    }
    else
    {
        [fromVC.view addGestureRecognizer:self.pan];
    }
}

#pragma mark - Gesture
- (void)handlePan:(UIPanGestureRecognizer *)sender
{
    CGPoint location = [sender locationInView:self.transitionContext.containerView];
    CGPoint velocity = [sender velocityInView:self.transitionContext.containerView];
    UIViewController *fromVC = [self.transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [self.transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    CGFloat touchStartHeight = 90.0;
    CGFloat touchLocationFromBottom = 20.0;
    
    if (sender.state == UIGestureRecognizerStateBegan)
    {
        CGPoint beginLocation = [sender locationInView:sender.view];
        if (self.isAppearing)
        {
            if (beginLocation.y <= touchStartHeight)
            {
                self.presentedVC.modalPresentationStyle = UIModalPresentationCustom;
                self.presentedVC.transitioningDelegate = self.transitionDelegate;
                [self.presentingVC presentViewController:self.presentedVC animated:YES completion:nil];
            }
        }
        else
        {
            if (beginLocation.y >= [sender locationInView:sender.view].y - touchStartHeight)
            {
                [self.presentedVC dismissViewControllerAnimated:true completion:nil];
            }
        }
    }
    else if (sender.state == UIGestureRecognizerStateChanged)
    {
        UIView *view = self.isAppearing ? toVC.view : fromVC.view;
        [UIView animateWithDuration:0.2 animations:^{
            CGRect frame = view.frame;
            frame.origin.y = location.y - CGRectGetHeight(view.bounds) + touchLocationFromBottom;
            view.frame = frame;
        }];
        [self.transitionContext updateInteractiveTransition:CGRectGetMaxY(view.frame) / CGRectGetHeight(view.frame)];
    }
    else if (sender.state == UIGestureRecognizerStateEnded || sender.state == UIGestureRecognizerStateCancelled)
    {
        UIView *view = self.isAppearing ? toVC.view : fromVC.view;
        BOOL isCancelled = self.isAppearing ? (velocity.y < 0.5 || view.center.y < 0.0) : (velocity.y > 0.5 || view.center.y > 0.0);
        [self addAttachmentBehaviorWithView:view isCancelled:isCancelled];
        [self addCollisionBehaviorWithView:view];
        [self addItemBehaviorWithView:view];
        
        [self.animator addBehavior:self];
        self.animator.delegate = self;
        
        __weak ShadeAnimator *weakSelf = self;
        self.action =
        ^{
            if (weakSelf.animator.elapsedTime > weakSelf.finishTime)
            {
                [weakSelf.animator removeAllBehaviors];
            }
            else
            {
                [weakSelf.transitionContext updateInteractiveTransition:CGRectGetMaxY(view.frame) / CGRectGetHeight(view.frame)];
            }
        };
    }
}

#pragma mark - UIViewControllerInteractiveTransitioning
- (void)startInteractiveTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    [self setupViewForTransitionWithContext:transitionContext];
}

#pragma mark - Behaviors
- (void)addCollisionBehaviorWithView:(UIView *)view
{
    UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc]initWithItems:@[view]];
    UIEdgeInsets insets = UIEdgeInsetsMake(-CGRectGetHeight(view.bounds), 0.0, 0.0, 0.0);
    [collisionBehavior setTranslatesReferenceBoundsIntoBoundaryWithInsets:insets];
    collisionBehavior.collisionDelegate = self;
    [self addChildBehavior:collisionBehavior];
}

- (void)addItemBehaviorWithView:(UIView *)view
{
    UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc]initWithItems:@[view]];
    itemBehavior.allowsRotation = NO;
    itemBehavior.elasticity = 0.6;
    [self addChildBehavior:itemBehavior];
}

- (void)addAttachmentBehaviorWithView:(UIView *)view isCancelled:(BOOL)isCancelled
{
    CGPoint anchor;
    if ((self.isAppearing && isCancelled) || (!self.isAppearing && isCancelled))
    {
        anchor = CGPointMake(view.center.x, -CGRectGetHeight(view.frame));
    }
    else
    {
        anchor = CGPointMake(view.center.x, -CGRectGetHeight(view.frame));
    }
    UIAttachmentBehavior *attachmentBehavior = [[UIAttachmentBehavior alloc]initWithItem:view attachedToAnchor:anchor];
    attachmentBehavior.damping = 0.1;
    attachmentBehavior.frequency = 3.0;
    attachmentBehavior.length = 0.5 * CGRectGetHeight(view.frame);
    [self addChildBehavior:attachmentBehavior];
}

#pragma mark - UICollisionBehaviorDelegate
- (void)collisionBehavior:(UICollisionBehavior *)behavior beganContactForItem:(id<UIDynamicItem>)item withBoundaryIdentifier:(id<NSCopying>)identifier atPoint:(CGPoint)p
{
    if (p.y > 0.0)
    {
        [self.impactFeedbackGenerator impactOccurred];
    }
}

#pragma mark - UIDynamicAnimatorDelegate
- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator
{
    UIViewController *fromVC = [self.transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [self.transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *view = self.isAppearing ? toVC.view : fromVC.view;
    if (view.center.y < 0.0 && (self.isAppearing ||  !self.isAppearing))
    {
        [view removeFromSuperview];
        [self.transitionContext finishInteractiveTransition];
        [self.transitionContext completeTransition:!self.isAppearing];
    }
    else if (view.center.y >= 0.0 && self.isAppearing)
    {
        toVC.view.frame = [self.transitionContext finalFrameForViewController:toVC];
        [self.transitionContext finishInteractiveTransition];
        [self.transitionContext completeTransition:YES];
    }
    else
    {
        fromVC.view.frame = [self.transitionContext initialFrameForViewController:fromVC];
        [self.transitionContext cancelInteractiveTransition];
        [self.transitionContext completeTransition:NO];
    }
    for (UIDynamicBehavior *behavior in self.childBehaviors)
    {
        [self removeChildBehavior:behavior];
    }
    [animator removeAllBehaviors];
    self.animator = nil;
    self.transitionContext = nil;
}

@end

protocol ResizableDynamicItem: UIDynamicItem
{
    var bounds: CGRect { set get }
}
extension UIView: ResizableDynamicItem {}

@protocol ResizableDynamicItem <UIDynamicItem>
@property (nonatomic, readwrite) CGRect bounds;
@end

final class PositionToBoundsMapping: NSObject, UIDynamicItem
{
    var target: ResizableDynamicItem
    
    init(target: ResizableDynamicItem)
    {
        self.target = target
        super.init()
    }
    
    var bounds: CGRect
    {
        get
        {
            return self.target.bounds
        }
    }
    
    var center: CGPoint
    {
        get
        {
            return CGPoint(x: self.target.bounds.width, y: self.target.bounds.height)
        }
        
        set
        {
            self.target.bounds = CGRect(x: 0.0, y: 0.0, width: newValue.x, height: newValue.y)
        }
    }
    
    var transform: CGAffineTransform
    {
        get
        {
            return self.target.transform
        }
        
        set
        {
            self.target.transform = newValue
        }
    }
}

@interface PositionToBoundsMapping ()
@property (nonatomic, strong) id<ResizableDynamicItem> target;
@end

@implementation PositionToBoundsMapping

- (instancetype)initWithTarget:(id<ResizableDynamicItem>)target
{
    self = [super init];
    if (self)
    {
        _target = target;
    }
    return self;
}

- (CGRect)bounds
{
    return self.target.bounds;
}

- (CGPoint)center
{
    return CGPointMake(self.target.bounds.size.width, self.target.bounds.size.height);
}

- (void)setCenter:(CGPoint)center
{
    self.target.bounds = CGRectMake(0, 0, center.x, center.y);
}

- (CGAffineTransform)transform
{
    return self.target.transform;
}

- (void)setTransform:(CGAffineTransform)transform
{
    self.target.transform = transform;
}

@end

final class ViewController: UIViewController
{
    lazy var button: UIButton =
    {
        let button = UIButton(frame: CGRect(x: 0.0, y: 0.0, width: 300.0, height: 200.0))
        button.backgroundColor = .red
        button.layer.cornerRadius = 15.0
        button.setTitle("Tap Me", for: .normal)
        self.view.addSubview(button)
        return button
    }()
    
    var buttonBounds = CGRect.zero
    var animator: UIDynamicAnimator?
    
    override func viewDidLoad() 
    {
        super.viewDidLoad()
        view.backgroundColor = .white
        button.addTarget(self, action: #selector(self.didPressButton(sender:)), for: .touchUpInside)
        buttonBounds = button.bounds
    }
    
    override func viewDidLayoutSubviews() 
    {
        super.viewDidLayoutSubviews()
        button.center = view.center
    }
    
    func didPressButton(sender: UIButton)
    {
        // Reset bounds so if button is press twice in a row, previous changes don't propogate
        button.bounds = buttonBounds
        let animator = UIDynamicAnimator(referenceView: view)
        
        // Create mapping
        let buttonBoundsDynamicItem = PositionToBoundsMapping(target: button)
        
        // Add Attachment behavior
        let attachmentBehavior = UIAttachmentBehavior(item: buttonBoundsDynamicItem, attachedToAnchor: buttonBoundsDynamicItem.center)
        
        // Higher frequency faster oscillation
        attachmentBehavior.frequency = 2.0
        
        // Lower damping longer oscillation lasts
        attachmentBehavior.damping = 0.1
        animator.addBehavior(attachmentBehavior)
        
        let pushBehavior = UIPushBehavior(items: [buttonBoundsDynamicItem], mode: .instantaneous)
        
        // Change angle to determine how much height/ width should change 45° means heigh:width is 1:1
        pushBehavior.angle = .pi / 4.0
        
        // Larger magnitude means bigger change
        pushBehavior.magnitude = 30.0
        animator.addBehavior(pushBehavior)
        pushBehavior.active = true
        
        // Hold refrence so animator is not released
        self.animator = animator
    }
}

@interface ViewController ()
@property (nonatomic, strong) UIButton *button;
@property (nonatomic, assign) CGRect buttonBounds;
@property (nonatomic, strong) UIDynamicAnimator *animator;
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    [self.button addTarget:self action:@selector(didTapButton:) forControlEvents:UIControlEventTouchUpInside];
    self.buttonBounds = self.button.bounds;
}

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    self.button.center = self.view.center;
}

- (UIButton *)button
{
    if (!_button)
    {
        _button = [[UIButton alloc]initWithFrame:CGRectMake(0.0, 0.0, 200.0, 200.0)];
        _button.backgroundColor = [UIColor redColor];
        _button.layer.cornerRadius = 15.0;
        [_button setTitle:@"Tap Me" forState:UIControlStateNormal];
        [self.view addSubview:_button];
    }
    return _button;
}

- (void)didTapButton:(id)sender
{
    self.button.bounds = self.buttonBounds;
    UIDynamicAnimator *animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];
    PositionToBoundsMapping *buttonBoundsDynamicItem = [[PositionToBoundsMapping alloc]initWithTarget:sender];
    UIAttachmentBehavior *attachmentBehavior = [[UIAttachmentBehavior alloc]initWithItem:buttonBoundsDynamicItem attachedToAnchor:buttonBoundsDynamicItem.center];
    [attachmentBehavior setFrequency:2.0];
    [attachmentBehavior setDamping:0.3];
    [animator addBehavior:attachmentBehavior];
    
    UIPushBehavior *pushBehavior = [[UIPushBehavior alloc] initWithItems:@[buttonBoundsDynamicItem] mode:UIPushBehaviorModeInstantaneous];
    pushBehavior.angle = M_PI_4;
    pushBehavior.magnitude = 2.0;
    [animator addBehavior:pushBehavior];
    
    [pushBehavior setActive:TRUE];
    
    self.animator = animator;
}

@end