IOS7ScrollViews/iOS7ScrollViews/SVSpringyFlowLayout.m

//
//  SVSpringyFlowLayout.m
//  iOS7ScrollViews
//
//  Created by Pierre Felgines on 20/06/13.
//  Copyright (c) 2013 Pierre Felgines. All rights reserved.
//

#import "SVSpringyFlowLayout.h"

@implementation SVSpringyFlowLayout {
    UIDynamicAnimator * _dynamicAnimator;
}

- (void)prepareLayout {
    [super prepareLayout];
    
    if (!_dynamicAnimator) {
        _dynamicAnimator = [[UIDynamicAnimator alloc] initWithCollectionViewLayout:self];
        
        CGSize contentSize = [self collectionViewContentSize];
        NSArray * items = [super layoutAttributesForElementsInRect:CGRectMake(0, 0, contentSize.width, contentSize.height)];
        
        for (UICollectionViewLayoutAttributes * item in items) {
            UIAttachmentBehavior * spring = [[UIAttachmentBehavior alloc] initWithItem:item attachedToAnchor:[item center]];
            spring.length = 0;
            spring.damping = 0.5f;
            spring.frequency = 0.8f;
            
            [_dynamicAnimator addBehavior:spring];
        }
    }
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    return [_dynamicAnimator itemsInRect:rect];
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    return [_dynamicAnimator layoutAttributesForCellAtIndexPath:indexPath];
}

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds {
    UIScrollView * scrollView = self.collectionView;
    CGFloat scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y;
    CGPoint touchLocation = [scrollView.panGestureRecognizer locationInView:scrollView];
    
    for (UIAttachmentBehavior * spring in _dynamicAnimator.behaviors) {
        CGPoint anchorPoint = spring.anchorPoint;
        CGFloat distanceFromTouch = fabsf(touchLocation.y - anchorPoint.y);
        CGFloat scrollResistance = distanceFromTouch / 500.0f;
        
        UICollectionViewLayoutAttributes * item = [spring.items firstObject];
        CGPoint center = item.center;
        
        /* Original Code from Apple in WWDC video */
//        center.y += MIN(scrollDelta, scrollDelta * scrollResistance);
        
        /* Correction */
        center.y += MIN(fabs(scrollDelta), fabs(scrollDelta * scrollResistance)) * scrollDelta / fabs(scrollDelta);
        item.center = center;
        
        [_dynamicAnimator updateItemFromCurrentState:item];
    }
    
    return NO;
}

@end