Cells order in UICollectionView

Question

I need to create a UICollectionView with cells of different sizes (1x1, 2x2, 2x1, 3x2.5). I've already code to add cells depending on which size they are using collectionView:layout:sizeForItemAtIndexPath:.

Here is expected result (cells 1 and 3 have to be on the left of cell 2) :

But current result is :

My (ugly) current code is (self.cellSize = screen width / 3) :

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {

    CGFloat size = self.cellSize;

    if (indexPath.item == 0) {
        return CGSizeMake(self.cellSize * 3.f, self.cellSize * 2.5f);
    }

    if (indexPath.item == 1) {
        return CGSizeMake(self.cellSize, self.cellSize);
    }

    if (indexPath.item == 2) {
        return CGSizeMake(self.cellSize * 2.f, self.cellSize * 2.f);
    }

    if (indexPath.item == 3) {
        return CGSizeMake(self.cellSize, self.cellSize);
    }

    if (indexPath.item == 4) {
        return CGSizeMake(self.cellSize * 2.f, self.cellSize);
    }

    if (indexPath.item == 5) {
        return CGSizeMake(self.cellSize, self.cellSize);
    }

    if (indexPath.item == 6) {
        return CGSizeMake(self.cellSize, self.cellSize);
    }

    if (indexPath.item == 7) {
        return CGSizeMake(self.cellSize * 2.f, self.cellSize);
    }

    if (indexPath.item == 8) {
        return CGSizeMake(self.cellSize * 3.f, self.cellSize * 2.5f);
    }

    return CGSizeMake(size, size);
}

Is there a way to specify that cell 3 has to be above cell 1, but on left of cell 2 ?

I don't want to do a static layout that will not change because each cell could be of different size in future. For example, cell 2 could be 2x1 sized ...

Which is the best approach to do that ? I was expecting to specify to UICollectionViewLayout a flow (like "from top"), but it doesn't work like that ...

Answer 1

A sample example by subclassing UICollectionViewLayout. All values are hard coded, just to explicit the logic behind it. Of course, that could be optimized.

@interface CustomCollectionViewLayout ()

@property (nonatomic, strong) NSMutableDictionary *cellLayouts;
@property (nonatomic, assign) CGSize              unitSize;

@end

@implementation CustomCollectionViewLayout


-(id)initWithSize:(CGSize)size
{
  self = [super init];
  if (self)
  {
    _unitSize = CGSizeMake(size.width/3,150);
    _cellLayouts = [[NSMutableDictionary alloc] init];
  }
  return self;
}
-(void)prepareLayout
{

  for (NSInteger i = 0; i < [[self collectionView] numberOfItemsInSection:0]; i ++)
  {
    NSIndexPath *indexPath = [NSIndexPath indexPathForItem:i inSection:0];
    UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
    CGRect frame;
    switch ([indexPath item])
    {
      case 0:
        frame = CGRectMake(0, 0, _unitSize.width*3, _unitSize.height*2.5);
        break;
      case 1:
        frame = CGRectMake(0, _unitSize.height*2.5, _unitSize.width, _unitSize.height);
        break;
      case 2:
        frame = CGRectMake(_unitSize.width, _unitSize.height*2.5, _unitSize.width*2, _unitSize.height*2);
        break;
      case 3:
        frame = CGRectMake(0, _unitSize.height*2.5+_unitSize.height, _unitSize.width, _unitSize.height);
        break;
      case 4:
        frame = CGRectMake(0, _unitSize.height*2.5+_unitSize.height+_unitSize.height, _unitSize.width*2, _unitSize.height);
        break;
      case 5:
        frame = CGRectMake(_unitSize.width*2, _unitSize.height*2.5+_unitSize.height+_unitSize.height, _unitSize.width, _unitSize.height);
        break;
      case 6:
        frame = CGRectMake(0, _unitSize.height*2.5+_unitSize.height+_unitSize.height+_unitSize.height, _unitSize.width, _unitSize.height);
        break;
      case 7:
        frame = CGRectMake(_unitSize.width, _unitSize.height*2.5+_unitSize.height+_unitSize.height+_unitSize.height, _unitSize.width*2, _unitSize.height);
        break;
      case 8:
        frame = CGRectMake(0, _unitSize.height*2.5+_unitSize.height+_unitSize.height+_unitSize.height+_unitSize.height, _unitSize.width*3, _unitSize.height*2.5);
        break;
      default:
        frame = CGRectZero;
        break;
    }
    [attributes setFrame:frame];
    [[self cellLayouts] setObject:attributes forKey:indexPath];
  }
}

-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
  NSMutableArray *retAttributes = [[NSMutableArray alloc] init];
  for (NSIndexPath *anIndexPath in [self cellLayouts])
  {
    UICollectionViewLayoutAttributes *attributes = [self cellLayouts][anIndexPath];
    if (CGRectIntersectsRect(rect, [attributes frame]))
    {
      [retAttributes addObject:attributes];
    }
  }
  return retAttributes;
}

-(UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{

  return [self cellLayouts][indexPath];
}

-(CGSize)collectionViewContentSize
{
  return CGSizeMake(_unitSize.width*3, _unitSize.height*9);
}

@end

Then, you just have to call :

CustomCollectionViewLayout *layout = [[CustomCollectionViewLayout alloc] initWithSize:self.myCollectionView.bounds.frame.size];
[self.myCollectionView setCollectionViewLayout:layout];

Rendering :

Answer 2

Here it's the same logic as Larme suggested. But with less hardcode and which give you the possibility of setting the number of items you want without adding new case:.

I call "pattern" a set of 5 items. So first I define constant values:

#define MAX_COLUMN              3 // Max columns in the pattern
#define MAX_LINE_PER_PATTERN    3 // Max lines in the pattern
#define PATTERN_ITEM_COUNT      5 // Max items in the pattern

Then I create a custom layout with 2 properties NSMutableArray *layoutAttributes and CGFloat contentHeight in which I need to override the methods:

- (void)prepareLayout{
    [super prepareLayout];

    if (!self.layoutAttributes){
        self.layoutAttributes = [[NSMutableArray alloc] init];

        CGFloat cellWidth = self.collectionView.frame.size.width / MAX_COLUMN;
        CGFloat cellHeight = cellWidth;
        self.contentHeight = 0.f;

        for (int item = 0 ; item < [self.collectionView numberOfItemsInSection:0] ; item ++){

            CGFloat width, height = 0.f;
            CGFloat xPos, yPos = 0.f;

            NSInteger patternCount = (NSInteger)((CGFloat)item / (CGFloat)PATTERN_ITEM_COUNT);
            NSInteger currentIndex = item % PATTERN_ITEM_COUNT;
            switch (currentIndex) {
                case 0:
                {
                    xPos = 0.f;
                    yPos = 0.f + MAX_LINE_PER_PATTERN * cellHeight * patternCount;
                    width = cellWidth;
                    height = cellHeight;
                    self.contentHeight += cellHeight;
                    break;
                }
                case 1:
                {
                    xPos = cellWidth;
                    yPos = 0.f + MAX_LINE_PER_PATTERN * cellHeight * patternCount;
                    width = cellWidth * 2.f;
                    height = cellHeight * 2.f;
                    self.contentHeight += cellHeight;
                    break;
                }
                case 2:
                {
                    xPos = 0.f;
                    yPos = cellHeight + MAX_LINE_PER_PATTERN * cellHeight * patternCount;
                    width = cellWidth;
                    height = cellHeight;
                    break;
                }
                case 3:
                {
                    xPos = 0.f;
                    yPos = 2.f * cellHeight + MAX_LINE_PER_PATTERN * cellHeight * patternCount;
                    width = cellWidth * 2.f;
                    height = cellHeight;
                    self.contentHeight += cellHeight;
                    break;
                }
                case 4:
                {
                    xPos = 2.f * cellWidth;
                    yPos = 2.f * cellHeight + MAX_LINE_PER_PATTERN * cellHeight * patternCount;
                    width = cellWidth;
                    height = cellHeight;
                    break;
                }
                default:
                    NSLog(@"error with index");
                    break;
            }
            UICollectionViewLayoutAttributes *attr = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:[NSIndexPath indexPathForRow:item inSection:0]];
            attr.frame = CGRectMake(xPos,
                                    yPos,
                                    width,
                                    height);
            [self.layoutAttributes addObject:attr];
        }
    }
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect{
    NSMutableArray *currentAttributes = [NSMutableArray new];
    for (UICollectionViewLayoutAttributes *attr in self.layoutAttributes) {
        if (CGRectIntersectsRect(attr.frame, rect))
        {
            [currentAttributes addObject:attr];
        }
    }
    return currentAttributes;
}

- (CGSize)collectionViewContentSize{
    return CGSizeMake(self.collectionView.frame.size.width, self.contentHeight);
}

Then assign this custom layout to self.collectionView.collectionViewLayout and that's it. You can find more information and swift version here.