Change the sequence of uicollectionviewcell

Question

I am trying to change the sequence to cells rendered in collectionview. By default in vertical layout collectionview cells are displayed in

1 2 3
4 5 6
7 8 9

What i am trying to do is change this sequence in following ways

1  2  3
6  5  4      <----- this order is reversed
7  8  9
12 11 10     <----- this order is reversed

Can any one tell me how do i achieve this kind of behaviour.

So far i have implemented UICollectionViewFlowLayout and trying to manipulate its methods but got no success

Note: All items must be loaded in one section only

Answer 1

I found the solution by inheriting UICollectionViewLayout and calculated the desired frame for each item. in prepareLayout method.

Still there are scope for improvement in below code.

-(void)prepareLayout{
    NSInteger itemCount = [self.collectionView numberOfItemsInSection:0];
    NSMutableDictionary *layoutAttr = [NSMutableDictionary dictionary];
    CGFloat y=0,x=0;
    for (int i = 0; i<itemCount; i++) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
        UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
        CGRect frame;
        if (indexPath.row == 0) {
            x= 0;
            y=0;
            frame = CGRectMake(x,  y, self.itemSize.width, self.itemSize.height);
            x += self.itemSize.width;
        }else if (indexPath.row % 2 == 0 ) {
            y += self.itemSize.height;
            x -=self.itemSize.width;
            frame = CGRectMake(x,  y, self.itemSize.width, self.itemSize.height);
            if (x <= 0) {
                x += self.itemSize.width;
            }else{
                x -= self.itemSize.width;
            }
        }else{
            frame = CGRectMake(x,  y, self.itemSize.width, self.itemSize.height);
            x += self.itemSize.width;
        }
        attributes.frame=frame;

        attributes.zIndex = i;
        layoutAttr[indexPath] = attributes;
    }
    layoutAttributes=layoutAttr;
}