Animated transitions and UIKit Dynamics

DismissGravity.h

@interface DismissGravity : NSObject <UIViewControllerAnimatedTransitioning>

@property (strong, nonatomic) UIDynamicAnimator *animator;

DismissGravity.m

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext;
{
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:[transitionContext containerView]];
    UIGravityBehavior *gravity = [[UIGravityBehavior alloc] initWithItems:@[fromVC.view]];
    
    gravity.gravityDirection = CGVectorMake(0, -12);
    
    [self.animator addBehavior:grabivty];
    
    gravity.action = ^{
        if (!CGRectIntersectsRect(fromVC.view.frame, [[transitionContext containerView] frame]
        )) {
            [self.animator removeAllBehaviors];
            [transitionContext completeTransition:YES];
        }
    }
}

- (void)

DropBounce.h

@interface DropBounce: NSObject <UIViewControllerAnimatedTransitioning>

@property (strong, nonatomic) UIDynamicAnimator *animator;
@property (strong, nonatomic) id<UIViewControllerContextTransitioning> transitionContext;

@end

DropBounce.m

- (void) animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    self.animateTransition = transitionContext;
    
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    
    CGRect fromVCStartFrame = [transitionContext initialFrameForViewController:fromVC];
    CGRect toVCStartFrame = CGRectMake(0,
        -CGRectGetHeight(startFrame),
        CGRectGetWidth(startFrame),
        CGRectGetHeight(startFrame));
    toVC.view.frame = toVCStartFrame;
    
    [[transitionContext containerView] addSubview:toVC.view];
    
    // We use the [transitionContext containerView] for the container here.
    // That way, the frame we refer to stays the same, regardless of what the toVC or fromVC are doing.
    self.animator = [[UIDynamicAnimator alloc] initWithContainer:[transitionContext containerView]];
    
    UIGravityBehavior *gravity =[[UIGravityBehavior alloc] initWithItems:@[toVC.view]];
    gravity.gravityDirection = CGVectorMake(0, 3);
    [self.animator addBehavior:gravity];
    
    // Now adding a boundary to just the bottom of the contianer view:
    UICollisionBehavior *boundary = [[UICollisionBehavior alloc] initWithItems:@[toVC.view];
    [boundary addBoundaryWithIdentifier@"bottomEdge"
        fromPoint:CGPointMake(0,
            CGRectGetMaxY(fromVC.view.frame))
        toPoint:CGPointMake(
            CGRectGetMaxX(fromVC.view.frame),
            CGRectGetMaxY(fromVC.view.frame),
    )];
    
    [self.animator addBehavior:boundary];
    self.animator.delegate = self;
}

- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator
{
    [self.transitionContext completeTransition:YES];
}

RootVC.m

-(void)showModal:(id)sender
{
    ModalViewController *modalVC = [[ModalViewController alloc] init];
    
    modalVC.modalPresentationStyle = UIPresentationCustom;
    settingsVC.transitioningDelegate = self;
    
    [self presentViewController:modalVC animated:YES completion:^{
        UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleTap:)];
        [modalVC.hideButton addGestureRecognizer:tapGesture];
    }];
}

-(id<UIViewControllerAnimatedTransitioning>) animatedViewControllerForPresentedController:(UIViewController *) presented
  presentingController:(UIViewController *)presenting
  sourceController:(UIViewController *)source
{
    // for presentation:
    
    DropBounce *dropBounce = [[DropBounce alloc] init];
    return DropBounce;
}

- (id <UIViewControllerAnimatedTransitioning>) animationControllerForDismissedController:(UIViewController *)dismissed
{
    // animation for dismissal:
    DismissGravity *dismissGravity = [[DismissGravity alloc] init];
    return dismissGravity;
}