//
//  LeftPagingCellCollectionViewFlowLayout.swift
//  CollectionViewCustomLayout
//
//  Created by Alexander Zimin on 17/03/2017.
//  Copyright © 2017 Alexander Zimin. All rights reserved.
//

import UIKit

class LeftPagingCellCollectionViewFlowLayout: UICollectionViewFlowLayout {

  override func prepare() {
    super.prepare()
    collectionView?.decelerationRate = UIScrollViewDecelerationRateFast
  }

  private var recentOffset = CGPoint.zero
  var additionalMargin: CGFloat = 0

  override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    guard let collectionView = collectionView else {
      return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
    }

    let cvBounds = CGRect(x: proposedContentOffset.x,
                          y: proposedContentOffset.y,
                          width: collectionView.bounds.width,
                          height: collectionView.bounds.height)

    guard let attributesForVisibleCells = self.layoutAttributesForElements(in: cvBounds) else {
      return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
    }

    var candidateAttributes : UICollectionViewLayoutAttributes?
    for attributes in attributesForVisibleCells {
      if attributes.representedElementCategory != UICollectionElementCategory.cell {
        continue
      }

      if attributes.center.x < proposedContentOffset.x {
        continue
      }

      candidateAttributes = attributes
      break
    }

    if let candidateAttributes = candidateAttributes {
      var margin = self.additionalMargin
      if candidateAttributes.indexPath.row == 0 {
        margin = collectionView.contentInset.left
      }
      recentOffset = CGPoint(x: candidateAttributes.frame.origin.x - margin,
                             y: proposedContentOffset.y)
      return recentOffset
    } else {
      return recentOffset
    }
  }
}