Tusker/Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift

//
//  DiffableTimelineLikeTableViewController.swift
//  Tusker
//
//  Created by Shadowfacts on 6/18/21.
//  Copyright © 2021 Shadowfacts. All rights reserved.
//

import UIKit
import Pachyderm

class DiffableTimelineLikeTableViewController<Section: Hashable & CaseIterable, Item: Hashable>: EnhancedTableViewController, RefreshableViewController {
    
    typealias Snapshot = NSDiffableDataSourceSnapshot<Section, Item>
    typealias LoadResult = Result<Snapshot, LoadError>
    
    private let pageSize = 20
    
    private(set) var state = State.unloaded
    private var lastLastVisibleRow: IndexPath?
    
    private(set) var dataSource: UITableViewDiffableDataSource<Section, Item>!
    
    init() {
        super.init(style: .plain)
        
        addKeyCommand(MenuController.refreshCommand(discoverabilityTitle: Self.refreshCommandTitle()))
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        dataSource = UITableViewDiffableDataSource(tableView: tableView, cellProvider: self.cellProvider)
        
        tableView.rowHeight = UITableView.automaticDimension
        tableView.estimatedRowHeight = 140
        
        #if !targetEnvironment(macCatalyst)
        self.refreshControl = UIRefreshControl()
        self.refreshControl!.addTarget(self, action: #selector(refresh), for: .valueChanged)
        #endif
        
        if let prefetchSource = self as? UITableViewDataSourcePrefetching {
            tableView.prefetchDataSource = prefetchSource
        }
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        loadInitial()
    }
    
    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        
        pruneOffscreenRows()
    }
    
    class func refreshCommandTitle() -> String {
        return "Refresh"
    }
    
    private func pruneOffscreenRows() {
        guard let lastVisibleRow = lastLastVisibleRow else {
            return
        }
        
        var snapshot = dataSource.snapshot()
        
        let lastVisibleRowSection = snapshot.sectionIdentifiers[lastVisibleRow.section]
        
        let contentSections = snapshot.sectionIdentifiers.filter { timelineContentSections().contains($0) }
        
        guard let lastVisibleContentSectionIndex = contentSections.lastIndex(of: lastVisibleRowSection) else {
            return
        }
        
        if lastVisibleContentSectionIndex < contentSections.count - 1 {
            // there are more content sections below the current last visible one
            
            let sectionsToRemove = contentSections[lastVisibleContentSectionIndex...]
            snapshot.deleteSections(Array(sectionsToRemove))
            
            willRemoveItems(sectionsToRemove.flatMap(snapshot.itemIdentifiers(inSection:)))
        } else if lastVisibleContentSectionIndex == contentSections.count - 1 {
            let items = snapshot.itemIdentifiers(inSection: lastVisibleRowSection)
            
            if lastVisibleRow.row < items.count - pageSize {
                let itemsToRemove = Array(items.suffix(pageSize))
                snapshot.deleteItems(itemsToRemove)
                
                willRemoveItems(itemsToRemove)
            } else {
                return
            }
        } else {
            return
        }
        
        dataSource.apply(snapshot, animatingDifferences: false)
    }
    
    private func loadInitial() {
        guard state == .unloaded else { return }
        // set loaded immediately so we don't trigger another request while the current one is running
        state = .loadingInitial
        
        loadInitialItems() { result in
            guard case let .success(snapshot) = result else {
                self.state = .unloaded
                return
            }
            DispatchQueue.main.async {
                self.dataSource.apply(snapshot, animatingDifferences: false)
                self.state = .loaded
            }
       }
    }
    
    func reloadInitial() {
        state = .unloaded
        loadInitial()
    }
    
    func loadOlder() {
        guard state != .loadingOlder else { return }
        
        state = .loadingOlder
        
        loadOlderItems(currentSnapshot: dataSource.snapshot()) { result in
            guard case let .success(snapshot) = result else {
                self.state = .loaded
                return
            }
            DispatchQueue.main.async {
                self.dataSource.apply(snapshot, animatingDifferences: false)
                self.state = .loaded
            }
        }
    }
    
    func cellHeightChanged() {
        // causes the table view to recalculate the cell heights
        tableView.beginUpdates()
        tableView.endUpdates()
    }
    
    // MARK: - UITableViewDelegate
    
    override func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
        // this assumes that indexPathsForVisibleRows is always in order
        lastLastVisibleRow = tableView.indexPathsForVisibleRows?.last
        
        let orderedContentSections = dataSource.snapshot().sectionIdentifiers.enumerated().filter { timelineContentSections().contains($0.element) }
        if let lastContentSection = orderedContentSections.last,
           indexPath.section == lastContentSection.offset,
           indexPath.row == tableView.numberOfRows(inSection: indexPath.section) - 1 {
            
            loadOlder()
        }
    }
    
    override func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {
        return true
    }
    
    override func tableView(_ tableView: UITableView, leadingSwipeActionsConfigurationForRowAt indexPath: IndexPath) -> UISwipeActionsConfiguration? {
        return (tableView.cellForRow(at: indexPath) as? TableViewSwipeActionProvider)?.leadingSwipeActionsConfiguration()
    }
    
    override func tableView(_ tableView: UITableView, trailingSwipeActionsConfigurationForRowAt indexPath: IndexPath) -> UISwipeActionsConfiguration? {
        return (tableView.cellForRow(at: indexPath) as? TableViewSwipeActionProvider)?.trailingSwipeActionsConfiguration()
    }
    
    // MARK: - RefreshableViewController
    
    func refresh() {
        guard state != .loadingNewer else { return }
        
        state = .loadingNewer
        
        let snapshot = dataSource.snapshot()
        
        var item: Item? = nil
        for section in timelineContentSections() {
            if let first = snapshot.itemIdentifiers(inSection: section).first {
                item = first
                break
            }
        }
        
        loadNewerItems(currentSnapshot: snapshot) { result in
            guard case let .success(snapshot) = result else {
                DispatchQueue.main.async {
                    self.refreshControl?.endRefreshing()
                    self.state = .loaded
                }
                return
            }
            
            DispatchQueue.main.async {
                self.refreshControl?.endRefreshing()
                self.dataSource.apply(snapshot, animatingDifferences: false)
                self.state = .loaded

                if let item = item,
                   let indexPath = self.dataSource.indexPath(for: item) {
                    // maintain the current position in the list (don't scroll to top)
                    self.tableView.scrollToRow(at: indexPath, at: .top, animated: false)
                }
            }
        }
    }
    
    // MARK: - Subclass Methods
    
    func cellProvider(_ tableView: UITableView, _ indexPath: IndexPath, _ item: Item) -> UITableViewCell? {
        fatalError("cellProvider(_:_:_:) must be implemented by subclasses")
    }
    
    func loadInitialItems(completion: @escaping (LoadResult) -> Void) {
        fatalError("loadInitialItems(completion:) must be implemented by subclasses")
    }
    
    func loadOlderItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
        fatalError("loadOlderItesm(completion:) must be implemented by subclasses")
    }
    
    func loadNewerItems(currentSnapshot: Snapshot, completion: @escaping (LoadResult) -> Void) {
        fatalError("loadNewerItems(completion:) must be implemented by subclasses")
    }
    
    func timelineContentSections() -> Section.AllCases {
        return Section.allCases
    }
    
    func willRemoveItems(_ items: [Item]) {
    }

}

extension DiffableTimelineLikeTableViewController {
    enum State: Equatable {
        case unloaded
        case loadingInitial
        case loaded
        case loadingNewer
        case loadingOlder
    }
}

extension DiffableTimelineLikeTableViewController {
    enum LoadError: LocalizedError {
        case noClient
        case noOlder
        case noNewer
        case client(Client.Error)
    }
}

extension DiffableTimelineLikeTableViewController: BackgroundableViewController {
    func sceneDidEnterBackground() {
        pruneOffscreenRows()
    }
}