increase image cache to 128 MiB and fix perf for calculating image cost

ImageViewer/ViewController.swift

@@ -1250,7 +1250,8 @@ class ImageLoader {
     private var loadingTasks: [String: [((UIImage?) -> Void)]] = [:]
 
     init() {
-        cache.totalCostLimit = 50 * 1024 * 1024
+        // 128 MiB
+        cache.totalCostLimit = 128 * 1024 * 1024
     }
 
     func preloadImage(at path: URL) {
@@ -1277,13 +1278,13 @@ class ImageLoader {
 
             let image = UIImage(contentsOfFile: path.path)
 
-            DispatchQueue.main.async {
-                if let image = image, let data = image.pngData() {
-                    self.cache.setObject(image, forKey: path.path as NSString, cost: data.count)
-                } else {
-                    print("no image to cache found")
-                }
+            if let image = image, let cost = imageByteSize(image) {
+                self.cache.setObject(image, forKey: path.path as NSString, cost: cost)
+            } else {
+                print("no image to cache found")
+            }
 
+            DispatchQueue.main.async {
                 self.loadingTasks[path.path]?.forEach { $0(image) }
                 self.loadingTasks.removeValue(forKey: path.path)
             }
@@ -1294,3 +1295,8 @@ class ImageLoader {
         return cache.object(forKey: path as NSString)
     }
 }
+
+func imageByteSize(_ image: UIImage) -> Int? {
+    guard let cgImage = image.cgImage else { return nil }
+    return cgImage.bytesPerRow * cgImage.height
+}