Left aligned flow layout for UICollectionView.

LeftAlignedFlowLayout.swift

import UIKit

class LeftAlignedFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)
        if let attributes = attributes {
            var previousItemFrame = CGRect.zero
            for attribute in attributes {
                if attribute.representedElementKind == UICollectionView.elementKindSectionFooter || attribute.representedElementKind == UICollectionView.elementKindSectionHeader {
                    continue
                }
                
                var sectionInset = self.sectionInset
                if let delegateFlowLayout = collectionView?.delegate as? UICollectionViewDelegateFlowLayout {
                    if let inset = delegateFlowLayout.collectionView?(self.collectionView!, layout: self, insetForSectionAt: attribute.indexPath.section) {
                        sectionInset = inset
                    }
                }
                
                var frame = attribute.frame
                if attribute.indexPath.item == 0 {
                    // 第一个
                    frame.origin.x = sectionInset.left
                    attribute.frame = frame
                    previousItemFrame = frame
                } else {
                    if previousItemFrame.minY == frame.minY {
                        // 同一行
                        // 同一行下一个紧跟上一个
                        frame.origin.x = previousItemFrame.maxX + minimumInteritemSpacing
                        attribute.frame = frame
                        previousItemFrame = frame
                    } else {
                        // 另一行
                        if frame.minX != sectionInset.left {
                            // 每行第一个要居左
                            frame.origin.x = sectionInset.left
                            attribute.frame = frame
                        }
                        previousItemFrame = frame
                    }
                }
            }
        }
        return attributes
    }
}