vds_ios/VDS/Components/Buttons/ButtonGroup/ButtonGroupPositionLayout.swift

//
//  ButtonGroupPositionLayout.swift
//  VDS
//
//  Created by Matt Bruce on 11/18/22.
//

import Foundation
import UIKit

class ButtonCollectionViewRow {
    var attributes = [UICollectionViewLayoutAttributes]()
    var spacing: CGFloat = 0

    init(spacing: CGFloat) {
        self.spacing = spacing
    }

    func add(attribute: UICollectionViewLayoutAttributes) {
        attributes.append(attribute)
    }

    var rowWidth: CGFloat {
        return attributes.reduce(0, { result, attribute -> CGFloat in
            return result + attribute.frame.width
        }) + CGFloat(attributes.count - 1) * spacing
    }

    func layout(for position: ButtonPosition, with collectionViewWidth: CGFloat){
        var offset = 0.0
        
        switch position {
        case .left:
            break
        case .center:
            offset = (collectionViewWidth - rowWidth) / 2
        case .right:
            offset = (collectionViewWidth - rowWidth)
        }
        
        for attribute in attributes {
            attribute.frame.origin.x = offset
            offset += attribute.frame.width + spacing
        }
    }
}

public enum ButtonPosition: String, CaseIterable {
    case left, center, right
}

protocol ButtongGroupPositionLayoutDelegate: AnyObject {
    func collectionView(_ collectionView: UICollectionView, sizeForItemAtIndexPath indexPath: IndexPath) -> CGSize
    func collectionView(_ collectionView: UICollectionView, insetsForItemsInSection section: Int) -> UIEdgeInsets
    func collectionView(_ collectionView: UICollectionView, itemSpacingInSection section: Int) -> CGFloat
}

class ButtonGroupPositionLayout: UICollectionViewLayout {
    
    weak var delegate: ButtongGroupPositionLayoutDelegate?
    
    // Total height of the content. Will be used to configure the scrollview content
    var layoutHeight: CGFloat = 0.0
    var position: ButtonPosition = .left
    private var itemCache: [UICollectionViewLayoutAttributes] = []
    
    override func prepare() {
        super.prepare()

        itemCache.removeAll()
        
        guard let collectionView = collectionView else {
            return
        }
        var itemSpacing = 0.0
        // Variable to track the width of the layout at the current state when the item is being drawn
        var layoutWidthIterator: CGFloat = 0.0
        
        for section in 0..<collectionView.numberOfSections {
            
            // Get the necessary data (if implemented) from the delegates else provide default values
            let insets: UIEdgeInsets = delegate?.collectionView(collectionView, insetsForItemsInSection: section) ?? UIEdgeInsets.zero
            let interItemSpacing: CGFloat = delegate?.collectionView(collectionView, itemSpacingInSection: section) ?? 0.0
            itemSpacing = interItemSpacing
            // Variables to track individual item width and cumultative height of all items as they are being laid out.
            var itemSize: CGSize = .zero
            
            layoutHeight += insets.top
            
            for item in 0..<collectionView.numberOfItems(inSection: section) {
                let indexPath = IndexPath(item: item, section: section)
                
                itemSize = delegate?.collectionView(collectionView, sizeForItemAtIndexPath: indexPath) ?? .zero
                
                if (layoutWidthIterator + itemSize.width + insets.left + insets.right) > collectionView.frame.width {
                    // If the current row width (after this item being laid out) is exceeding the width of the collection view content, put it in the next line
                    layoutWidthIterator = 0.0
                    layoutHeight += itemSize.height + interItemSpacing
                }
                
                let frame = CGRect(x: layoutWidthIterator + insets.left, y: layoutHeight, width: itemSize.width, height: itemSize.height)
                let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                attributes.frame = frame
                itemCache.append(attributes)
                layoutWidthIterator = layoutWidthIterator + frame.width + interItemSpacing
            }
            
            layoutHeight += itemSize.height + insets.bottom
            layoutWidthIterator = 0.0
        }
        
        //Turn into rows and re-calculate
        var rows = [ButtonCollectionViewRow]()
        var currentRowY: CGFloat = -1

        for attribute in itemCache {
            if currentRowY != attribute.frame.midY {
                currentRowY = attribute.frame.midY
                rows.append(ButtonCollectionViewRow(spacing: itemSpacing))
            }
            rows.last?.add(attribute: attribute)
        }

        //recalculate rows based off of positions
        rows.forEach { $0.layout(for: position, with: collectionView.frame.width) }
        let rowAttributes = rows.flatMap { $0.attributes }
        itemCache = rowAttributes

    }
    
    override func layoutAttributesForElements(in rect: CGRect)-> [UICollectionViewLayoutAttributes]? {
        super.layoutAttributesForElements(in: rect)
        
        var visibleLayoutAttributes: [UICollectionViewLayoutAttributes] = []
        
        for attributes in itemCache {
            if attributes.frame.intersects(rect) {
                visibleLayoutAttributes.append(attributes)
            }
        }
        
        return visibleLayoutAttributes
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        super.layoutAttributesForItem(at: indexPath)
        return itemCache[indexPath.row]
    }
    
    override var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: layoutHeight)
    }
    
    private var contentWidth: CGFloat {
        guard let collectionView = collectionView else {
            return 0
        }
        let insets = collectionView.contentInset
        return collectionView.bounds.width - (insets.left + insets.right)
    }
    
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        layoutHeight = 0.0
        return true
    }
}