Left Align Cells in UICollectionView

Question

I am using a UICollectionView in my project, where there are multiple cells of differing widths on a line. According to: https://developer.apple.com/library/content/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/UsingtheFlowLayout/UsingtheFlowLayout.html

it spreads the cells out across the line with equal padding. This happens as expected, except I want to left justify them, and hard code a padding width.

I figure I need to subclass UICollectionViewFlowLayout, however after reading some of the tutorials etc online I just don't seem to get how this works.

Answer 1

Based on all answers, I change a bit and it works good for me

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    let attributes = super.layoutAttributesForElements(in: rect)

    var leftMargin = sectionInset.left
    var maxY: CGFloat = -1.0


    attributes?.forEach { layoutAttribute in
        if layoutAttribute.frame.origin.y >= maxY
                   || layoutAttribute.frame.origin.x == sectionInset.left {
            leftMargin = sectionInset.left
        }

        if layoutAttribute.frame.origin.x == sectionInset.left {
            leftMargin = sectionInset.left
        }
        else {
            layoutAttribute.frame.origin.x = leftMargin
        }

        leftMargin += layoutAttribute.frame.width
        maxY = max(layoutAttribute.frame.maxY, maxY)
    }

    return attributes
}

Answer 2

The question has been up a while but there's no answer and it's a good question. The answer is to override one method in the UICollectionViewFlowLayout subclass:

@implementation MYFlowLayoutSubclass

//Note, the layout's minimumInteritemSpacing (default 10.0) should not be less than this. 
#define ITEM_SPACING 10.0f

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {

    NSArray *attributesForElementsInRect = [super layoutAttributesForElementsInRect:rect];
    NSMutableArray *newAttributesForElementsInRect = [[NSMutableArray alloc] initWithCapacity:attributesForElementsInRect.count];

    CGFloat leftMargin = self.sectionInset.left; //initalized to silence compiler, and actaully safer, but not planning to use.

    //this loop assumes attributes are in IndexPath order
    for (UICollectionViewLayoutAttributes *attributes in attributesForElementsInRect) {
        if (attributes.frame.origin.x == self.sectionInset.left) {
            leftMargin = self.sectionInset.left; //will add outside loop
        } else {
            CGRect newLeftAlignedFrame = attributes.frame;
            newLeftAlignedFrame.origin.x = leftMargin;
            attributes.frame = newLeftAlignedFrame;
        }

        leftMargin += attributes.frame.size.width + ITEM_SPACING;
        [newAttributesForElementsInRect addObject:attributes];
    }   

    return newAttributesForElementsInRect;
}

@end

As recommended by Apple, you get the layout attributes from super and iterate over them. If it's the first in the row (defined by its origin.x being on the left margin), you leave it alone and reset the x to zero. Then for the first cell and every cell, you add the width of that cell plus some margin. This gets passed to the next item in the loop. If it's not the first item, you set it's origin.x to the running calculated margin, and add new elements to the array.

Answer 3

The problem with UICollectionView is that it tries to automatically fit the cells in the available area. I have done this by first defining number of rows and columns and then defining the cell size for that row and column

1) To define Sections (Rows) of my UICollectionView:

(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView

2) To define number of items in a section. You can define different number of items for every section. you can get section number using 'section' parameter.

(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section

3) To define Cell size for each section and row separately. You can get section number and row number using the 'indexPath' parameter i.e. [indexPath section] for section number and [indexPath row] for row number.

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

4) Then you can display your cells in rows and sections using:

(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath

NOTE: In UICollectionView

Section == Row
IndexPath.Row == Column

Answer 4

Mike Sand's answer is good but i had experienced some issues with this code (Like lengthy cell clipped out). And new code:

#define ITEM_SPACE 7.0f

@implementation LeftAlignedCollectionViewFlowLayout
- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray* attributesToReturn = [super layoutAttributesForElementsInRect:rect];
    for (UICollectionViewLayoutAttributes* attributes in attributesToReturn) {
        if (nil == attributes.representedElementKind) {
            NSIndexPath* indexPath = attributes.indexPath;
            attributes.frame = [self layoutAttributesForItemAtIndexPath:indexPath].frame;
        }
    }
    return attributesToReturn;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes* currentItemAttributes =
    [super layoutAttributesForItemAtIndexPath:indexPath];

    UIEdgeInsets sectionInset = [(UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout sectionInset];

    if (indexPath.item == 0) { // first item of section
        CGRect frame = currentItemAttributes.frame;
        frame.origin.x = sectionInset.left; // first item of the section should always be left aligned
        currentItemAttributes.frame = frame;

        return currentItemAttributes;
    }

    NSIndexPath* previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item-1 inSection:indexPath.section];
    CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
    CGFloat previousFrameRightPoint = previousFrame.origin.x + previousFrame.size.width + ITEM_SPACE;

    CGRect currentFrame = currentItemAttributes.frame;
    CGRect strecthedCurrentFrame = CGRectMake(0,
                                              currentFrame.origin.y,
                                              self.collectionView.frame.size.width,
                                              currentFrame.size.height);

    if (!CGRectIntersectsRect(previousFrame, strecthedCurrentFrame)) { // if current item is the first item on the line
        // the approach here is to take the current frame, left align it to the edge of the view
        // then stretch it the width of the collection view, if it intersects with the previous frame then that means it
        // is on the same line, otherwise it is on it's own new line
        CGRect frame = currentItemAttributes.frame;
        frame.origin.x = sectionInset.left; // first item on the line should always be left aligned
        currentItemAttributes.frame = frame;
        return currentItemAttributes;
    }

    CGRect frame = currentItemAttributes.frame;
    frame.origin.x = previousFrameRightPoint;
    currentItemAttributes.frame = frame;
    return currentItemAttributes;
}

Answer 5

The other solutions in here not work properly when the line is composed by only 1 item or are over complicated.

Based on the example given by Ryan, I changed the code to detect a new line by inspecting the Y position of the new element. Very simple and quick in performance.

Swift:

class LeftAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)

        var leftMargin = sectionInset.left
        var maxY: CGFloat = -1.0
        attributes?.forEach { layoutAttribute in
            if layoutAttribute.frame.origin.y >= maxY {
                leftMargin = sectionInset.left
            }

            layoutAttribute.frame.origin.x = leftMargin

            leftMargin += layoutAttribute.frame.width + minimumInteritemSpacing
            maxY = max(layoutAttribute.frame.maxY , maxY)
        }

        return attributes
    }
}

If you want to have supplementary views keep their size, add the following at the top of the closure in the forEach call:

guard layoutAttribute.representedElementCategory == .cell else {
    return
}

Objective-C:

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray *attributes = [super layoutAttributesForElementsInRect:rect];

    CGFloat leftMargin = self.sectionInset.left; //initalized to silence compiler, and actaully safer, but not planning to use.
    CGFloat maxY = -1.0f;

    //this loop assumes attributes are in IndexPath order
    for (UICollectionViewLayoutAttributes *attribute in attributes) {
        if (attribute.frame.origin.y >= maxY) {
            leftMargin = self.sectionInset.left;
        }

        attribute.frame = CGRectMake(leftMargin, attribute.frame.origin.y, attribute.frame.size.width, attribute.frame.size.height);

        leftMargin += attribute.frame.size.width + self.minimumInteritemSpacing;
        maxY = MAX(CGRectGetMaxY(attribute.frame), maxY);
    }

    return attributes;
}