block-employee-directory/EmployeeDirectory/ViewControllers/EmployeesViewController.swift

//
//  ViewController.swift
//  EmployeeDirectory
//
//  Created by Matt Bruce on 1/20/25.
//

import UIKit
import Combine

class EmployeesViewController: UIViewController {
    
    // MARK: - Properties

    
    /// List for the employees
    private let tableView = UITableView()
    
    /// Will only show when fetching data occurs
    private let activityIndicator = UIActivityIndicatorView(style: .large)
    
    /// Allows the user to pick between service modes
    private let modeSegmentedControl = UISegmentedControl(items: EmployeeServiceMode.allCases.map{ $0.rawValue } )
    
    /// ViewModel in which drives the screen
    private let viewModel = EmployeesViewModel()
    
    /// Holds onto the ViewModels Subscribers
    private var cancellables = Set<AnyCancellable>()
    
    /// Will show specific state of the viewModel
    private var footerView: TableFooterView?
    
    // Prevents multiple calls during rapid scrolling
    private var isFetchingNextPage: Bool = false

    private var dataSource: UITableViewDiffableDataSource<Int, Employee>!

    // MARK: - Public Methods

    public override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        setupDataSource()
        bindViewModel()
        viewModel.fetchEmployees()
    }
    
    // MARK: - Private Methods
    
    
    /// Setting up the TableView Datasource
    private func setupDataSource() {
        dataSource = UITableViewDiffableDataSource<Int, Employee>(tableView: tableView) { tableView, indexPath, employee in
            guard let cell = tableView.dequeueReusableCell(withIdentifier: EmployeeTableViewCell.identifier,for: indexPath) as? EmployeeTableViewCell else {
                return UITableViewCell()
            }
            cell.configure(with: EmployeeCellViewModel(employee: employee))
            return cell
        }
    }
    
    //Snapshot Handling
    private func applySnapshot(employees: [Employee]) {
        var snapshot = NSDiffableDataSourceSnapshot<Int, Employee>()
        snapshot.appendSections([0])
        snapshot.appendItems(employees, toSection: 0)
        dataSource.apply(snapshot, animatingDifferences: true)
    }
    
    /// Setup the UI by adding the views to the main view
    private func setupUI() {
        view.backgroundColor = .white
        
        // Configure TableView
        tableView.register(EmployeeTableViewCell.self, forCellReuseIdentifier: EmployeeTableViewCell.identifier)
        tableView.delegate = self
        view.addSubview(tableView)
        tableView.frame = view.bounds

        //add pull to refresh
        tableView.refreshControl = UIRefreshControl()
        tableView.refreshControl?.addTarget(self, action: #selector(didPullToRefresh), for: .valueChanged)
        
        // Configure Activity Indicator
        activityIndicator.center = view.center
        view.addSubview(activityIndicator)

        // Configure Mode Selector
        modeSegmentedControl.selectedSegmentIndex = 0
        modeSegmentedControl.addTarget(self, action: #selector(onServiceModeChange), for: .valueChanged)
        navigationItem.titleView = modeSegmentedControl
        
        let sortButton = UIBarButtonItem(title: "Sort", style: .plain, target: self, action: #selector(showSortOptions))
        navigationItem.rightBarButtonItem = sortButton
    }
    
    private func presentSortingActionSheet() {
        let alert = UIAlertController(title: "Sort Employees", message: "Select a sort option", preferredStyle: .actionSheet)

        alert.addAction(UIAlertAction(title: "Full Name (Asc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .fullName
            self?.viewModel.sortOrder = .ascending
        })
        alert.addAction(UIAlertAction(title: "Full Name (Desc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .fullName
            self?.viewModel.sortOrder = .descending
        })
        alert.addAction(UIAlertAction(title: "Team (Asc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .team
            self?.viewModel.sortOrder = .ascending
        })
        alert.addAction(UIAlertAction(title: "Team (Desc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .team
            self?.viewModel.sortOrder = .descending
        })
        alert.addAction(UIAlertAction(title: "Employee Type (Asc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .employeeType
            self?.viewModel.sortOrder = .ascending
        })
        alert.addAction(UIAlertAction(title: "Employee Type (Desc)", style: .default) { [weak self] _ in
            self?.viewModel.sortField = .employeeType
            self?.viewModel.sortOrder = .ascending
        })
        alert.addAction(UIAlertAction(title: "Cancel", style: .cancel, handler: nil))
        present(alert, animated: true, completion: nil)
        
    }
    
    /// Using the ViewModel setup combine handlers
    private func bindViewModel() {
        viewModel.$employees
            .receive(on: RunLoop.main)
            .sink { [weak self] employees in
                self?.applySnapshot(employees: employees)
            }
            .store(in: &cancellables)
        
        viewModel.$isLoading
            .receive(on: RunLoop.main)
            .sink { [weak self] isLoading in
                if isLoading {
                    self?.activityIndicator.startAnimating()
                } else {
                    self?.activityIndicator.stopAnimating()
                    self?.tableView.refreshControl?.endRefreshing() // End refresh control
                }
            }
            .store(in: &cancellables)
        
        viewModel.$errorMessage
            .receive(on: RunLoop.main)
            .sink { [weak self] _ in
                self?.updateFooter()
            }
            .store(in: &cancellables)
    }
    
    private func animateEmployeeChanges(from oldEmployees: [Employee], to newEmployees: [Employee]) {
        let oldCount = oldEmployees.count
        let newCount = newEmployees.count

        // Case: Removing all employees
        if oldCount > 0 && newCount == 0 {
            let indexPaths = (0..<oldCount).map { IndexPath(row: $0, section: 0) }
            tableView.performBatchUpdates {
                tableView.deleteRows(at: indexPaths, with: .fade)
            }
            return
        }

        // Case: Resetting with new data (or switching modes)
        if newCount > 0 {
            tableView.reloadData()
            return
        }
    }
    
    /// Show state in specific use-cases for the EmployeesViewModel
    private func updateFooter() {
        var footerMessage: String?
        
        // Check for error messages or empty state first
        if let message = viewModel.errorMessage ?? (viewModel.employees.isEmpty && !viewModel.isLoading ? "No employees found, please try to refresh." : nil) {
            footerMessage = message
            
        }        
        // Show loading footer if there are more pages to load
        else if (viewModel.isLoading || isFetchingNextPage) && viewModel.hasMorePages {
            footerMessage = "Loading more employees..."
        }
        
        if let footerMessage {
            // Lazy initialize footerView if needed
            if footerView == nil {
                footerView = TableFooterView(message: footerMessage)
            } else {
                footerView?.update(message: footerMessage)
            }
            footerView?.frame = CGRect(x: 0, y: 0, width: tableView.frame.width, height: 150)
            tableView.tableFooterView = footerView

        } else {
            tableView.tableFooterView = nil
        }
    }
}

// MARK: - UITableViewDelegate
extension EmployeesViewController: UITableViewDelegate {
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let offsetY = scrollView.contentOffset.y
        let contentHeight = scrollView.contentSize.height
        let scrollViewHeight = scrollView.frame.size.height

        if offsetY > contentHeight - scrollViewHeight - 100 && !isFetchingNextPage {
            isFetchingNextPage = true
            updateFooter()
            viewModel.loadNextPage()

            // Reset the flag after a short delay to allow new fetches
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) { [weak self] in
                self?.isFetchingNextPage = false
            }
        }
    }
}


// Mark: - Objective-C Methods
extension EmployeesViewController {
    
    /// Show sort options
    @objc private func showSortOptions() {
        presentSortingActionSheet()
    }

    /// Fetch the Employees
    @objc private func didPullToRefresh() {
        viewModel.fetchEmployees()
    }
    
    /// This will handle services changes to test conditions to ensure UI works correctly.
    /// - Parameter sender: Mode in which to test
    @objc private func onServiceModeChange(_ sender: UISegmentedControl) {
        let selectedMode: EmployeeServiceMode
        switch sender.selectedSegmentIndex {
        case 0: selectedMode = .production
        case 1: selectedMode = .malformed
        case 2: selectedMode = .empty
        default: return
        }
        viewModel.changeMode(to: selectedMode)
    }
}