SwiftUICollectionView.swift

//
//  SwiftUICollectionView.swift
//  Sentry
//
//  Created by Simon Westerlund on 2019-12-29.
//  Copyright © 2019 Simon Westerlund. All rights reserved.
//

import SwiftUI

final class UIHostingControllerCollectionViewCell<Content: View>: UICollectionViewCell {
    private var view: UIView? {
        willSet {
            view?.removeFromSuperview()
        }
    }
    
    override func prepareForReuse() {
        super.prepareForReuse()
        
        view?.removeFromSuperview()
    }
    
    func configureForDisplay(with item: Content) {
        view = UIHostingController(rootView: item).view
        embedView(view)
    }
    
    func select(with item: Content) {
        let selectedItem = item.modifier(SelectedViewModifier(selected: true))
        view = UIHostingController(rootView: selectedItem).view
        embedView(view)
    }
    
    private func embedView(_ view: UIView?) {
        guard let view = view else {
            return
        }
        
        contentView.addSubview(view)
        contentView.backgroundColor = .clear
        backgroundColor = .clear
        view.frame = contentView.bounds
        view.autoresizingMask = [.flexibleHeight, .flexibleWidth]
    }
}

final class DataSource<Content: View>: NSObject, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    
    @Binding var selectedIndexPath: IndexPath
    
    let items: [Content]
    init(items: [Content], selectedIndexPath: Binding<IndexPath>) {
        self.items = items
        self._selectedIndexPath = selectedIndexPath
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return items.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! UIHostingControllerCollectionViewCell<Content>
        
        let item = items[indexPath.row]
        
        if selectedIndexPath == indexPath {
            cell.select(with: item)
        } else {
            cell.configureForDisplay(with: item)
        }
        
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 0, left: 16, bottom: 0, right: 16)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        
        let height = collectionView.frame.height
        return CGSize(width: height * 4 / 3, height: height)
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        
        if let cell = collectionView.cellForItem(at: selectedIndexPath) as? UIHostingControllerCollectionViewCell<Content> {
            let item = items[selectedIndexPath.row]
            cell.configureForDisplay(with: item)
        }
        
        selectedIndexPath = indexPath

        if let cell = collectionView.cellForItem(at: selectedIndexPath) as? UIHostingControllerCollectionViewCell<Content> {
            let item = items[selectedIndexPath.row]
            cell.select(with: item)
        }
    }
}

struct SwiftUICollectionView<Content: View>: UIViewRepresentable {
    typealias UIViewType = UICollectionView
    
    let dataSource: DataSource<Content>
    let collectionView: UIViewType
    init(cells: [Content], selectedIndexPath: Binding<IndexPath>) {
        dataSource = DataSource(items: cells, selectedIndexPath: selectedIndexPath)
        
        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .horizontal
        
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    }
    
    func makeUIView(context: UIViewRepresentableContext<SwiftUICollectionView>) -> UICollectionView {
        collectionView.register(UIHostingControllerCollectionViewCell<Content>.self, forCellWithReuseIdentifier: "cell")
        collectionView.dataSource = dataSource
        collectionView.delegate = dataSource
        collectionView.backgroundColor = .clear
        
        return collectionView
    }
    
    func updateUIView(_ uiView: UICollectionView, context: UIViewRepresentableContext<SwiftUICollectionView>) {}
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
    
    class Coordinator: NSObject {
        var control: SwiftUICollectionView
        init(_ control: SwiftUICollectionView) {
            self.control = control
        }
    }
}