TimelineLayout.swift

//
//  TimelineLayout.swift
//  OKVideo
//
//  Created by Pim Coumans on 12/11/2020.
//  Copyright © 2020 pixelrock. All rights reserved.
//

import UIKit

protocol UICollectionViewTimelineLayoutDelegate: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, durationForClipAtIndexPath indexPath: IndexPath) -> TimeInterval
}

class TimelineLayout: UICollectionViewLayout {
    
    /// Relative offset from center, 1 would be bottom, -1 would be top
    var relativeOffset: CGFloat = 0 {
        didSet {
            invalidateLayout()
        }
    }
    
    let cellSpacing: CGFloat = 4
    let cellHeight: CGFloat = 80
    
    private let minWidth: CGFloat = 40
    private let widthPerSecond: Double = 20
    
    private var cache: [UICollectionViewLayoutAttributes] = []
    private var contentWidth: CGFloat = 0
    
    override var collectionViewContentSize: CGSize {
        CGSize(width: contentWidth, height: cellHeight)
    }
    
    override func prepare() {
        guard cache.isEmpty, let collectionView = collectionView else {
            return
        }
        
        var xOffset: CGFloat = 0
        
        let visibleBounds = collectionView.bounds.inset(by: collectionView.adjustedContentInset)
        let yCenter = visibleBounds.midY
        let yValue = yCenter - (cellHeight / 2)
        let yOffset = yCenter + (yValue * relativeOffset) - (cellHeight / 2)
        
        for itemIndex in 0..<collectionView.numberOfItems(inSection: 0) {
            let indexPath = IndexPath(item: itemIndex, section: 0)
            
            let duration = (collectionView.delegate as? UICollectionViewTimelineLayoutDelegate)?.collectionView(collectionView, durationForClipAtIndexPath: indexPath) ?? 0
            
            let frame = CGRect(x: xOffset + cellSpacing,
                               y: yOffset,
                               width: max(CGFloat(duration * widthPerSecond), minWidth),
                               height: cellHeight)
            xOffset = frame.maxX
            
            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = frame
            cache.append(attributes)
            
        }
        
        contentWidth = xOffset
    }
    
    override func invalidateLayout() {
        cache.removeAll()
        super.invalidateLayout()
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return cache.filter { $0.frame.intersects(rect) }
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cache[indexPath.item]
    }
}