Texture Chat Application UICollectionView Flow Layout Example

ChatFlowLayoutExample.swift

import Foundation
import AsyncDisplayKit

class ChatFlowLayoutExample: UICollectionViewFlowLayout {
    private var topVisibleItem =  Int.max
    private var bottomVisibleItem = -Int.max
    private var offset: CGFloat = 0.0
    private var visibleAttributes: [UICollectionViewLayoutAttributes]?
    private var isPrepend: Bool = false
    
    override open func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        // Reset offset and prepend scope
        visibleAttributes = super.layoutAttributesForElements(in: rect)
        offset = 0.0
        isPrepend = false
        return visibleAttributes
    }
    
    override open func prepare(forCollectionViewUpdates updateItems: [UICollectionViewUpdateItem]) {
        guard let collectionView = self.collectionView else { return }
        guard let visibleAttributes = self.visibleAttributes else { return }
      
        // Calculate Bottom and Top Visible Item Count
        bottomVisibleItem = -Int.max
        topVisibleItem = Int.max
        var containerHeight: CGFloat = collectionView.frame.size.height
        containerHeight -= collectionView.contentInset.top
        containerHeight -= collectionView.contentInset.bottom
        let container = CGRect(x: collectionView.contentOffset.x,
                               y: collectionView.contentOffset.y,
                               width: collectionView.frame.size.width,
                               height: containerHeight)
        for attributes in visibleAttributes {
            if attributes.frame.intersects(container) {
                let item = attributes.indexPath.item
                
                if item < topVisibleItem {
                    topVisibleItem = item
                }
                
                if item > bottomVisibleItem {
                    bottomVisibleItem = item
                }
            }
        }
        
        super.prepare(forCollectionViewUpdates: updateItems)
        // Check: Initial Load or Load More
        let isInitialLoading: Bool = bottomVisibleItem + topVisibleItem == 0
        
        // Chack: Pre-Append or Append
        if updateItems.first?.indexPathAfterUpdate?.item ?? -1 == 0,
            updateItems.first?.updateAction == .insert,
            !isInitialLoading {
            self.isPrepend = true
        } else {
            return
        }
        
        // Calculate Offset
        offset = updateItems.filter { $0.updateAction == .insert }
            .map { $0.indexPathAfterUpdate }.filterNil()
            .filter { topVisibleItem + updateItems.count > $0.item }
            .map { self.layoutAttributesForItem(at: $0) }.filterNil()
            .map { $0.size.height + self.minimumLineSpacing }
            .reduce(0.0, { $0 + $1 })
        
        let contentHeight = collectionView.contentSize.height
        var frameHeight = collectionView.frame.size.height
        frameHeight -= collectionView.contentInset.top
        frameHeight -= collectionView.contentInset.bottom
        guard contentHeight + offset > frameHeight else {
            // Exception
            self.isPrepend = false
            return
        }
        CATransaction.begin()
        CATransaction.setDisableActions(true)
    }
    
    override open func finalizeCollectionViewUpdates() {
        guard let collectionView = self.collectionView, isPrepend else { return }
        // Adjust Content Offset
        let newContentOffset = CGPoint(x: collectionView.contentOffset.x,
                                       y: collectionView.contentOffset.y + self.offset)
        collectionView.contentOffset = newContentOffset
        CATransaction.commit()
    }
}