animateTransition, Top and bottom halves slide to reveal next view underneath.

gistfile1.m

- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    // Get transition context views and things
    UIViewController *fromVC = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIView *fromView = fromVC.view;

    UIViewController *toVC = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    UIView *toView = toVC.view;
    
    //here is where you do the transition
    UIView *inView = [transitionContext containerView];
    
    // Create snapshot views of the halfs of the current view
    CGRect topHalf, bottomHalf;
    CGRectDivide(fromView.bounds, &topHalf, &bottomHalf,
                 fromView.bounds.size.height/2.0, CGRectMinYEdge);
    UIView *topHalfView = [fromView resizableSnapshotViewFromRect:topHalf
                                               afterScreenUpdates:NO
                                                    withCapInsets:UIEdgeInsetsZero];
    UIView *bottomHalfView = [fromView resizableSnapshotViewFromRect:bottomHalf
                                               afterScreenUpdates:NO
                                                    withCapInsets:UIEdgeInsetsZero];
    bottomHalfView.frame = bottomHalf;
    
    // Add the split views and destination view to the container
    [inView addSubview:toView];
    [inView addSubview:topHalfView];
    [inView addSubview:bottomHalfView];
    
    [UIView animateKeyframesWithDuration:1.0 delay:0.0 options:0 animations:^{
        [UIView addKeyframeWithRelativeStartTime:0.0 relativeDuration:0.5 animations:^{
            topHalfView.center = CGPointMake(0.0, topHalfView.center.y);
            bottomHalfView.center = CGPointMake(bottomHalf.size.width,
                                             bottomHalfView.center.y);

        }];
        [UIView addKeyframeWithRelativeStartTime:0.5 relativeDuration:0.5 animations:^{
            topHalfView.center = CGPointMake(0.0,
                                             inView.bounds.size.height + topHalf.size.height/2.0);
            bottomHalfView.center = CGPointMake(bottomHalf.size.width, -bottomHalf.size.height/2.0);
        }];
    } completion:^(BOOL finished) {
        [topHalfView removeFromSuperview];
        [bottomHalfView removeFromSuperview];
        
        [transitionContext completeTransition:YES];
    }];
}