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()
    
    /// Filtering the employees
    private let searchBar = UISearchBar()
    
    /// 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?
    
    private var dataSource: UITableViewDiffableDataSource<Int, Employee>!

    // MARK: - Public Methods

    public override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        setupDataSource()
        bindViewModel()
        viewModel.fetchEmployees()
    }
    
    // MARK: - Private Methods
    
    
    /// Setup 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.translatesAutoresizingMaskIntoConstraints = false
        tableView.register(EmployeeTableViewCell.self, forCellReuseIdentifier: EmployeeTableViewCell.identifier)
        tableView.estimatedRowHeight = 80 // Provide a reasonable default height
        tableView.rowHeight = UITableView.automaticDimension // Enable dynamic row height
        
        // Configure Segmented Control
        let segmentedContainer = UIView()
        segmentedContainer.translatesAutoresizingMaskIntoConstraints = false
        segmentedContainer.addSubview(modeSegmentedControl)
        
        modeSegmentedControl.selectedSegmentIndex = 0
        modeSegmentedControl.addTarget(self, action: #selector(onServiceModeChange), for: .valueChanged)
        modeSegmentedControl.translatesAutoresizingMaskIntoConstraints = false

        let stackView = UIStackView(arrangedSubviews: [segmentedContainer, tableView])
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.axis = .vertical
        stackView.distribution = .fill
        stackView.spacing = 5
        
        view.addSubview(stackView)
        
        NSLayoutConstraint.activate([
            stackView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            stackView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            stackView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
            stackView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
            modeSegmentedControl.topAnchor.constraint(equalTo: segmentedContainer.topAnchor),
            modeSegmentedControl.bottomAnchor.constraint(equalTo: segmentedContainer.bottomAnchor, constant: -5),
            modeSegmentedControl.leadingAnchor.constraint(equalTo: segmentedContainer.leadingAnchor, constant: 16),
            modeSegmentedControl.trailingAnchor.constraint(equalTo: segmentedContainer.trailingAnchor, constant: -16)
        ])
                                    
        //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)

        /// setup Search
        searchBar.placeholder = "Search employees"
        searchBar.delegate = self
        navigationItem.titleView = searchBar
    }
    
    /// Using the ViewModel setup combine handlers
    private func bindViewModel() {
        viewModel.$employees
            .receive(on: RunLoop.main)
            .sink { [weak self] employees in
                self?.updateFooter()
                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()
                    self?.updateFooter()
                }
            }
            .store(in: &cancellables)
        
        viewModel.$errorMessage
            .receive(on: RunLoop.main)
            .sink { [weak self] _ in
                self?.updateFooter()
            }
            .store(in: &cancellables)
    }
    
    /// Show state in specific use-cases for the EmployeesViewModel
    private func updateFooter() {
        var message: String? {
            guard !viewModel.isLoading else { return nil }
            return viewModel.errorMessage ?? (viewModel.employees.isEmpty ? "No employees found" : nil)
        }
        
        if let message, !viewModel.isLoading {
            // Lazy initialize footerView if needed
            if footerView == nil {
                footerView = TableFooterView(message: message)
            } else { // Update the message
                footerView?.update(message: message)
            }
            footerView?.frame = CGRect(x: 0, y: 0, width: tableView.frame.width, height: 150)
            tableView.tableFooterView = footerView
        } else {
            tableView.tableFooterView = nil
        }
    }
}

// Mark: - Objective-C Methods
extension EmployeesViewController {
    
    /// 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.changeService(to: selectedMode.service)
    }
}

extension EmployeesViewController: UISearchBarDelegate {
    public func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
        viewModel.searchText = searchText // Updates search text in ViewModel
    }
    
    func searchBarCancelButtonClicked(_ searchBar: UISearchBar) {
        searchBar.text = ""
        searchBar.resignFirstResponder()
        viewModel.searchText = "" // Clear search and reset list
    }
}