gistfile1.m

- (CGPoint)targetContentOffsetForProposedContentOffset:(CGPoint)proposedContentOffset withScrollingVelocity:(CGPoint)velocity
{
	CGFloat offSetAdjustment = MAXFLOAT;
	CGFloat horizontalCenter = (CGFloat) (proposedContentOffset.x + (self.collectionView.bounds.size.width / 2.0));

	CGRect targetRect = CGRectMake(proposedContentOffset.x, 0.0, self.collectionView.bounds.size.width, self.collectionView.bounds.size.height);

	NSArray *array = [self layoutAttributesForElementsInRect:targetRect];
	for (UICollectionViewLayoutAttributes *layoutAttributes in array)
	{
		if(layoutAttributes.representedElementCategory == UICollectionElementCategoryCell)
		{
			CGFloat itemHorizontalCenter = layoutAttributes.center.x;
			if (ABS(itemHorizontalCenter - horizontalCenter) < ABS(offSetAdjustment))
			{
				offSetAdjustment = itemHorizontalCenter - horizontalCenter;
			}
		}
	}

	CGFloat nextOffset = proposedContentOffset.x + offSetAdjustment;

	do {
		proposedContentOffset.x = nextOffset;
		CGFloat deltaX = proposedContentOffset.x - self.collectionView.contentOffset.x;
		CGFloat velX = velocity.x;

		if(deltaX == 0.0 || velX == 0 || (velX > 0.0 && deltaX > 0.0) || (velX < 0.0 && deltaX < 0.0))
		{
			break;
		}

		if(velocity.x > 0.0)
		{
			nextOffset += [self snapStep];
		}
		else if(velocity.x < 0.0)
		{
			nextOffset -= [self snapStep];
		}
	} while ([self isValidOffset:nextOffset]);

	proposedContentOffset.y = 0.0;
	
	return proposedContentOffset;
}

- (BOOL)isValidOffset:(CGFloat)offset
{
    return (offset >= [self minContentOffset] && offset <= [self maxContentOffset]);
}

- (CGFloat)minContentOffset
{
    return -self.collectionView.contentInset.left;
}

- (CGFloat)maxContentOffset
{
	return [self minContentOffset] + self.collectionView.contentSize.width - self.itemSize.width;
}

- (CGFloat)snapStep
{
	return self.itemSize.width + self.minimumLineSpacing;
}