Add loading indicator to Trends screen

Tusker/Screens/Explore/TrendsViewController.swift

@@ -42,6 +42,12 @@ class TrendsViewController: UIViewController, CollectionViewController {
         let layout = UICollectionViewCompositionalLayout { [unowned self] sectionIndex, environment in
             let sectionIdentifier = self.dataSource.snapshot().sectionIdentifiers[sectionIndex]
             switch sectionIdentifier {
+            case .loadingIndicator:
+                var listConfig = UICollectionLayoutListConfiguration(appearance: .grouped)
+                listConfig.backgroundColor = .appGroupedBackground
+                listConfig.showsSeparators = false
+                return .list(using: listConfig, layoutEnvironment: environment)
+                
             case .trendingHashtags:
                 var listConfig = UICollectionLayoutListConfiguration(appearance: .grouped)
                 listConfig.headerMode = .supplementary
@@ -107,6 +113,9 @@ class TrendsViewController: UIViewController, CollectionViewController {
             headerView.contentConfiguration = config
         }
         
+        let loadingCell = UICollectionView.CellRegistration<LoadingCollectionViewCell, Void> { cell, indexPath, itemIdentifier in
+            cell.indicator.startAnimating()
+        }
         let trendingHashtagCell = UICollectionView.CellRegistration<TrendingHashtagCollectionViewCell, Hashtag> { (cell, indexPath, hashtag) in
             cell.updateUI(hashtag: hashtag)
         }
@@ -125,7 +134,7 @@ class TrendsViewController: UIViewController, CollectionViewController {
         let moreCell = UICollectionView.SupplementaryRegistration<MoreTrendsFooterCollectionViewCell>(elementKind: UICollectionView.elementKindSectionFooter) { [unowned self] supplementaryView, elementKind, indexPath in
             supplementaryView.delegate = self
             switch self.dataSource.sectionIdentifier(for: indexPath.section) {
-            case nil, .trendingStatuses:
+            case nil, .loadingIndicator, .trendingStatuses:
                 fatalError()
             case .trendingHashtags:
                 supplementaryView.updateUI(.hashtags)
@@ -138,6 +147,9 @@ class TrendsViewController: UIViewController, CollectionViewController {
         
         let dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView) { collectionView, indexPath, item in
             switch item {
+            case .loadingIndicator:
+                return collectionView.dequeueConfiguredReusableCell(using: loadingCell, for: indexPath, item: ())
+                
             case let .tag(hashtag):
                 return collectionView.dequeueConfiguredReusableCell(using: trendingHashtagCell, for: indexPath, item: hashtag)
                 
@@ -189,6 +201,11 @@ class TrendsViewController: UIViewController, CollectionViewController {
         }
         
         var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+        snapshot.appendSections([.loadingIndicator])
+        snapshot.appendItems([.loadingIndicator])
+        await apply(snapshot: snapshot)
+        
+        snapshot = NSDiffableDataSourceSnapshot()
 
         let hashtagsReq = Client.getTrendingHashtags(limit: 5)
         let hashtags = try? await mastodonController.run(hashtagsReq).0
@@ -271,13 +288,16 @@ class TrendsViewController: UIViewController, CollectionViewController {
 
 extension TrendsViewController {
     enum Section {
+        case loadingIndicator
         case trendingHashtags
         case trendingLinks
         case profileSuggestions
         case trendingStatuses
         
-        var title: String {
+        var title: String? {
             switch self {
+            case .loadingIndicator:
+                return nil
             case .trendingHashtags:
                 return "Trending Hashtags"
             case .trendingLinks:
@@ -290,6 +310,7 @@ extension TrendsViewController {
         }
     }
     enum Item: Equatable, Hashable {
+        case loadingIndicator
         case status(String, CollapseState)
         case tag(Hashtag)
         case link(Card)
@@ -297,6 +318,8 @@ extension TrendsViewController {
         
         static func == (lhs: Item, rhs: Item) -> Bool {
             switch (lhs, rhs) {
+            case (.loadingIndicator, .loadingIndicator):
+                return true
             case let (.status(a, _), .status(b, _)):
                 return a == b
             case let (.tag(a), .tag(b)):
@@ -312,6 +335,8 @@ extension TrendsViewController {
         
         func hash(into hasher: inout Hasher) {
             switch self {
+            case .loadingIndicator:
+                hasher.combine("loadingIndicator")
             case let .status(id, _):
                 hasher.combine("status")
                 hasher.combine(id)
@@ -326,15 +351,31 @@ extension TrendsViewController {
                 hasher.combine(id)
             }
         }
+        
+        var shouldSelect: Bool {
+            switch self {
+            case .loadingIndicator:
+                return false
+            default:
+                return true
+            }
+        }
     }
 }
 
 extension TrendsViewController: UICollectionViewDelegate {
+    func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool {
+        return dataSource.itemIdentifier(for: indexPath)?.shouldSelect ?? false
+    }
+    
     func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
         guard let item = dataSource.itemIdentifier(for: indexPath) else {
             return
         }
         switch item {
+        case .loadingIndicator:
+            return
+            
         case let .tag(hashtag):
             show(HashtagTimelineViewController(for: hashtag, mastodonController: mastodonController), sender: nil)
             
@@ -358,6 +399,9 @@ extension TrendsViewController: UICollectionViewDelegate {
         }
         
         switch item {
+        case .loadingIndicator:
+            return nil
+            
         case let .tag(hashtag):
             return UIContextMenuConfiguration(identifier: nil) {
                 HashtagTimelineViewController(for: hashtag, mastodonController: self.mastodonController)
@@ -443,6 +487,9 @@ extension TrendsViewController: UICollectionViewDragDelegate {
                   return []
               }
         switch item {
+        case .loadingIndicator:
+            return []
+            
         case let .tag(hashtag):
             guard let url = URL(hashtag.url) else {
                 return []