Tusker/Tusker/Screens/Asset Picker/AssetCollectionViewController.swift

//
//  AssetCollectionViewController.swift
//  Tusker
//
//  Created by Shadowfacts on 1/1/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import UIKit
import Photos

private let reuseIdentifier = "assetCell"
private let cameraReuseIdentifier = "showCameraCell"

protocol AssetCollectionViewControllerDelegate: class {
    func shouldSelectAsset(_ asset: PHAsset) -> Bool
    func didSelectAssets(_ assets: [PHAsset])
    func captureFromCamera()
}

class AssetCollectionViewController: UICollectionViewController {

    weak var delegate: AssetCollectionViewControllerDelegate?
    
    private var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
    private var flowLayout: UICollectionViewFlowLayout {
        return collectionViewLayout as! UICollectionViewFlowLayout
    }
    
    private var availableWidth: CGFloat!
    private var thumbnailSize: CGSize!
    
    private let imageManager = PHCachingImageManager()
    private var fetchResult: PHFetchResult<PHAsset>!
    
    var selectedAssets: [PHAsset] {
        return collectionView.indexPathsForSelectedItems?.compactMap { (indexPath) in
            guard case let .asset(asset) = dataSource.itemIdentifier(for: indexPath) else { return nil }
            return asset
        } ?? []
    }
    
    init() {
        super.init(collectionViewLayout: UICollectionViewFlowLayout())
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // use the safe area layout guide instead of letting it automatically use the safe area insets
        // because otherwise, when presented in a popover with the arrow on the left or right side,
        // the collection view content will be cut off by the width of the arrow because the popover
        // doesn't respect safe area insets
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            view.safeAreaLayoutGuide.leadingAnchor.constraint(equalTo: collectionView.leadingAnchor),
            view.safeAreaLayoutGuide.trailingAnchor.constraint(equalTo: collectionView.trailingAnchor),
            // top ignores safe area because when presented in the sheet container, it simplifies the top content offset
            view.topAnchor.constraint(equalTo: collectionView.topAnchor),
            // bottom ignores safe area because we want cells to underflow bottom of the screen on notched iPhones
            view.bottomAnchor.constraint(equalTo: collectionView.bottomAnchor),
        ])
        view.backgroundColor = .systemBackground
        
        navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .done, target: self, action: #selector(donePressed))

        collectionView.alwaysBounceVertical = true
        collectionView.allowsMultipleSelection = true
        
        collectionView.register(UINib(nibName: "AssetCollectionViewCell", bundle: .main), forCellWithReuseIdentifier: reuseIdentifier)
        collectionView.register(UINib(nibName: "ShowCameraCollectionViewCell", bundle: .main), forCellWithReuseIdentifier: cameraReuseIdentifier)
        
        let scale = UIScreen.main.scale
        let cellSize = flowLayout.itemSize
        thumbnailSize = CGSize(width: cellSize.width * scale, height: cellSize.height * scale)
        
        dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView, cellProvider: { (collectionView, indexPath, item) -> UICollectionViewCell? in
            switch item {
            case .showCamera:
                return collectionView.dequeueReusableCell(withReuseIdentifier: cameraReuseIdentifier, for: indexPath)
            case let .asset(asset):
                let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! AssetCollectionViewCell
        
                cell.updateUI(asset: asset)
                self.imageManager.requestImage(for: asset, targetSize: self.thumbnailSize, contentMode: .aspectFill, options: nil) { (image, _) in
                    guard let image = image else { return }
                    DispatchQueue.main.async {
                        guard cell.assetIdentifier == asset.localIdentifier else { return }
                        cell.thumbnailImage = image
                    }
                }

                return cell
            }
        })
        
        setEditing(true, animated: false)
        
        updateItemsSelectedCount()

        if let singleFingerPanGesture = collectionView.gestureRecognizers?.first(where: {
                $0.name == "multi-select.singleFingerPanGesture"
            }),
            let interactivePopGesture = navigationController?.interactivePopGestureRecognizer {
            singleFingerPanGesture.require(toFail: interactivePopGesture)
        }
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        loadAssets()
    }
    
    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        
        let availableWidth = view.bounds.inset(by: view.safeAreaInsets).width
        
        if self.availableWidth != availableWidth {
            self.availableWidth = availableWidth
            
            let size = (availableWidth - 8) / 3
            flowLayout.itemSize = CGSize(width: size, height: size)
            flowLayout.minimumInteritemSpacing = 4
            flowLayout.minimumLineSpacing = 4
        }
    }
    
    private func loadAssets() {
        switch PHPhotoLibrary.authorizationStatus(for: .readWrite) {
        case .notDetermined:
            PHPhotoLibrary.requestAuthorization(for: .readWrite) { (_) in
                self.loadAssets()
            }
            return
            
        case .restricted, .denied:
            // todo: better UI for this
            return
            
        case .authorized, .limited:
            // todo: show "add more" button for limited access
            break
            
        @unknown default:
            // who knows, just try anyways
            break
        }
        
        let options = PHFetchOptions()
        options.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: false)]
        fetchResult = fetchAssets(with: options)
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        snapshot.appendSections([.assets])
        var items: [Item] = [.showCamera]
        fetchResult.enumerateObjects { (asset, _, _) in
            items.append(.asset(asset))
        }
        snapshot.appendItems(items)
        dataSource.apply(snapshot, animatingDifferences: false)
    }
    
    open func fetchAssets(with options: PHFetchOptions) -> PHFetchResult<PHAsset> {
        return PHAsset.fetchAssets(with: options)
    }
    
    func updateItemsSelectedCount() {
        let selected = collectionView.indexPathsForSelectedItems?.count ?? 0
        
        navigationItem.title = "\(selected) selected"
    }

    // MARK: UICollectionViewDelegate

    override func collectionView(_ collectionView: UICollectionView, shouldBeginMultipleSelectionInteractionAt indexPath: IndexPath) -> Bool {
        return true
    }
    
    override func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool {
        guard let item = dataSource.itemIdentifier(for: indexPath) else { return false }
        if let delegate = delegate,
           case let .asset(asset) = item {
            return delegate.shouldSelectAsset(asset)
        }
        return true
    }
    
    override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        guard let item = dataSource.itemIdentifier(for: indexPath) else { return }
        switch item {
        case .showCamera:
            collectionView.deselectItem(at: indexPath, animated: false)
            delegate?.captureFromCamera()
        case .asset(_):
            updateItemsSelectedCount()
        }
    }
    
    override func collectionView(_ collectionView: UICollectionView, didDeselectItemAt indexPath: IndexPath) {
        updateItemsSelectedCount()
    }
    
    override func collectionView(_ collectionView: UICollectionView, contextMenuConfigurationForItemAt indexPath: IndexPath, point: CGPoint) -> UIContextMenuConfiguration? {
        guard case let .asset(asset) = dataSource.itemIdentifier(for: indexPath) else { return nil }
        return UIContextMenuConfiguration(identifier: indexPath as NSIndexPath, previewProvider: { () -> UIViewController? in
            return AssetPreviewViewController(asset: asset)
        }, actionProvider: nil)
    }
    
    override func collectionView(_ collectionView: UICollectionView, previewForHighlightingContextMenuWithConfiguration configuration: UIContextMenuConfiguration) -> UITargetedPreview? {
        if let indexPath = (configuration.identifier as? NSIndexPath) as IndexPath?,
            let cell = collectionView.cellForItem(at: indexPath) as? AssetCollectionViewCell {
            let parameters = UIPreviewParameters()
            parameters.backgroundColor = .black
            return UITargetedPreview(view: cell.imageView, parameters: parameters)
        } else {
            return nil
        }
    }
    
    // MARK: - Interaction
    
    @objc func donePressed() {
        delegate?.didSelectAssets(selectedAssets)
        dismiss(animated: true)
    }
    
}

extension AssetCollectionViewController {
    enum Section: Hashable {
        case assets
    }
    enum Item: Hashable {
        case showCamera
        case asset(PHAsset)
    }
}