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

In swift. According to Michaels answer

override func layoutAttributesForElementsInRect(rect: CGRect) ->     [UICollectionViewLayoutAttributes]? {
    guard let oldAttributes = super.layoutAttributesForElementsInRect(rect) else {
        return super.layoutAttributesForElementsInRect(rect)
    }
    let spacing = CGFloat(50) // REPLACE WITH WHAT SPACING YOU NEED
    var newAttributes = [UICollectionViewLayoutAttributes]()
    var leftMargin = self.sectionInset.left
    for attributes in oldAttributes {
        if (attributes.frame.origin.x == self.sectionInset.left) {
            leftMargin = self.sectionInset.left
        } else {
            var newLeftAlignedFrame = attributes.frame
            newLeftAlignedFrame.origin.x = leftMargin
            attributes.frame = newLeftAlignedFrame
        }

        leftMargin += attributes.frame.width + spacing
        newAttributes.append(attributes)
    }
    return newAttributes
}

Answer 2

The above code works for me. I would like to share the respective Swift 3.0 code.

class SFFlowLayout: UICollectionViewFlowLayout {

    let itemSpacing: CGFloat = 3.0

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        let attriuteElementsInRect = super.layoutAttributesForElements(in: rect)
        var newAttributeForElement: Array<UICollectionViewLayoutAttributes> = []
        var leftMargin = self.sectionInset.left
        for tempAttribute in attriuteElementsInRect! {
            let attribute = tempAttribute 
            if attribute.frame.origin.x == self.sectionInset.left {
                leftMargin = self.sectionInset.left
            }
            else {
                var newLeftAlignedFrame = attribute.frame
                newLeftAlignedFrame.origin.x = leftMargin
                attribute.frame = newLeftAlignedFrame
            }
            leftMargin += attribute.frame.size.width + itemSpacing
            newAttributeForElement.append(attribute)
        }
        return newAttributeForElement
    }
}

Answer 3

Based on answers here, but fixed crashes and aligning problems when your collection view is also got headers or footers. Aligning left only cells:

class LeftAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {

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

        var leftMargin = sectionInset.left
        var prevMaxY: CGFloat = -1.0
        attributes?.forEach { layoutAttribute in

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

            if layoutAttribute.frame.origin.y >= prevMaxY {
                leftMargin = sectionInset.left
            }

            layoutAttribute.frame.origin.x = leftMargin

            leftMargin += layoutAttribute.frame.width + minimumInteritemSpacing
            prevMaxY = layoutAttribute.frame.maxY
        }

        return attributes
    }
}

Answer 4

Thanks for the Michael Sand's answer. I modified it to a solution of multiple rows (the same alignment Top y of each row) that is Left Alignment, even spacing to each item.

static CGFloat const ITEM_SPACING = 10.0f;

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    CGRect contentRect = {CGPointZero, self.collectionViewContentSize};

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

    CGFloat leftMargin = self.sectionInset.left; //initalized to silence compiler, and actaully safer, but not planning to use.
    NSMutableDictionary *leftMarginDictionary = [[NSMutableDictionary alloc] init];

    for (UICollectionViewLayoutAttributes *attributes in attributesForElementsInRect) {
        UICollectionViewLayoutAttributes *attr = attributes.copy;

        CGFloat lastLeftMargin = [[leftMarginDictionary valueForKey:[[NSNumber numberWithFloat:attributes.frame.origin.y] stringValue]] floatValue];
        if (lastLeftMargin == 0) lastLeftMargin = leftMargin;

        CGRect newLeftAlignedFrame = attr.frame;
        newLeftAlignedFrame.origin.x = lastLeftMargin;
        attr.frame = newLeftAlignedFrame;

        lastLeftMargin += attr.frame.size.width + ITEM_SPACING;
        [leftMarginDictionary setObject:@(lastLeftMargin) forKey:[[NSNumber numberWithFloat:attributes.frame.origin.y] stringValue]];
        [newAttributesForElementsInRect addObject:attr];
    }

    return newAttributesForElementsInRect;
}

Answer 5

Building on Michael Sand's answer, I created a subclassed UICollectionViewFlowLayout library to do left, right, or full (basically the default) horizontal justification—it also lets you set the absolute distance between each cell. I plan on adding horizontal center justification and vertical justification to it, too.

https://github.com/eroth/ERJustifiedFlowLayout

Answer 6

With Swift 4.1 and iOS 11, according to your needs, you may choose one of the 2 following complete implementations in order to fix your problem.

---

#1. Left align autoresizing UICollectionViewCells

The implementation below shows how to use UICollectionViewLayout's layoutAttributesForElements(in:), UICollectionViewFlowLayout's estimatedItemSize and UILabel's preferredMaxLayoutWidth in order to left align autoresizing cells in a UICollectionView:

CollectionViewController.swift

import UIKit

class CollectionViewController: UICollectionViewController {

    let array = ["1", "1 2", "1 2 3 4 5 6 7 8", "1 2 3 4 5 6 7 8 9 10 11", "1 2 3", "1 2 3 4", "1 2 3 4 5 6", "1 2 3 4 5 6 7 8 9 10", "1 2 3 4", "1 2 3 4 5 6 7", "1 2 3 4 5 6 7 8 9", "1", "1 2 3 4 5", "1", "1 2 3 4 5 6"]

    let columnLayout = FlowLayout(
        minimumInteritemSpacing: 10,
        minimumLineSpacing: 10,
        sectionInset: UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    )

    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView?.collectionViewLayout = columnLayout
        collectionView?.contentInsetAdjustmentBehavior = .always
        collectionView?.register(CollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return array.count
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CollectionViewCell
        cell.label.text = array[indexPath.row]
        return cell
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        collectionView?.collectionViewLayout.invalidateLayout()
        super.viewWillTransition(to: size, with: coordinator)
    }

}

FlowLayout.swift

import UIKit

class FlowLayout: UICollectionViewFlowLayout {

    required init(minimumInteritemSpacing: CGFloat = 0, minimumLineSpacing: CGFloat = 0, sectionInset: UIEdgeInsets = .zero) {
        super.init()

        estimatedItemSize = UICollectionViewFlowLayoutAutomaticSize
        self.minimumInteritemSpacing = minimumInteritemSpacing
        self.minimumLineSpacing = minimumLineSpacing
        self.sectionInset = sectionInset
        sectionInsetReference = .fromSafeArea
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let layoutAttributes = super.layoutAttributesForElements(in: rect)!.map { $0.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ $0.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { ($0.center.y / 10).rounded(.up) * 10 }) {
            // Set the initial left inset
            var leftInset = sectionInset.left

            // Loop on cells to adjust each cell's origin and prepare leftInset for the next cell
            for attribute in attributes {
                attribute.frame.origin.x = leftInset
                leftInset = attribute.frame.maxX + minimumInteritemSpacing
            }
        }

        return layoutAttributes
    }

}

CollectionViewCell.swift

import UIKit

class CollectionViewCell: UICollectionViewCell {

    let label = UILabel()

    override init(frame: CGRect) {
        super.init(frame: frame)

        contentView.backgroundColor = .orange
        label.preferredMaxLayoutWidth = 120
        label.numberOfLines = 0

        contentView.addSubview(label)
        label.translatesAutoresizingMaskIntoConstraints = false
        contentView.layoutMarginsGuide.topAnchor.constraint(equalTo: label.topAnchor).isActive = true
        contentView.layoutMarginsGuide.leadingAnchor.constraint(equalTo: label.leadingAnchor).isActive = true
        contentView.layoutMarginsGuide.trailingAnchor.constraint(equalTo: label.trailingAnchor).isActive = true
        contentView.layoutMarginsGuide.bottomAnchor.constraint(equalTo: label.bottomAnchor).isActive = true
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}

Expected result:

---

#2. Left align UICollectionViewCells with fixed size

The implementation below shows how to use UICollectionViewLayout's layoutAttributesForElements(in:) and UICollectionViewFlowLayout's itemSize in order to left align cells with predefined size in a UICollectionView:

CollectionViewController.swift

import UIKit

class CollectionViewController: UICollectionViewController {

    let columnLayout = FlowLayout(
        itemSize: CGSize(width: 140, height: 140),
        minimumInteritemSpacing: 10,
        minimumLineSpacing: 10,
        sectionInset: UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
    )

    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView?.collectionViewLayout = columnLayout
        collectionView?.contentInsetAdjustmentBehavior = .always
        collectionView?.register(CollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
    }

    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 7
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! CollectionViewCell
        return cell
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        collectionView?.collectionViewLayout.invalidateLayout()
        super.viewWillTransition(to: size, with: coordinator)
    }

}

FlowLayout.swift

import UIKit

class FlowLayout: UICollectionViewFlowLayout {

    required init(itemSize: CGSize, minimumInteritemSpacing: CGFloat = 0, minimumLineSpacing: CGFloat = 0, sectionInset: UIEdgeInsets = .zero) {
        super.init()

        self.itemSize = itemSize
        self.minimumInteritemSpacing = minimumInteritemSpacing
        self.minimumLineSpacing = minimumLineSpacing
        self.sectionInset = sectionInset
        sectionInsetReference = .fromSafeArea
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let layoutAttributes = super.layoutAttributesForElements(in: rect)!.map { $0.copy() as! UICollectionViewLayoutAttributes }
        guard scrollDirection == .vertical else { return layoutAttributes }

        // Filter attributes to compute only cell attributes
        let cellAttributes = layoutAttributes.filter({ $0.representedElementCategory == .cell })

        // Group cell attributes by row (cells with same vertical center) and loop on those groups
        for (_, attributes) in Dictionary(grouping: cellAttributes, by: { ($0.center.y / 10).rounded(.up) * 10 }) {
            // Set the initial left inset
            var leftInset = sectionInset.left

            // Loop on cells to adjust each cell's origin and prepare leftInset for the next cell
            for attribute in attributes {
                attribute.frame.origin.x = leftInset
                leftInset = attribute.frame.maxX + minimumInteritemSpacing
            }
        }

        return layoutAttributes
    }

}

CollectionViewCell.swift

import UIKit

class CollectionViewCell: UICollectionViewCell {

    override init(frame: CGRect) {
        super.init(frame: frame)

        contentView.backgroundColor = .cyan
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}

Expected result: