Add activity indicator to instance selector

Tusker/Screens/Onboarding/InstanceSelectorTableViewController.swift

@@ -29,6 +29,8 @@ class InstanceSelectorTableViewController: UITableViewController {
     var urlHandler: AnyCancellable?
     var currentQuery: String?
     
+    private var activityIndicator: UIActivityIndicatorView!
+    
     override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
         if UIDevice.current.userInterfaceIdiom == .phone {
             return .portrait
@@ -56,9 +58,9 @@ class InstanceSelectorTableViewController: UITableViewController {
         navigationItem.scrollEdgeAppearance = appearance
 
         tableView.register(UINib(nibName: "InstanceTableViewCell", bundle: .main), forCellReuseIdentifier: instanceCell)
-        
         tableView.rowHeight = UITableView.automaticDimension
         tableView.estimatedRowHeight = 120
+        createActivityIndicatorHeader()
         
         dataSource = DataSource(tableView: tableView, cellProvider: { (tableView, indexPath, item) -> UITableViewCell? in
             switch item {
@@ -78,12 +80,19 @@ class InstanceSelectorTableViewController: UITableViewController {
         searchController.obscuresBackgroundDuringPresentation = false
         searchController.searchBar.searchTextField.autocapitalizationType = .none
         navigationItem.searchController = searchController
+        navigationItem.hidesSearchBarWhenScrolling = false
         definesPresentationContext = true
         
         urlHandler = urlCheckerSubject
-            .debounce(for: .seconds(1), scheduler: RunLoop.main)
             .compactMap { $0?.trimmingCharacters(in: .whitespacesAndNewlines) }
-            .sink(receiveValue: updateSpecificInstance)
+            .map { [weak self] (s) -> String in
+                if !s.isEmpty {
+                    self?.activityIndicator.startAnimating()
+                }
+                return s
+            }
+            .debounce(for: .seconds(1), scheduler: RunLoop.main)
+            .sink { [weak self] in self?.updateSpecificInstance(domain: $0) }
         
         loadRecommendedInstances()
     }
@@ -117,6 +126,8 @@ class InstanceSelectorTableViewController: UITableViewController {
     }
     
     private func updateSpecificInstance(domain: String) {
+        activityIndicator.startAnimating()
+        
         let components = parseURLComponents(input: domain)
         let url = components.url!
 
@@ -138,7 +149,13 @@ class InstanceSelectorTableViewController: UITableViewController {
                 snapshot.appendItems([.selected(url, instance)], toSection: .selected)
                 
                 DispatchQueue.main.async {
-                    self.dataSource.apply(snapshot)
+                    self.dataSource.apply(snapshot) {
+                        self.activityIndicator.stopAnimating()
+                    }
+                }
+            } else {
+                DispatchQueue.main.async {
+                    self.activityIndicator.stopAnimating()
                 }
             }
         }
@@ -158,6 +175,28 @@ class InstanceSelectorTableViewController: UITableViewController {
         }
     }
     
+    private func createActivityIndicatorHeader() {
+        let header = UITableViewHeaderFooterView()
+        header.translatesAutoresizingMaskIntoConstraints = false
+        header.contentView.backgroundColor = .secondarySystemBackground
+        
+        activityIndicator = UIActivityIndicatorView(style: .large)
+        activityIndicator.translatesAutoresizingMaskIntoConstraints = false
+        header.contentView.addSubview(activityIndicator)
+        
+        NSLayoutConstraint.activate([
+            activityIndicator.centerXAnchor.constraint(equalTo: header.contentView.centerXAnchor),
+            activityIndicator.topAnchor.constraint(equalTo: header.contentView.topAnchor, constant: 4),
+            activityIndicator.bottomAnchor.constraint(equalTo: header.contentView.bottomAnchor, constant: -4),
+        ])
+        
+        let fittingSize = CGSize(width: tableView.bounds.width - (tableView.safeAreaInsets.left + tableView.safeAreaInsets.right), height: 0)
+        let size = header.systemLayoutSizeFitting(fittingSize, withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel)
+        header.frame = CGRect(origin: .zero, size: size)
+        
+        tableView.tableHeaderView = header
+    }
+    
     private func showRecommendationsError(_ error: Client.Error) {
         let footer = UITableViewHeaderFooterView()
         footer.translatesAutoresizingMaskIntoConstraints = false