Example using pan gestures and push collision

push_collision.m

UIView *view = [panGesture view];

if (panGesture.state == UIGestureRecognizerStateBegan) {
    self.collectionView.panGestureRecognizer.enabled = NO;
    self.offsetY = view.frame.origin.y;
} else if (panGesture.state == UIGestureRecognizerStateBegan || panGesture.state == UIGestureRecognizerStateChanged)
{
    CGPoint translation = [panGesture translationInView:[view superview]];
    if (view.frame.origin.y <= 0 && translation.y < 0) {
        return;
    }
    [view setCenter:CGPointMake([view center].x, [view center].y + translation.y)];
    [panGesture setTranslation:CGPointZero inView:view];
} else if (panGesture.state == UIGestureRecognizerStateEnded) {
    CGPoint velocity = [panGesture velocityInView:view];
    UIPushBehavior *pushBehavior = [[UIPushBehavior alloc] init];
    UICollisionBehavior *collision = [[UICollisionBehavior alloc] init];
    collision.collisionDelegate = self;
    [collision addItem:self.collectionView];

    if (self.offsetY < view.frame.origin.y) {
        pushBehavior.pushDirection = CGVectorMake(0, 1.0);
        [collision addBoundaryWithIdentifier:@"max" fromPoint:CGPointMake(0, 2*self.view.frame.size.height + 1) toPoint:CGPointMake(self.view.frame.size.width, 2*self.view.frame.size.height + 1)];
    } else {
        pushBehavior.pushDirection = CGVectorMake(0, -1.0);
        [collision addBoundaryWithIdentifier:@"min" fromPoint:CGPointMake(0, 0) toPoint:CGPointMake(self.view.frame.size.width, 0)];
    }
    [pushBehavior addItem:self.collectionView];

    pushBehavior.magnitude = fabs(velocity.y);

    [self.animator addBehavior:pushBehavior];
    [self.animator addBehavior:collision];
    [self.animator updateItemUsingCurrentState:self.collectionView];

    self.collectionView.panGestureRecognizer.enabled = YES;
}