TPSpringyModalTransition - modal transition that bounces off the top like a spring

TPSpringyModalTransition.h

//  TPSpringyModalTransition.h
//  iPeru
//
//  Created by Pietro Rea on 5/4/14.
//  Copyright (c) 2014 Pietro Rea. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface TPSpringyModalTransition : NSObject <UIViewControllerAnimatedTransitioning>

@property (assign, nonatomic) BOOL presenting;

@end

TPSpringyModalTransition.m

//
//  TPSpringyModalTransition.m
//  iPeru
//
//  Created by Pietro Rea on 5/4/14.
//  Copyright (c) 2014 Pietro Rea. All rights reserved.
//

#import "TPSpringyModalTransition.h"
#import <FrameAccessor/FrameAccessor.h>

#define kHeightDelta 30

@implementation TPSpringyModalTransition

- (NSTimeInterval)transitionDuration:(id <UIViewControllerContextTransitioning>)transitionContext
{
    return 0.45;
}

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    UIViewController *presentingVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *presentedVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    if (self.presenting) {
        
        [transitionContext.containerView addSubview:presentingVC.view];
        [transitionContext.containerView addSubview:presentedVC.view];
        
        presentedVC.view.height += kHeightDelta;
        presentedVC.view.top += presentedVC.view.height;
        
        [UIView animateWithDuration:[self transitionDuration:transitionContext]
                              delay:0.0
             usingSpringWithDamping:0.70
              initialSpringVelocity:0.9
                            options:0
                         animations:^{
                
                             presentedVC.view.top = presentingVC.view.top;
                             
                         } completion:^(BOOL finished) {
                             
                             [transitionContext completeTransition:YES];
                             presentedVC.view.height -= kHeightDelta;
                             
                         }];
        
        
    } else {
        
        [UIView animateWithDuration:[self transitionDuration:transitionContext] animations:^{
            
            presentingVC.view.frame = CGRectOffset(presentingVC.view.frame, 0, presentingVC.view.frame.size.height);
            
        } completion:^(BOOL finished) {
            [transitionContext.containerView addSubview:presentingVC.view];
            [transitionContext.containerView addSubview:presentedVC.view];
            [transitionContext completeTransition:YES];
        }];
        
    }
}

@end