[Framework] loading indicator for the Feed list (#14)

This PR contains the work done to integrate a loading indicator to the `FeedListViewController` view controller.

Reviewed-on: rock-n-code/app-reviews#14
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Frameworks/Feed/Bundle/Sources/UI/View Controllers/FeedListViewController.swift

@@ -22,6 +22,14 @@ public class FeedListViewController: UITableViewController {
     private var cancellables: Set<AnyCancellable> = []
     
     // MARK: Outlets
+    private lazy var activityIndicator = {
+        let indicator = UIActivityIndicatorView()
+        
+        indicator.translatesAutoresizingMaskIntoConstraints = false
+
+        return indicator
+    }()
+
     private lazy var filterButton = {
         UIBarButtonItem(
             title: NSLocalizedString(
@@ -92,6 +100,7 @@ public class FeedListViewController: UITableViewController {
 
         setNavigationBar()
         setView()
+        setLayout()
         registerTableCells()
         bindViewModel()
         
@@ -163,11 +172,20 @@ private extension FeedListViewController {
             }
             .store(in: &cancellables)
         
+        viewModel.$isLoading
+            .filter { $0 == true }
+            .receive(on: RunLoop.main)
+            .sink { [weak self] _ in
+                self?.activityIndicator.startAnimating()
+            }
+            .store(in: &cancellables)
+
         viewModel.$isLoading
             .dropFirst()
             .filter { $0 == false }
             .receive(on: RunLoop.main)
             .sink { [weak self] _ in
+                self?.activityIndicator.stopAnimating()
                 self?.pullControl.endRefreshing()
                 self?.tableView.reloadData()
                 self?.tableView.scrollToRow(
@@ -232,6 +250,13 @@ private extension FeedListViewController {
         TopWordsCell.register(in: tableView)
     }
     
+    func setLayout() {
+        NSLayoutConstraint.activate([
+            activityIndicator.centerXAnchor.constraint(equalTo: view.centerXAnchor),
+            activityIndicator.centerYAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerYAnchor),
+        ])
+    }
+    
     func setNavigationBar() {
         navigationController?.navigationBar.prefersLargeTitles = true
         navigationController?.navigationBar.isTranslucent = true
@@ -246,6 +271,11 @@ private extension FeedListViewController {
     
     func setView() {
         tableView.refreshControl = pullControl
+        
+        view.addSubview(activityIndicator)
+        view.bringSubviewToFront(activityIndicator)
+        
+        activityIndicator.startAnimating()
     }
     
     func updateFilterMenu(_ option: FilterOption) {