block-employee-directory-interview/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?
    
    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.register(EmployeeTableViewCell.self, forCellReuseIdentifier: EmployeeTableViewCell.identifier)
        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
    }
    
    /// 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()
                }
            }
            .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, please try to refresh." : 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.changeMode(to: selectedMode)
    }
}