Carousel.swift

import UIKit
import RxSwift
import RxCocoa
import RxDataSources

private let reuseIdentifier = "ProductImageItemCell"

protocol ProductHeaderCarouselDelegate {
    func didTapItem(indexPath: IndexPath, selectedImage: ImageView)
}

class ProductHeaderCarouselController: UICollectionViewController {
    var viewModel: ProductHeaderViewModel!
    var delegate: ProductHeaderCarouselDelegate?
    
    lazy var pageControl: UIPageControl = {
        let pc = UIPageControl()
        pc.currentPage = 0
        pc.currentPageIndicatorTintColor =  UIColor.primaryDark()
        pc.pageIndicatorTintColor = UIColor.secondary()
        return pc
    }()
    
    override func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let x = targetContentOffset.pointee.x
        pageControl.currentPage = Int(x / view.frame.width)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        collectionView.register(cellWithClass: ProductImageItemCell.self)

        setupBottomControls()
        
        collectionView?.backgroundColor = .white
        collectionView?.register(ProductImageItemCell.self, forCellWithReuseIdentifier: "cellId")
        if let layout = collectionViewLayout as? UICollectionViewFlowLayout {
            layout.scrollDirection = .horizontal
            layout.minimumLineSpacing = 0
            layout.minimumInteritemSpacing = 0
        }
        collectionView?.isPagingEnabled = true
    }
    
    func setupBottomControls() {
        view.addSubview(pageControl)
        
        pageControl.snp.makeConstraints { (make) -> Void in
            make.leading.equalTo(view.safeAreaLayoutGuide.snp.leading)
            make.trailing.equalTo(view.safeAreaLayoutGuide.snp.trailing)
            make.bottom.equalTo(view.safeAreaLayoutGuide.snp.bottom)
            make.height.equalTo(50)
        }
    }
    
    func bindViewModel() {

        viewModel.transform().asDriver(onErrorJustReturn: [])
            .drive(self.collectionView.rx.items(cellIdentifier: reuseIdentifier, cellType: ProductImageItemCell.self)) { collectionView, viewModel, cell in
                cell.bind(to: viewModel)
            }.disposed(by: rx.disposeBag)
        
        viewModel.map{
            pageControl.numberOfPages = $0.elements.value.count
        }
    }
    
    override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        let selectedCell = collectionView.cellForItem(at: indexPath) as! ProductImageItemCell

        delegate?.didTapItem(indexPath: indexPath, selectedImage: selectedCell.productImageView)
    }
}

extension ProductHeaderCarouselController {
    
    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        
        coordinator.animate(alongsideTransition: { (_) in
            self.collectionViewLayout.invalidateLayout()
            
            if self.pageControl.currentPage == 0 {
                self.collectionView?.contentOffset = .zero
            } else {
                let indexPath = IndexPath(item: self.pageControl.currentPage, section: 0)
                self.collectionView?.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
            }
            
        }) { (_) in
            
        }
    }
}

extension ProductHeaderCarouselController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: view.frame.width, height: view.frame.height)
    }
}