Avoid loading cached data into memory when prefetching

Tusker/Caching/ImageCache.swift

@@ -51,13 +51,7 @@ class ImageCache {
                 }
                 return nil
             } else {
-                let group = RequestGroup(url: url) { (data, image) in
+                let group = createGroup(url: url)
-                    if let data = data {
-                        try? self.cache.set(key, data: data)
-                    }
-                    self.groups.removeValueWithoutReturning(forKey: url)
-                }
-                groups[url] = group
                 let request = group.addCallback(completion)
                 group.run()
                 return request
@@ -65,6 +59,28 @@ class ImageCache {
         }
     }
     
+    func fetchIfNotCached(_ url: URL) {
+        // if caching is disabled, don't bother fetching since nothing will be done with the result
+        guard !ImageCache.disableCaching else { return }
+        
+        if !((try? cache.has(url.absoluteString)) ?? false),
+           !groups.contains(key: url) {
+            let group = createGroup(url: url)
+            group.run()
+        }
+    }
+    
+    private func createGroup(url: URL) -> RequestGroup {
+        let group = RequestGroup(url: url) { (data, image) in
+            if let data = data {
+                try? self.cache.set(url.absoluteString, data: data)
+            }
+            self.groups.removeValueWithoutReturning(forKey: url)
+        }
+        groups[url] = group
+        return group
+    }
+    
     func getData(_ url: URL) -> Data? {
         return try? cache.getData(url.absoluteString)
     }

Tusker/MultiThreadDictionary.swift

@@ -47,4 +47,12 @@ class MultiThreadDictionary<Key: Hashable, Value> {
         }
         return value
     }
+    
+    func contains(key: Key) -> Bool {
+        var value: Bool!
+        queue.sync {
+            value = dict.keys.contains(key)
+        }
+        return value
+    }
 }

Tusker/Screens/Notifications/NotificationsTableViewController.swift

@@ -213,7 +213,7 @@ extension NotificationsTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             for notification in item(for: indexPath).notifications {
-                _ = ImageCache.avatars.get(notification.account.avatar, completion: nil)
+                ImageCache.avatars.fetchIfNotCached(notification.account.avatar)
             }
         }
     }

Tusker/Screens/Utilities/StatusTablePrefetching.swift

@@ -21,9 +21,9 @@ extension StatusTablePrefetching {
                 return
             }
             for status in statuses {
-                _ = ImageCache.avatars.get(status.account.avatar, completion: nil)
+                ImageCache.avatars.fetchIfNotCached(status.account.avatar)
                 for attachment in status.attachments where attachment.kind == .image {
-                    _ = ImageCache.attachments.get(attachment.url, completion: nil)
+                    ImageCache.attachments.fetchIfNotCached(attachment.url)
                 }
             }
         }