Use background image preparation apis on iOS 15

Closes #128

Tusker/Caching/ImageCache.swift

@@ -22,6 +22,7 @@ class ImageCache {
     #endif
 
     private let cache: ImageDataCache
+    private let desiredPixelSize: CGSize?
     
     private var groups = MultiThreadDictionary<URL, RequestGroup>(name: "ImageCache request groups")
     
@@ -30,28 +31,47 @@ class ImageCache {
     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))
+        self.desiredPixelSize = pixelSize
         self.cache = ImageDataCache(name: name, memoryExpiry: memoryExpiry, diskExpiry: diskExpiry, storeOriginalDataInMemory: diskExpiry == nil, desiredPixelSize: pixelSize)
     }
     
     func get(_ url: URL, loadOriginal: Bool = false, completion: ((Data?, UIImage?) -> Void)?) -> Request? {
         let key = url.absoluteString
+        
+        let wrappedCompletion: ((Data?, UIImage?) -> Void)?
+        if let completion = completion {
+            wrappedCompletion = { (data, image) in
+                if #available(iOS 15.0, *) {
+                    if !loadOriginal,
+                       let size = self.desiredPixelSize {
+                        image?.prepareThumbnail(of: size, completionHandler: {
+                            completion(data, $0)
+                        })
+                    } else {
+                        image?.prepareForDisplay {
+                            completion(data, $0)
+                        }
+                    }
+                } else {
+                    self.backgroundQueue.async {
+                        completion(data, image)
+                    }
+                }
+            }
+        } else {
+            wrappedCompletion = nil
+        }
+        
         if !ImageCache.disableCaching,
            let entry = try? cache.get(key, loadOriginal: loadOriginal) {
-            if let completion = completion {
-                backgroundQueue.async {
-                    completion(entry.data, entry.image)
-                }
-            }
+            wrappedCompletion?(entry.data, entry.image)
             return nil
         } else {
             if let group = groups[url] {
-                if let completion = completion {
-                    return group.addCallback(completion)
-                }
-                return nil
+                return group.addCallback(wrappedCompletion)
             } else {
                 let group = createGroup(url: url)
-                let request = group.addCallback(completion)
+                let request = group.addCallback(wrappedCompletion)
                 group.run()
                 return request
             }
@@ -122,21 +142,15 @@ class ImageCache {
             task!.resume()
         }
         
-        private func updatePriority() {
-            task?.priority = max(1.0, URLSessionTask.defaultPriority + 0.1 * Float(requests.filter { !$0.cancelled }.count))
-        }
-        
         func addCallback(_ completion: ((Data?, UIImage?) -> 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()
             }
         }
         
@@ -145,8 +159,6 @@ class ImageCache {
             if remaining <= 0 {
                 task?.cancel()
                 complete(with: nil)
-            } else {
-                updatePriority()
             }
         }