Ditch custom image request grouping, rely on URLSession's

Tusker/Caching/ImageCache.swift

@@ -24,10 +24,6 @@ class ImageCache {
     private let cache: ImageDataCache
     private let desiredPixelSize: CGSize?
     
-    private var groups = MultiThreadDictionary<URL, RequestGroup>()
-    
-    private var backgroundQueue = DispatchQueue(label: "ImageCache completion queue", qos: .default)
-    
     init(name: String, memoryExpiry: CacheExpiry, diskExpiry: CacheExpiry? = nil, desiredSize: CGSize? = nil) {
         // todo: might not always want to use UIScreen.main for this, e.g. Catalyst?
         let pixelSize = desiredSize?.applying(.init(scaleX: UIScreen.main.scale, y: UIScreen.main.scale))
@@ -61,14 +57,9 @@ class ImageCache {
             wrappedCompletion?(entry.data, entry.image)
             return nil
         } else {
-            if let group = groups[url] {
+            let task = dataTask(url: url, completion: wrappedCompletion)
-                return group.addCallback(wrappedCompletion)
+            task.resume()
-            } else {
+            return task
-                let group = createGroup(url: url)
-                let request = group.addCallback(wrappedCompletion)
-                group.run()
-                return request
-            }
         }
     }
     
@@ -85,22 +76,23 @@ class ImageCache {
         // 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),
+        if !((try? cache.has(url.absoluteString)) ?? false) {
-           !groups.contains(key: url) {
+            let task = dataTask(url: url) { data, image in
-            let group = createGroup(url: url)
+                guard let data else { return }
-            group.run()
+                try? self.cache.set(url.absoluteString, data: data, image: image)
+            }
+            task.resume()
         }
     }
     
-    private func createGroup(url: URL) -> RequestGroup {
+    private func dataTask(url: URL, completion: ((Data?, UIImage?) -> Void)?) -> URLSessionDataTask {
-        let group = RequestGroup(url: url) { (data, image) in
+        return URLSession.shared.dataTask(with: url) { data, response, error in
-            if let data = data {
+            guard error == nil,
-                try? self.cache.set(url.absoluteString, data: data, image: image)
+                  let data else {
+                return
             }
-            _ = self.groups.removeValue(forKey: url)
+            completion?(data, UIImage(data: data))
         }
-        groups[url] = group
-        return group
     }
     
     func getData(_ url: URL) -> Data? {
@@ -110,88 +102,11 @@ class ImageCache {
     func get(_ url: URL, loadOriginal: Bool = false) -> ImageDataCache.Entry? {
         return try? cache.get(url.absoluteString, loadOriginal: loadOriginal)
     }
-
-    func cancelWithoutCallback(_ url: URL) {
-        groups[url]?.cancelWithoutCallback()
-    }
     
     func reset() throws {
         try cache.removeAll()
     }
     
-    private class RequestGroup {
+    typealias Request = URLSessionDataTask
-        let url: URL
-        private let onFinished: (Data?, UIImage?) -> Void
-        private var task: URLSessionDataTask?
-        private var requests = [Request]()
-
-        init(url: URL, onFinished: @escaping (Data?, UIImage?) -> Void) {
-            self.url = url
-            self.onFinished = onFinished
-        }
-
-        deinit {
-            task?.cancel()
-        }
-        
-        func run() {
-            task = URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
-                guard error == nil, let data = data else {
-                    self.complete(with: nil)
-                    return
-                }
-                self.complete(with: data)
-            })
-            task!.resume()
-        }
-        
-        func addCallback(_ completion: ((Data?, UIImage?) -> Void)?) -> Request {
-            let request = Request(callback: completion)
-            requests.append(request)
-            return request
-        }
-        
-        func cancelWithoutCallback() {
-            if let request = requests.first(where: { $0.callback == nil && !$0.cancelled }) {
-                request.cancel()
-            }
-        }
-        
-        fileprivate func requestCancelled() {
-            let remaining = requests.filter { !$0.cancelled }.count
-            if remaining <= 0 {
-                task?.cancel()
-                complete(with: nil)
-            }
-        }
-
-        func complete(with data: Data?) {
-            let image = data != nil ? UIImage(data: data!) : nil
-            
-            requests.filter { !$0.cancelled }.forEach {
-                if let callback = $0.callback {
-                    callback(data, image)
-                }
-            }
-            
-            self.onFinished(data, image)
-        }
-    }
-    
-    class Request {
-        private weak var group: RequestGroup?
-        private(set) var callback: ((Data?, UIImage?) -> Void)?
-        private(set) var cancelled: Bool = false
-        
-        init(callback: ((Data?, UIImage?) -> Void)?) {
-            self.callback = callback
-        }
-        
-        func cancel() {
-            cancelled = true
-            callback = nil
-            group?.requestCancelled()
-        }
-    }
     
 }

Tusker/Screens/Bookmarks/BookmarksTableViewController.swift

@@ -175,14 +175,4 @@ extension BookmarksTableViewController: UITableViewDataSourcePrefetching, Status
         let ids = indexPaths.map { statuses[$0.row].id }
         prefetchStatuses(with: ids)
     }
-    
-    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
-        let ids: [String] = indexPaths.compactMap {
-            guard $0.row < statuses.count else {
-                return nil
-            }
-            return statuses[$0.row].id
-        }
-        cancelPrefetchingStatuses(with: ids)
-    }
 }

Tusker/Screens/Conversation/ConversationTableViewController.swift

@@ -467,11 +467,6 @@ extension ConversationTableViewController: UITableViewDataSourcePrefetching, Sta
         let ids: [String] = indexPaths.compactMap { item(for: $0)?.id }
         prefetchStatuses(with: ids)
     }
-    
-    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
-        let ids: [String] = indexPaths.compactMap { item(for: $0)?.id }
-        cancelPrefetchingStatuses(with: ids)
-    }
 }
 
 extension ConversationTableViewController: ToastableViewController {

Tusker/Screens/Notifications/NotificationsTableViewController.swift

@@ -294,14 +294,4 @@ extension NotificationsTableViewController: UITableViewDataSourcePrefetching {
             }
         }
     }
-    
-    func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
-        for indexPath in indexPaths {
-            guard let group = dataSource.itemIdentifier(for: indexPath)?.group else { continue }
-            for notification in group.notifications {
-                guard let avatar = notification.account.avatar else { continue }
-                ImageCache.avatars.cancelWithoutCallback(avatar)
-            }
-        }
-    }
 }

Tusker/Screens/Utilities/StatusTablePrefetching.swift

@@ -30,22 +30,6 @@ extension StatusTablePrefetching {
         }
     }
     
-    func cancelPrefetchingStatuses(with ids: [String]) {
-        let context = apiController.persistentContainer.prefetchBackgroundContext
-        context.perform {
-            guard let statuses = getStatusesWith(ids: ids, in: context) else {
-                return
-            }
-            for status in statuses {
-                guard let avatar = status.account.avatar else { continue }
-                ImageCache.avatars.cancelWithoutCallback(avatar)
-                for attachment in status.attachments where attachment.kind == .image {
-                    ImageCache.attachments.cancelWithoutCallback(attachment.url)
-                }
-            }
-        }
-    }
-    
 }
 
 fileprivate func getStatusesWith(ids: [String], in context: NSManagedObjectContext) -> [StatusMO]? {