Add prefetching for images in table views

Tusker/Screens/Conversation/ConversationTableViewController.swift

@@ -40,6 +40,8 @@ class ConversationTableViewController: EnhancedTableViewController {
         tableView.register(UINib(nibName: "StatusTableViewCell", bundle: nil), forCellReuseIdentifier: "statusCell")
         tableView.register(UINib(nibName: "ConversationMainStatusTableViewCell", bundle: nil), forCellReuseIdentifier: "mainStatusCell")
         
+        tableView.prefetchDataSource = self
+        
         statusIDs = [mainStatusID]
         
         guard let mainStatus = MastodonCache.status(for: mainStatusID) else { fatalError("Missing cached status \(mainStatusID!)") }
@@ -147,3 +149,25 @@ class ConversationTableViewController: EnhancedTableViewController {
 }
 
 extension ConversationTableViewController: StatusTableViewCellDelegate {}
+
+extension ConversationTableViewController: UITableViewDataSourcePrefetching {
+    func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            for attachment in status.attachments {
+                ImageCache.attachments.get(attachment.url, completion: nil)
+            }
+        }
+    }
+    
+    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.cancel(status.account.avatar)
+            for attachment in status.attachments {
+                ImageCache.attachments.cancel(attachment.url)
+            }
+        }
+    }
+}

Tusker/Screens/Notifications/NotificationsTableViewController.swift

@@ -44,6 +44,8 @@ class NotificationsTableViewController: EnhancedTableViewController {
         tableView.register(UINib(nibName: "ActionNotificationTableViewCell", bundle: nil), forCellReuseIdentifier: "actionCell")
         tableView.register(UINib(nibName: "FollowNotificationTableViewCell", bundle: nil), forCellReuseIdentifier: "followCell")
         
+        tableView.prefetchDataSource = self
+        
         let request = MastodonController.client.getNotifications()
         MastodonController.client.run(request) { result in
             guard case let .success(notifications, pagination) = result else { fatalError() }
@@ -150,3 +152,19 @@ class NotificationsTableViewController: EnhancedTableViewController {
 }
 
 extension NotificationsTableViewController: StatusTableViewCellDelegate {}
+
+extension NotificationsTableViewController: UITableViewDataSourcePrefetching {
+    func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            let notification = notifications[indexPath.row]
+            ImageCache.avatars.get(notification.account.avatar, completion: nil)
+        }
+    }
+    
+    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            let notification = notifications[indexPath.row]
+            ImageCache.avatars.cancel(notification.account.url)
+        }
+    }
+}

Tusker/Screens/Profile/ProfileTableViewController.swift

@@ -59,6 +59,8 @@ class ProfileTableViewController: EnhancedTableViewController, PreferencesAdapti
         tableView.register(UINib(nibName: "StatusTableViewCell", bundle: nil), forCellReuseIdentifier: "statusCell")
         tableView.register(UINib(nibName: "ProfileHeaderTableViewCell", bundle: nil), forCellReuseIdentifier: "headerCell")
         
+        tableView.prefetchDataSource = self
+        
         if let accountID = accountID {
             if MastodonCache.account(for: accountID) != nil {
                 updateAccountUI()
@@ -283,3 +285,27 @@ extension ProfileTableViewController: ProfileHeaderTableViewCellDelegate {
         }
     }
 }
+
+extension ProfileTableViewController: UITableViewDataSourcePrefetching {
+    func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard indexPath.section == 1,
+                let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            for attachment in status.attachments {
+                ImageCache.attachments.get(attachment.url, completion: nil)
+            }
+        }
+    }
+    
+    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard indexPath.section == 1,
+                let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.cancel(status.account.avatar)
+            for attachment in status.attachments {
+                ImageCache.attachments.cancel(attachment.url)
+            }
+        }
+    }
+}

Tusker/Screens/Timeline/TimelineTableViewController.swift

@@ -71,6 +71,8 @@ class TimelineTableViewController: EnhancedTableViewController {
         
         tableView.register(UINib(nibName: "StatusTableViewCell", bundle: nil), forCellReuseIdentifier: "statusCell")
         
+        tableView.prefetchDataSource = self
+        
         guard MastodonController.client?.accessToken != nil else { return }
         let request = MastodonController.client.getStatuses(timeline: timeline)
         MastodonController.client.run(request) { response in
@@ -168,3 +170,25 @@ class TimelineTableViewController: EnhancedTableViewController {
 }
 
 extension TimelineTableViewController: StatusTableViewCellDelegate {}
+
+extension TimelineTableViewController: UITableViewDataSourcePrefetching {
+    func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            for attachment in status.attachments {
+                ImageCache.attachments.get(attachment.url, completion: nil)
+            }
+        }
+    }
+    
+    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
+        for indexPath in indexPaths {
+            guard let status = MastodonCache.status(for: statusIDs[indexPath.row]) else { continue }
+            ImageCache.avatars.cancel(status.account.avatar)
+            for attachment in status.attachments {
+                ImageCache.attachments.cancel(attachment.url)
+            }
+        }
+    }
+}