UICollection View Flow Layout Vertical Align

By default, when you are using the flow layout in a collection view, cells are centered vertically. Is there a way to change this alignment ?

Solution 1:

Swift 4 with functional oriented approach:

class TopAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)?
            .map { $0.copy() } as? [UICollectionViewLayoutAttributes]

        attributes?
            .reduce([CGFloat: (CGFloat, [UICollectionViewLayoutAttributes])]()) {
                guard $1.representedElementCategory == .cell else { return $0 }
                return $0.merging([ceil($1.center.y): ($1.frame.origin.y, [$1])]) {
                    ($0.0 < $1.0 ? $0.0 : $1.0, $0.1 + $1.1)
                }
            }
            .values.forEach { minY, line in
                line.forEach {
                    $0.frame = $0.frame.offsetBy(
                        dx: 0,
                        dy: minY - $0.frame.origin.y
                    )
                }
            }

        return attributes
    }
}

Solution 2:

following code worked for me

@interface TopAlignedCollectionViewFlowLayout : UICollectionViewFlowLayout

- (void)alignToTopForSameLineElements:(NSArray *)sameLineElements;

@end

@implementation TopAlignedCollectionViewFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect;
{
    NSArray *attrs = [super layoutAttributesForElementsInRect:rect];
    CGFloat baseline = -2;
    NSMutableArray *sameLineElements = [NSMutableArray array];
    for (UICollectionViewLayoutAttributes *element in attrs) {
        if (element.representedElementCategory == UICollectionElementCategoryCell) {
            CGRect frame = element.frame;
            CGFloat centerY = CGRectGetMidY(frame);
            if (ABS(centerY - baseline) > 1) {
                baseline = centerY;
                [self alignToTopForSameLineElements:sameLineElements];
                [sameLineElements removeAllObjects];
            }
            [sameLineElements addObject:element];
        }
    }
    [self alignToTopForSameLineElements:sameLineElements];//align one more time for the last line
    return attrs;
}

- (void)alignToTopForSameLineElements:(NSArray *)sameLineElements
{
    if (sameLineElements.count == 0) {
        return;
    }
    NSArray *sorted = [sameLineElements sortedArrayUsingComparator:^NSComparisonResult(UICollectionViewLayoutAttributes *obj1, UICollectionViewLayoutAttributes *obj2) {
        CGFloat height1 = obj1.frame.size.height;
        CGFloat height2 = obj2.frame.size.height;
        CGFloat delta = height1 - height2;
        return delta == 0. ? NSOrderedSame : ABS(delta)/delta;
    }];
    UICollectionViewLayoutAttributes *tallest = [sorted lastObject];
    [sameLineElements enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *obj, NSUInteger idx, BOOL *stop) {
        obj.frame = CGRectOffset(obj.frame, 0, tallest.frame.origin.y - obj.frame.origin.y);
    }];
}

@end

Solution 3:

@DongXu: Your solution worked for me too. Here is the SWIFT version if it:

class TopAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout
{
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]?
    {
        if let attrs = super.layoutAttributesForElementsInRect(rect)
        {
            var baseline: CGFloat = -2
            var sameLineElements = [UICollectionViewLayoutAttributes]()
            for element in attrs
            {
                if element.representedElementCategory == .Cell
                {
                    let frame = element.frame
                    let centerY = CGRectGetMidY(frame)
                    if abs(centerY - baseline) > 1
                    {
                        baseline = centerY
                        TopAlignedCollectionViewFlowLayout.alignToTopForSameLineElements(sameLineElements)
                        sameLineElements.removeAll()
                    }
                    sameLineElements.append(element)
                }
            }
            TopAlignedCollectionViewFlowLayout.alignToTopForSameLineElements(sameLineElements) // align one more time for the last line
            return attrs
        }
        return nil
    }

    private class func alignToTopForSameLineElements(sameLineElements: [UICollectionViewLayoutAttributes])
    {
        if sameLineElements.count < 1
        {
            return
        }
        let sorted = sameLineElements.sort { (obj1: UICollectionViewLayoutAttributes, obj2: UICollectionViewLayoutAttributes) -> Bool in

            let height1 = obj1.frame.size.height
            let height2 = obj2.frame.size.height
            let delta = height1 - height2
            return delta <= 0
        }
        if let tallest = sorted.last
        {
            for obj in sameLineElements
            {
                obj.frame = CGRectOffset(obj.frame, 0, tallest.frame.origin.y - obj.frame.origin.y)
            }
        }
    }
}

Solution 4:

Swift 3 Version in case someone just wants to Copy & Paste:

class TopAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        if let attrs = super.layoutAttributesForElements(in: rect) {
            var baseline: CGFloat = -2
            var sameLineElements = [UICollectionViewLayoutAttributes]()
            for element in attrs {
                if element.representedElementCategory == .cell {
                    let frame = element.frame
                    let centerY = frame.midY
                    if abs(centerY - baseline) > 1 {
                        baseline = centerY
                        alignToTopForSameLineElements(sameLineElements: sameLineElements)
                        sameLineElements.removeAll()
                    }
                    sameLineElements.append(element)
                }
            }
            alignToTopForSameLineElements(sameLineElements: sameLineElements) // align one more time for the last line
            return attrs
        }
        return nil
    }

    private func alignToTopForSameLineElements(sameLineElements: [UICollectionViewLayoutAttributes]) {
        if sameLineElements.count < 1 { return }
        let sorted = sameLineElements.sorted { (obj1: UICollectionViewLayoutAttributes, obj2: UICollectionViewLayoutAttributes) -> Bool in
            let height1 = obj1.frame.size.height
            let height2 = obj2.frame.size.height
            let delta = height1 - height2
            return delta <= 0
        }
        if let tallest = sorted.last {
            for obj in sameLineElements {
                obj.frame = obj.frame.offsetBy(dx: 0, dy: tallest.frame.origin.y - obj.frame.origin.y)
            }
        }
    }
}

Solution 5:

I have used something similar to the before answers. In my case I want to align cells by colum with different heights.

import UIKit

class AlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        if let attributes = super.layoutAttributesForElements(in: rect) {
            let sectionElements: [Int : [UICollectionViewLayoutAttributes]] = attributes
                .filter {
                    return $0.representedElementCategory == .cell //take cells only
                }.groupBy {
                    return $0.indexPath.section //group attributes by section
            }

            sectionElements.forEach { (section, elements) in
                //get suplementary view (header) to align each section
                let suplementaryView = attributes.first {
                    return $0.representedElementCategory == .supplementaryView && $0.indexPath.section == section
                }
                //call align method
                alignToTopSameSectionElements(elements, with: suplementaryView)
            }

            return attributes
        }

        return super.layoutAttributesForElements(in: rect)
    }

    private func alignToTopSameSectionElements(_ elements: [UICollectionViewLayoutAttributes], with suplementaryView: UICollectionViewLayoutAttributes?) {
        //group attributes by colum 
        let columElements: [Int : [UICollectionViewLayoutAttributes]] = elements.groupBy {
            return Int($0.frame.midX)
        }

        columElements.enumerated().forEach { (columIndex, object) in
            let columElement = object.value.sorted {
                return $0.indexPath < $1.indexPath
            }

            columElement.enumerated().forEach { (index, element) in
                var frame = element.frame

                if columIndex == 0 {
                    frame.origin.x = minimumLineSpacing
                }

                switch index {
                case 0:
                    if let suplementaryView = suplementaryView {
                        frame.origin.y = suplementaryView.frame.maxY
                    }
                default:
                    let beforeElement = columElement[index-1]
                    frame.origin.y = beforeElement.frame.maxY + minimumInteritemSpacing
                }

                element.frame = frame
            }
        }
    }
}

public extension Array {

    func groupBy <U> (groupingFunction group: (Element) -> U) -> [U: Array] {

        var result = [U: Array]()

        for item in self {

            let groupKey = group(item)

            if result.has(groupKey) {
                result[groupKey]! += [item]
            } else {
                result[groupKey] = [item]
            }
        }

        return result
    }
}

This is the result of this layout: