Improve ImageCache loading

Keep track of the number of requests and only cancel the underlying
URLSessionTask if there are no concrete requsts remaining.

Closes #81

Tusker/Caching/ImageCache.swift

@@ -12,13 +12,13 @@ import Cache
 class ImageCache {
     
     static let avatars = ImageCache(name: "Avatars", memoryExpiry: .seconds(60 * 60), diskExpiry: .seconds(60 * 60 * 24))
-    static let headers = ImageCache(name: "Headers", memoryExpiry: .seconds(60 * 60), diskExpiry: .seconds(60 * 60 * 24))
+    static let headers = ImageCache(name: "Headers", memoryExpiry: .seconds(60 * 5), diskExpiry: .seconds(60 * 60))
     static let attachments = ImageCache(name: "Attachments", memoryExpiry: .seconds(60 * 2))
     static let emojis = ImageCache(name: "Emojis", memoryExpiry: .seconds(60 * 5), diskExpiry: .seconds(60 * 60))
 
     let cache: Cache<Data>
     
-    var requests = [URL: Request]()
+    var requests = [URL: RequestGroup]()
     
     init(name: String, memoryExpiry expiry: Expiry) {
         let storage = MemoryStorage<Data>(config: MemoryConfig(expiry: expiry))
@@ -36,20 +36,22 @@ class ImageCache {
         self.cache = .hybrid(HybridStorage(memoryStorage: memory, diskStorage: disk))
     }
     
-    func get(_ url: URL, completion: ((Data?) -> Void)?) {
+    func get(_ url: URL, completion: ((Data?) -> Void)?) -> Request? {
         let key = url.absoluteString
         if (try? cache.existsObject(forKey: key)) ?? false,
             let data = try? cache.object(forKey: key) {
             completion?(data)
+            return nil
         } else {
-            if let completion = completion, let request = requests[url] {
+            if let completion = completion, let group = requests[url] {
-                request.callbacks.append(completion)
+                return group.addCallback(completion)
             } else {
-                let request = Request(url: url, completion: completion)
+                let group = RequestGroup(url: url)
-                requests[url] = request
+                let request = group.addCallback(completion)
-                request.run { (data) in
+                group.run { (data) in
                     try? self.cache.setObject(data, forKey: key)
                 }
+                return request
             }
         }
     }
@@ -58,24 +60,23 @@ class ImageCache {
         return try? cache.object(forKey: url.absoluteString)
     }
 
-    func cancel(_ url: URL) {
+    func cancelWithoutCallback(_ url: URL) {
-        requests[url]?.cancel()
+        requests[url]?.cancelWithoutCallback()
     }
     
-    class Request {
+    class RequestGroup {
         let url: URL
-        var task: URLSessionDataTask?
+        private var task: URLSessionDataTask?
-        var callbacks: [(Data?) -> Void]
+        private var requests = [Request]()
 
-        init(url: URL, completion: ((Data?) -> Void)?) {
+        init(url: URL) {
-            if let completion = completion {
-                self.callbacks = [completion]
-            } else {
-                self.callbacks = []
-            }
             self.url = url
         }
 
+        deinit {
+            task?.cancel()
+        }
+        
         func run(cache: @escaping (Data) -> Void) {
             task = URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
                 guard error == nil, let data = data else {
@@ -88,13 +89,56 @@ class ImageCache {
             task!.resume()
         }
         
-        func cancel() {
+        private func updatePriority() {
+            task?.priority = max(1.0, URLSessionTask.defaultPriority + 0.1 * Float(requests.filter { !$0.cancelled }.count))
+        }
+        
+        func addCallback(_ completion: ((Data?) -> Void)?) -> Request {
+            let request = Request(callback: completion)
+            requests.append(request)
+            updatePriority()
+            return request
+        }
+        
+        func cancelWithoutCallback() {
+            if let request = requests.first(where: { $0.callback == nil && !$0.cancelled }) {
+                request.cancel()
+                updatePriority()
+            }
+        }
+        
+        fileprivate func requestCancelled() {
+            let remaining = requests.filter { !$0.cancelled }.count
+            if remaining <= 0 {
                 task?.cancel()
                 complete(with: nil)
+            } else {
+                updatePriority()
+            }
         }
 
         func complete(with data: Data?) {
-            callbacks.forEach { $0(data) }
+            requests.filter { !$0.cancelled }.forEach {
+                if let callback = $0.callback {
+                    callback(data)
+                }
+            }
+        }
+    }
+    
+    class Request {
+        weak var group: RequestGroup?
+        private(set) var callback: ((Data?) -> Void)?
+        private(set) var cancelled: Bool = false
+        
+        init(callback: ((Data?) -> Void)?) {
+            self.callback = callback
+        }
+        
+        func cancel() {
+            cancelled = true
+            callback = nil
+            group?.requestCancelled()
         }
     }
     

Tusker/Screens/Attachment/AttachmentViewController.swift

@@ -15,6 +15,8 @@ class AttachmentViewController: UIViewController {
     var largeImageVC: LargeImageViewController?
     var loadingVC: LoadingViewController?
     
+    var attachmentRequest: ImageCache.Request?
+    
     private var initialControlsVisible: Bool = true
     var controlsVisible: Bool {
         get {
@@ -51,15 +53,25 @@ class AttachmentViewController: UIViewController {
         } else {
             loadingVC = LoadingViewController()
             embedChild(loadingVC!)
-            ImageCache.attachments.get(attachment.url) { [weak self] (data) in
+            attachmentRequest = ImageCache.attachments.get(attachment.url) { [weak self] (data) in
+                guard let self = self else { return }
+                self.attachmentRequest = nil
                 DispatchQueue.main.async {
-                    self?.loadingVC?.removeViewAndController()
+                    self.loadingVC?.removeViewAndController()
-                    self?.createLargeImage(data: data!)
+                    self.createLargeImage(data: data!)
                 }
             }
         }
     }
     
+    override func didMove(toParent parent: UIViewController?) {
+        super.didMove(toParent: parent)
+        
+        if parent == nil {
+            attachmentRequest?.cancel()
+        }
+    }
+    
     func createLargeImage(data: Data) {
         guard let image = UIImage(data: data) else { return }
         largeImageVC = LargeImageViewController(image: image, description: attachment.description, sourceInfo: nil)

Tusker/Screens/Bookmarks/BookmarksTableViewController.swift

@@ -159,9 +159,9 @@ extension BookmarksTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statuses[indexPath.row].id) else { continue }
-            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            _ = ImageCache.avatars.get(status.account.avatar, completion: nil)
             for attachment in status.attachments where attachment.kind == .image {
-                ImageCache.attachments.get(attachment.url, completion: nil)
+                _ = ImageCache.attachments.get(attachment.url, completion: nil)
             }
         }
     }
@@ -169,9 +169,9 @@ extension BookmarksTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statuses[indexPath.row].id) else { continue }
-            ImageCache.avatars.cancel(status.account.avatar)
+            ImageCache.avatars.cancelWithoutCallback(status.account.avatar)
             for attachment in status.attachments where attachment.kind == .image {
-                ImageCache.attachments.cancel(attachment.url)
+                ImageCache.attachments.cancelWithoutCallback(attachment.url)
             }
         }
     }

Tusker/Screens/Conversation/ConversationTableViewController.swift

@@ -170,9 +170,9 @@ extension ConversationTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statuses[indexPath.row].id) else { continue }
-            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            _ = ImageCache.avatars.get(status.account.avatar, completion: nil)
             for attachment in status.attachments {
-                ImageCache.attachments.get(attachment.url, completion: nil)
+                _ = ImageCache.attachments.get(attachment.url, completion: nil)
             }
         }
     }
@@ -180,9 +180,9 @@ extension ConversationTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statuses[indexPath.row].id) else { continue }
-            ImageCache.avatars.cancel(status.account.avatar)
+            ImageCache.avatars.cancelWithoutCallback(status.account.avatar)
             for attachment in status.attachments {
-                ImageCache.attachments.cancel(attachment.url)
+                ImageCache.attachments.cancelWithoutCallback(attachment.url)
             }
         }
     }

Tusker/Screens/Notifications/NotificationsTableViewController.swift

@@ -240,7 +240,7 @@ extension NotificationsTableViewController: UITableViewDataSourcePrefetching {
         for indexPath in indexPaths {
             for notificationID in groups[indexPath.row].notificationIDs {
                 guard let notification = mastodonController.cache.notification(for: notificationID) else { continue }
-                ImageCache.avatars.get(notification.account.avatar, completion: nil)
+                _ = ImageCache.avatars.get(notification.account.avatar, completion: nil)
             }
         }
     }
@@ -249,7 +249,7 @@ extension NotificationsTableViewController: UITableViewDataSourcePrefetching {
         for indexPath in indexPaths {
             for notificationID in groups[indexPath.row].notificationIDs {
                 guard let notification = mastodonController.cache.notification(for: notificationID) else { continue }
-                ImageCache.avatars.cancel(notification.account.avatar)
+                ImageCache.avatars.cancelWithoutCallback(notification.account.avatar)
             }
         }
     }

Tusker/Screens/Profile/MyProfileTableViewController.swift

@@ -21,8 +21,8 @@ class MyProfileTableViewController: ProfileTableViewController {
         mastodonController.getOwnAccount { (account) in
             self.accountID = account.id
             
-            ImageCache.avatars.get(account.avatar, completion: { (data) in
+            _ = ImageCache.avatars.get(account.avatar, completion: { [weak self] (data) in
-                guard let data = data, let image = UIImage(data: data) else { return }
+                guard let self = self, let data = data, let image = UIImage(data: data) else { return }
                 DispatchQueue.main.async {
                     let size = CGSize(width: 30, height: 30)
                     let tabBarImage = UIGraphicsImageRenderer(size: size).image { (_) in

Tusker/Screens/Profile/ProfileTableViewController.swift

@@ -294,9 +294,9 @@ extension ProfileTableViewController: UITableViewDataSourcePrefetching {
         for indexPath in indexPaths where indexPath.section > 1 {
             let statusID = timelineSegments[indexPath.section - 2][indexPath.row].id
             guard let status = mastodonController.cache.status(for: statusID) else { continue }
-            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            _ = ImageCache.avatars.get(status.account.avatar, completion: nil)
             for attachment in status.attachments {
-                ImageCache.attachments.get(attachment.url, completion: nil)
+                _ = ImageCache.attachments.get(attachment.url, completion: nil)
             }
         }
     }
@@ -305,9 +305,9 @@ extension ProfileTableViewController: UITableViewDataSourcePrefetching {
         for indexPath in indexPaths where indexPath.section > 1 {
             let statusID = timelineSegments[indexPath.section - 2][indexPath.row].id
             guard let status = mastodonController.cache.status(for: statusID) else { continue }
-            ImageCache.avatars.cancel(status.account.avatar)
+            ImageCache.avatars.cancelWithoutCallback(status.account.avatar)
             for attachment in status.attachments {
-                ImageCache.attachments.cancel(attachment.url)
+                ImageCache.attachments.cancelWithoutCallback(attachment.url)
             }
         }
     }

Tusker/Screens/Timeline/TimelineTableViewController.swift

@@ -165,9 +165,9 @@ extension TimelineTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statusID(for: indexPath)) else { continue }
-            ImageCache.avatars.get(status.account.avatar, completion: nil)
+            _ = ImageCache.avatars.get(status.account.avatar, completion: nil)
             for attachment in status.attachments {
-                ImageCache.attachments.get(attachment.url, completion: nil)
+                _ = ImageCache.attachments.get(attachment.url, completion: nil)
             }
         }
     }
@@ -175,9 +175,9 @@ extension TimelineTableViewController: UITableViewDataSourcePrefetching {
     func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) {
         for indexPath in indexPaths {
             guard let status = mastodonController.cache.status(for: statusID(for: indexPath)) else { continue }
-            ImageCache.avatars.cancel(status.account.avatar)
+            ImageCache.avatars.cancelWithoutCallback(status.account.avatar)
             for attachment in status.attachments {
-                ImageCache.attachments.cancel(attachment.url)
+                ImageCache.attachments.cancelWithoutCallback(attachment.url)
             }
         }
     }

Tusker/Views/Account Cell/AccountTableViewCell.swift

@@ -19,7 +19,7 @@ class AccountTableViewCell: UITableViewCell {
     
     var accountID: String!
     
-    var avatarURL: URL?
+    var avatarRequest: ImageCache.Request?
     
     override func awakeFromNib() {
         super.awakeFromNib()
@@ -44,9 +44,9 @@ class AccountTableViewCell: UITableViewCell {
             fatalError("Missing cached account \(accountID)")
         }
         
-        self.avatarURL = account.avatar
+        avatarRequest = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-        ImageCache.avatars.get(account.avatar) { (data) in
+            guard let self = self, let data = data, self.accountID == accountID else { return }
-            guard let data = data, self.avatarURL == account.avatar else { return }
+            self.avatarRequest = nil
             DispatchQueue.main.async {
                 self.avatarImageView.image = UIImage(data: data)
             }
@@ -57,6 +57,12 @@ class AccountTableViewCell: UITableViewCell {
         updateUIForPrefrences()
     }
     
+    override func prepareForReuse() {
+        super.prepareForReuse()
+        
+        avatarRequest?.cancel()
+    }
+
 }
 
 extension AccountTableViewCell: SelectableTableViewCell {

Tusker/Views/Account Detail/LargeAccountDetailView.swift

@@ -15,6 +15,8 @@ class LargeAccountDetailView: UIView {
     var displayNameLabel = UILabel()
     var usernameLabel = UILabel()
     
+    var avatarRequest: ImageCache.Request?
+    
     required init?(coder aDecoder: NSCoder) {
         super.init(coder: aDecoder)
         
@@ -49,6 +51,10 @@ class LargeAccountDetailView: UIView {
         NotificationCenter.default.addObserver(self, selector: #selector(updateUIForPreferences), name: .preferencesChanged, object: nil)
     }
     
+    deinit {
+        avatarRequest?.cancel()
+    }
+    
     override func layoutSubviews() {
         super.layoutSubviews()
         
@@ -63,8 +69,9 @@ class LargeAccountDetailView: UIView {
         displayNameLabel.text = account.realDisplayName
         usernameLabel.text = "@\(account.acct)"
         
-        ImageCache.avatars.get(account.avatar) { (data) in
+        avatarRequest = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-            guard let data = data else { return }
+            guard let self = self, let data = data else { return }
+            self.avatarRequest = nil
             DispatchQueue.main.async {
                 self.avatarImageView.image = UIImage(data: data)
             }

Tusker/Views/Attachments/AttachmentView.swift

@@ -24,6 +24,8 @@ class AttachmentView: UIImageView, GIFAnimatable {
     var attachment: Attachment!
     var index: Int!
     
+    private var attachmentRequest: ImageCache.Request?
+    
     var gifData: Data?
 
     public lazy var animator: Animator? = Animator(withDelegate: self)
@@ -37,6 +39,10 @@ class AttachmentView: UIImageView, GIFAnimatable {
         loadAttachment()
     }
     
+    deinit {
+        attachmentRequest?.cancel()
+    }
+    
     required init?(coder aDecoder: NSCoder) {
         super.init(coder: aDecoder)
         commonInit()
@@ -71,8 +77,9 @@ class AttachmentView: UIImageView, GIFAnimatable {
     }
     
     func loadImage() {
-        ImageCache.attachments.get(attachment.url) { [weak self] (data) in
+        attachmentRequest = ImageCache.attachments.get(attachment.url) { [weak self] (data) in
             guard let self = self, let data = data else { return }
+            self.attachmentRequest = nil
             DispatchQueue.main.async {
                 if self.attachment.url.pathExtension == "gif" {
                     self.animate(withGIFData: data)
@@ -87,12 +94,14 @@ class AttachmentView: UIImageView, GIFAnimatable {
     }
     
     func loadVideo() {
+        let attachmentURL = self.attachment.url
         DispatchQueue.global(qos: .userInitiated).async {
-            let asset = AVURLAsset(url: self.attachment.url)
+            let asset = AVURLAsset(url: attachmentURL)
             let generator = AVAssetImageGenerator(asset: asset)
             generator.appliesPreferredTrackTransform = true
             guard let image = try? generator.copyCGImage(at: CMTime(seconds: 0, preferredTimescale: 1), actualTime: nil) else { return }
             DispatchQueue.main.async {
+                guard self.attachment.url == attachmentURL else { return }
                 self.image = UIImage(cgImage: image)
             }
         }

Tusker/Views/Compose Status Reply/ComposeStatusReplyView.swift

@@ -18,10 +18,16 @@ class ComposeStatusReplyView: UIView {
     @IBOutlet weak var usernameLabel: UILabel!
     @IBOutlet weak var statusContentTextView: StatusContentTextView!
     
+    var avatarRequest: ImageCache.Request?
+    
     static func create() -> ComposeStatusReplyView {
         return UINib(nibName: "ComposeStatusReplyView", bundle: nil).instantiate(withOwner: nil, options: nil).first as! ComposeStatusReplyView
     }
     
+    deinit {
+        avatarRequest?.cancel()
+    }
+
     override func awakeFromNib() {
         super.awakeFromNib()
         
@@ -39,8 +45,8 @@ class ComposeStatusReplyView: UIView {
         statusContentTextView.overrideMastodonController = mastodonController
         statusContentTextView.statusID = status.id
         
-        ImageCache.avatars.get(status.account.avatar) { (data) in
+        avatarRequest = ImageCache.avatars.get(status.account.avatar) { [weak self] (data) in
-            guard let data = data else { return }
+            guard let self = self, let data = data else { return }
             DispatchQueue.main.async {
                 self.avatarImageView.image = UIImage(data: data)
             }

Tusker/Views/ContentTextView.swift

@@ -49,7 +49,7 @@ class ContentTextView: LinkTextView {
         
         for emoji in emojis {
             group.enter()
-            ImageCache.emojis.get(emoji.url) { (data) in
+            _ = ImageCache.emojis.get(emoji.url) { (data) in
                 defer { group.leave() }
                 guard let data = data, let image = UIImage(data: data) else {
                     return

Tusker/Views/Instance Cell/InstanceTableViewCell.swift

@@ -20,6 +20,7 @@ class InstanceTableViewCell: UITableViewCell {
     var selectorInstance: InstanceSelector.Instance?
     
     var thumbnailURL: URL?
+    var thumbnailRequest: ImageCache.Request?
     
     override func awakeFromNib() {
         super.awakeFromNib()
@@ -59,12 +60,21 @@ class InstanceTableViewCell: UITableViewCell {
     private func updateThumbnail(url: URL) {
         thumbnailImageView.image = nil
         thumbnailURL = url
-        ImageCache.attachments.get(url) { (data) in
+        thumbnailRequest = ImageCache.attachments.get(url) { [weak self] (data) in
-            guard self.thumbnailURL == url, let data = data, let image = UIImage(data: data) else { return }
+            guard let self = self, self.thumbnailURL == url, let data = data, let image = UIImage(data: data) else { return }
+            self.thumbnailRequest = nil
             DispatchQueue.main.async {
                 self.thumbnailImageView.image = image
             }
         }
     }
     
+    override func prepareForReuse() {
+        super.prepareForReuse()
+        
+        thumbnailRequest?.cancel()
+        instance = nil
+        selectorInstance = nil
+    }
+
 }

Tusker/Views/Notifications/ActionNotificationGroupTableViewCell.swift

@@ -24,7 +24,7 @@ class ActionNotificationGroupTableViewCell: UITableViewCell {
     var group: NotificationGroup!
     var statusID: String!
     
-    var authorAvatarURL: URL?
+    var avatarRequests = [String: ImageCache.Request]()
     var updateTimestampWorkItem: DispatchWorkItem?
     
     deinit {
@@ -75,9 +75,10 @@ class ActionNotificationGroupTableViewCell: UITableViewCell {
             imageView.translatesAutoresizingMaskIntoConstraints = false
             imageView.layer.masksToBounds = true
             imageView.layer.cornerRadius = Preferences.shared.avatarStyle.cornerRadiusFraction * 30
-            ImageCache.avatars.get(account.avatar) { (data) in
+            avatarRequests[account.id] = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-                guard let data = data, self.group.id == group.id else { return }
+                guard let self = self, let data = data, self.group.id == group.id else { return }
                 DispatchQueue.main.async {
+                    self.avatarRequests.removeValue(forKey: account.id)
                     imageView.image = UIImage(data: data)
                 }
             }
@@ -149,10 +150,7 @@ class ActionNotificationGroupTableViewCell: UITableViewCell {
     override func prepareForReuse() {
         super.prepareForReuse()
         
-        if let authorAvatarURL = authorAvatarURL {
+        avatarRequests.values.forEach { $0.cancel() }
-            ImageCache.avatars.cancel(authorAvatarURL)
-        }
-        
         updateTimestampWorkItem?.cancel()
         updateTimestampWorkItem = nil
     }

Tusker/Views/Notifications/FollowNotificationGroupTableViewCell.swift

@@ -20,6 +20,7 @@ class FollowNotificationGroupTableViewCell: UITableViewCell {
     
     var group: NotificationGroup!
     
+    var avatarRequests = [String: ImageCache.Request]()
     var updateTimestampWorkItem: DispatchWorkItem?
     
     deinit {
@@ -55,9 +56,10 @@ class FollowNotificationGroupTableViewCell: UITableViewCell {
             imageView.translatesAutoresizingMaskIntoConstraints = false
             imageView.layer.masksToBounds = true
             imageView.layer.cornerRadius = Preferences.shared.avatarStyle.cornerRadiusFraction * 30
-            ImageCache.avatars.get(account.avatar) { (data) in
+            avatarRequests[account.id] = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-                guard let data = data, self.group.id == group.id else { return }
+                guard let self = self, let data = data, self.group.id == group.id else { return }
                 DispatchQueue.main.async {
+                    self.avatarRequests.removeValue(forKey: account.id)
                     imageView.image = UIImage(data: data)
                 }
             }
@@ -114,6 +116,7 @@ class FollowNotificationGroupTableViewCell: UITableViewCell {
     override func prepareForReuse() {
         super.prepareForReuse()
         
+        avatarRequests.values.forEach { $0.cancel() }
         updateTimestampWorkItem?.cancel()
         updateTimestampWorkItem = nil
     }

Tusker/Views/Notifications/FollowRequestNotificationTableViewCell.swift

@@ -25,6 +25,7 @@ class FollowRequestNotificationTableViewCell: UITableViewCell {
     var notification: Pachyderm.Notification?
     var account: Account!
     
+    var avatarRequest: ImageCache.Request?
     var updateTimestampWorkItem: DispatchWorkItem?
     
     deinit {
@@ -53,8 +54,9 @@ class FollowRequestNotificationTableViewCell: UITableViewCell {
     func updateUI(account: Account) {
         self.account = account
         actionLabel.text = "Request to follow from \(account.realDisplayName)"
-        ImageCache.avatars.get(account.avatar) { (data) in
+        avatarRequest = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-            guard self.account == account, let data = data, let image = UIImage(data: data) else { return }
+            guard let self = self, self.account == account, let data = data, let image = UIImage(data: data) else { return }
+            self.avatarRequest = nil
             DispatchQueue.main.async {
                 self.avatarImageView.image = image
             }
@@ -88,6 +90,7 @@ class FollowRequestNotificationTableViewCell: UITableViewCell {
     override func prepareForReuse() {
         super.prepareForReuse()
         
+        avatarRequest?.cancel()
         updateTimestampWorkItem?.cancel()
         updateTimestampWorkItem = nil
     }

Tusker/Views/Profile Header/ProfileHeaderTableViewCell.swift

@@ -32,8 +32,8 @@ class ProfileHeaderTableViewCell: UITableViewCell {
     
     var accountID: String!
     
-    var avatarURL: URL?
+    var avatarRequest: ImageCache.Request?
-    var headerURL: URL?
+    var headerRequest: ImageCache.Request?
     
     override func awakeFromNib() {
         avatarContainerView.layer.masksToBounds = true
@@ -63,19 +63,19 @@ class ProfileHeaderTableViewCell: UITableViewCell {
         usernameLabel.text = "@\(account.acct)"
         
         avatarImageView.image = nil
-        avatarURL = account.avatar
+        avatarRequest = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-        ImageCache.avatars.get(account.avatar) { (data) in
+            guard let self = self, let data = data, self.accountID == accountID else { return }
-            guard let data = data else { return }
+            self.avatarRequest = nil
             DispatchQueue.main.async {
                 self.avatarImageView.image = UIImage(data: data)
-                self.avatarURL = nil
             }
         }
-        ImageCache.headers.get(account.header) { (data) in
+        headerImageView.image = nil
-            guard let data = data else { return }
+        headerRequest = ImageCache.headers.get(account.header) { [weak self] (data) in
+            guard let self = self, let data = data, self.accountID == accountID else { return }
+            self.headerRequest = nil
             DispatchQueue.main.async {
                 self.headerImageView.image = UIImage(data: data)
-                self.headerURL = nil
             }
         }
         
@@ -131,12 +131,8 @@ class ProfileHeaderTableViewCell: UITableViewCell {
     }
     
     override func prepareForReuse() {
-        if let url = avatarURL {
+        avatarRequest?.cancel()
-            ImageCache.avatars.cancel(url)
+        headerRequest?.cancel()
-        }
-        if let url = headerURL {
-            ImageCache.headers.cancel(url)
-        }
     }
     
     @objc func morePressed() {

Tusker/Views/Status/BaseStatusTableViewCell.swift

@@ -64,8 +64,7 @@ class BaseStatusTableViewCell: UITableViewCell {
     }
     var showStatusAutomatically = false
     
-    var avatarURL: URL?
+    private var avatarRequest: ImageCache.Request?
-    var attachmentDataTasks: [URLSessionDataTask] = []
 
     private var statusUpdater: Cancellable?
     private var accountUpdater: Cancellable?
@@ -180,12 +179,10 @@ class BaseStatusTableViewCell: UITableViewCell {
     func updateUI(account: Account) {
         usernameLabel.text = "@\(account.acct)"
         avatarImageView.image = nil
-        avatarURL = account.avatar
+        avatarRequest = ImageCache.avatars.get(account.avatar) { [weak self] (data) in
-        ImageCache.avatars.get(account.avatar) { (data) in
+            guard let self = self, let data = data, self.accountID == account.id else { return }
-            guard let data = data, self.avatarURL == account.avatar else { return }
             DispatchQueue.main.async {
                 self.avatarImageView.image = UIImage(data: data)
-                self.avatarURL = nil
             }
         }
     }
@@ -200,9 +197,7 @@ class BaseStatusTableViewCell: UITableViewCell {
     override func prepareForReuse() {
         super.prepareForReuse()
         
-        if let avatarURL = avatarURL {
+        avatarRequest?.cancel()
-            ImageCache.avatars.cancel(avatarURL)
-        }
         attachmentsView.attachmentViews.allObjects.forEach { $0.removeFromSuperview() }
         showStatusAutomatically = false
     }

Tusker/Views/Status/TimelineStatusTableViewCell.swift

@@ -95,7 +95,7 @@ class TimelineStatusTableViewCell: BaseStatusTableViewCell {
     }
     
     func updateTimestamp() {
-        guard let status = mastodonController.cache.status(for: statusID) else { fatalError("Missing cached status \(statusID!)") }
+        guard let mastodonController = mastodonController, let status = mastodonController.cache.status(for: statusID) else { fatalError("Missing cached status \(statusID!)") }
         
         timestampLabel.text = status.createdAt.timeAgoString()
         timestampLabel.accessibilityLabel = TimelineStatusTableViewCell.relativeDateFormatter.localizedString(for: status.createdAt, relativeTo: Date())