Tusker/Tusker/Screens/Utilities/TimelineLikeTableViewController.swift

//
//  TimelineLikeTableViewController.swift
//  Tusker
//
//  Created by Shadowfacts on 11/15/20.
//  Copyright © 2020 Shadowfacts. All rights reserved.
//

import UIKit

/// A table view controller that manages common functionality  between timeline-like UIs.
/// For example, this class handles loading new items when the user scrolls to the end,
/// refreshing, and pruning offscreen rows automatically.
class TimelineLikeTableViewController<Item>: EnhancedTableViewController, RefreshableViewController {

    private(set) var loaded = false
    
    var sections: [[Item]] = []
    
    private let pageSize = 20
    
    private var lastLastVisibleRow: IndexPath?
    
    init() {
        super.init(style: .plain)
        
        addKeyCommand(MenuController.refreshCommand(discoverabilityTitle: Self.refreshCommandTitle()))
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func item(for indexPath: IndexPath) -> Item {
        return sections[indexPath.section][indexPath.row]
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        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 viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        
        pruneOffscreenRows()
    }
    
    func loadInitial() {
        guard !loaded else { return }
        // set loaded immediately so we don't trigger another request while the current one is running
        loaded = true
        
        loadInitialItems() { (items) in
            DispatchQueue.main.async {
                guard items.count > 0 else {
                    // set loaded back to false so the next time the VC appears, we try to load again
                    // todo: this should probably retry automatically
                    self.loaded = false
                    return
                }
                
                if self.sections.count < self.headerSectionsCount() {
                    self.sections.insert(contentsOf: Array(repeating: [], count: self.headerSectionsCount() - self.sections.count), at: 0)
                }
                
                self.sections.append(items)
                
                self.tableView.reloadData()
            }
        }
    }
    
    func reloadInitialItems() {
        loaded = false
        sections = []
        loadInitial()
    }
    
    func cellHeightChanged() {
        // causes the table view to recalculate the cell heights
        tableView.beginUpdates()
        tableView.endUpdates()
    }
    
    class func refreshCommandTitle() -> String {
        return "Refresh"
    }
    
    // todo: these three should use Result<[Item], Client.Error> so we can differentiate between failed requests and there actually being no results
    
    func loadInitialItems(completion: @escaping ([Item]) -> Void) {
        fatalError("loadInitialItems(completion:) must be implemented by subclasses")
    }
    
    func loadOlder(completion: @escaping ([Item]) -> Void) {
        fatalError("loadOlder(completion:) must be implemented by subclasses")
    }
    
    func loadNewer(completion: @escaping ([Item]) -> Void) {
        fatalError("loadNewer(completion:) must be implemented by subclasses")
    }
    
    func willRemoveRows(at indexPaths: [IndexPath]) {
    }
    
    func headerSectionsCount() -> Int {
        return 0
    }
    
    private func pruneOffscreenRows() {
        guard let lastVisibleRow = lastLastVisibleRow,
              // never remove the last section
              sections.count - headerSectionsCount() > 1 else {
            return
        }
        let lastSectionIndex = sections.count - 1
        
        if lastVisibleRow.section < lastSectionIndex {
            // if there is a section below the last visible one
            
            let sectionsToRemove = (lastVisibleRow.section + 1)...lastSectionIndex
            
            let indexPathsToRemove = sectionsToRemove.flatMap { (section) in
                sections[section].indices.map { (row) in
                    IndexPath(row: row, section: section)
                }
            }
            willRemoveRows(at: indexPathsToRemove)
            
            UIView.performWithoutAnimation {
                tableView.deleteSections(IndexSet(sectionsToRemove), with: .none)
            }
            
            sections.removeSubrange(sectionsToRemove)
        } else if lastVisibleRow.section == lastSectionIndex {
            let lastSection = sections.last!
            let lastRowIndex = lastSection.count - 1
            
            if lastVisibleRow.row < lastRowIndex - pageSize {
                // if there are more than pageSize rows in the current section below the last visible one
                
                let rowIndicesInLastSectionToRemove = (lastVisibleRow.row + pageSize)..<lastSection.count
                
                let indexPathsToRemove = rowIndicesInLastSectionToRemove.map {
                    IndexPath(row: $0, section: lastSectionIndex)
                }
                willRemoveRows(at: indexPathsToRemove)
                
                sections[lastSectionIndex].removeSubrange(rowIndicesInLastSectionToRemove)
                
                UIView.performWithoutAnimation {
                    tableView.deleteRows(at: indexPathsToRemove, with: .none)
                }
            }
        }
        
    }
    
    // MARK: - UITableViewDataSource
    override func numberOfSections(in tableView: UITableView) -> Int {
        return sections.count
    }
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return sections[section].count
    }
    
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        fatalError("tableView(_:cellForRowAt:) must be implemented by subclasses")
    }
    
    // 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
        
        if indexPath.section == sections.count - 1,
           indexPath.row == sections[indexPath.section].count - 1 {
            loadOlder() { (newItems) in
                guard newItems.count > 0 else { return }
                
                DispatchQueue.main.async {
                    let newRows = self.sections.last!.count..<(self.sections.last!.count + newItems.count)
                    let newIndexPaths = newRows.map { IndexPath(row: $0, section: self.sections.count - 1) }
                    
                    self.sections[self.sections.count - 1].append(contentsOf: newItems)
                    
                    UIView.performWithoutAnimation {
                        self.tableView.insertRows(at: newIndexPaths, with: .none)
                    }
                }
            }
        }
    }
    
    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() {
        loadNewer() { (newItems) in
            DispatchQueue.main.async {
                self.refreshControl?.endRefreshing()
                
                guard newItems.count > 0 else { return }
                
                let firstNonHeaderSection = self.headerSectionsCount()
                
                self.sections[firstNonHeaderSection].insert(contentsOf: newItems, at: 0)
                
                let newIndexPaths = (0..<newItems.count).map { IndexPath(row: $0, section: firstNonHeaderSection) }
                UIView.performWithoutAnimation {
                    self.tableView.insertRows(at: newIndexPaths, with: .none)
                }

                // maintain the current position in the list (don't scroll to top)
                self.tableView.scrollToRow(at: IndexPath(row: newItems.count, section: firstNonHeaderSection), at: .top, animated: false)
            }
        }
    }
}

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