mastodon-ios/Mastodon/Scene/Discovery/News/DiscoveryNewsViewController.swift

//
//  DiscoveryNewsViewController.swift
//  Mastodon
//
//  Created by MainasuK on 2022-4-13.
//

import UIKit
import Combine
import MastodonCore
import MastodonUI

final class DiscoveryNewsViewController: UIViewController, NeedsDependency, MediaPreviewableViewController {
    
    weak var context: AppContext! { willSet { precondition(!isViewLoaded) } }
    weak var coordinator: SceneCoordinator! { willSet { precondition(!isViewLoaded) } }
    
    var disposeBag = Set<AnyCancellable>()
    var viewModel: DiscoveryNewsViewModel!
    
    let mediaPreviewTransitionController = MediaPreviewTransitionController()

    lazy var tableView: UITableView = {
        let tableView = UITableView()
        tableView.rowHeight = UITableView.automaticDimension
        tableView.estimatedRowHeight = 100
        tableView.separatorStyle = .none
        tableView.backgroundColor = .clear
        return tableView
    }()
    
    let refreshControl = RefreshControl()
}

extension DiscoveryNewsViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .secondarySystemBackground
        
        tableView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(tableView)
        tableView.pinToParent()

        tableView.delegate = self
        viewModel.setupDiffableDataSource(
            tableView: tableView
        )
        
        tableView.refreshControl = refreshControl
        refreshControl.addTarget(self, action: #selector(DiscoveryNewsViewController.refreshControlValueChanged(_:)), for: .valueChanged)
        viewModel.didLoadLatest
            .receive(on: DispatchQueue.main)
            .sink { [weak self] _ in
                guard let self = self else { return }
                self.refreshControl.endRefreshing()
            }
            .store(in: &disposeBag)
        
        // setup batch fetch
        viewModel.listBatchFetchViewModel.setup(scrollView: tableView)
        viewModel.listBatchFetchViewModel.shouldFetch
            .receive(on: DispatchQueue.main)
            .sink { [weak self] _ in
                guard let self = self else { return }
                guard self.view.window != nil else { return }
                self.viewModel.stateMachine.enter(DiscoveryNewsViewModel.State.Loading.self)
            }
            .store(in: &disposeBag)
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        refreshControl.endRefreshing()
        tableView.deselectRow(with: transitionCoordinator, animated: animated)
    }

}

extension DiscoveryNewsViewController {
    
    @objc private func refreshControlValueChanged(_ sender: RefreshControl) {
        guard viewModel.stateMachine.enter(DiscoveryNewsViewModel.State.Reloading.self) else {
            sender.endRefreshing()
            return
        }
    }
    
}

// MARK: - UITableViewDelegate
extension DiscoveryNewsViewController: UITableViewDelegate {

    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        guard case let .link(link) = viewModel.diffableDataSource?.itemIdentifier(for: indexPath) else { return }
        guard let url = URL(string: link.url) else { return }
        _ = coordinator.present(
            scene: .safari(url: url),
            from: self,
            transition: .safariPresent(animated: true, completion: nil)
        )
    }

}

// MARK: ScrollViewContainer
extension DiscoveryNewsViewController: ScrollViewContainer {
    var scrollView: UIScrollView { tableView }
}

extension DiscoveryNewsViewController {
    override var keyCommands: [UIKeyCommand]? {
        return navigationKeyCommands
    }
}

extension DiscoveryNewsViewController: TableViewControllerNavigateable {
    
    func navigate(direction: TableViewNavigationDirection) {
        if let indexPathForSelectedRow = tableView.indexPathForSelectedRow {
            // navigate up/down on the current selected item
            navigateToLink(direction: direction, indexPath: indexPathForSelectedRow)
        } else {
            // set first visible item selected
            navigateToFirstVisibleLink()
        }
    }
    
    private func navigateToLink(direction: TableViewNavigationDirection, indexPath: IndexPath) {
        guard let diffableDataSource = viewModel.diffableDataSource else { return }
        let items = diffableDataSource.snapshot().itemIdentifiers
        guard let selectedItem = diffableDataSource.itemIdentifier(for: indexPath),
              let selectedItemIndex = items.firstIndex(of: selectedItem) else {
            return
        }

        let _navigateToItem: DiscoveryItem? = {
            var index = selectedItemIndex
            while 0..<items.count ~= index {
                index = {
                    switch direction {
                    case .up:   return index - 1
                    case .down: return index + 1
                    }
                }()
                guard 0..<items.count ~= index else { return nil }
                let item = items[index]
                
                guard Self.validNavigateableItem(item) else { continue }
                return item
            }
            return nil
        }()
        
        guard let item = _navigateToItem, let indexPath = diffableDataSource.indexPath(for: item) else { return }
        let scrollPosition: UITableView.ScrollPosition = overrideNavigationScrollPosition ?? Self.navigateScrollPosition(tableView: tableView, indexPath: indexPath)
        tableView.selectRow(at: indexPath, animated: true, scrollPosition: scrollPosition)
    }
    
    private func navigateToFirstVisibleLink() {
        guard let indexPathsForVisibleRows = tableView.indexPathsForVisibleRows else { return }
        guard let diffableDataSource = viewModel.diffableDataSource else { return }
        
        var visibleItems: [DiscoveryItem] = indexPathsForVisibleRows.sorted().compactMap { indexPath in
            guard let item = diffableDataSource.itemIdentifier(for: indexPath) else { return nil }
            guard Self.validNavigateableItem(item) else { return nil }
            return item
        }
        if indexPathsForVisibleRows.first?.row != 0, visibleItems.count > 1 {
            // drop first when visible not the first cell of table
            visibleItems.removeFirst()
        }
        guard let item = visibleItems.first, let indexPath = diffableDataSource.indexPath(for: item) else { return }
        let scrollPosition: UITableView.ScrollPosition = overrideNavigationScrollPosition ?? Self.navigateScrollPosition(tableView: tableView, indexPath: indexPath)
        tableView.selectRow(at: indexPath, animated: true, scrollPosition: scrollPosition)
    }
    
    static func validNavigateableItem(_ item: DiscoveryItem) -> Bool {
        switch item {
        case .link:
            return true
        default:
            return false
        }
    }
    
    func open() {
        guard let indexPathForSelectedRow = tableView.indexPathForSelectedRow else { return }
        guard let diffableDataSource = viewModel.diffableDataSource else { return }
        guard let item = diffableDataSource.itemIdentifier(for: indexPathForSelectedRow) else { return }
        
        guard case let .link(link) = item else { return }
        guard let url = URL(string: link.url) else { return }
        _ = coordinator.present(
            scene: .safari(url: url),
            from: self,
            transition: .safariPresent(animated: true, completion: nil)
        )
    }
    
    func navigateKeyCommandHandlerRelay(_ sender: UIKeyCommand) {
        navigateKeyCommandHandler(sender)
    }

}