introduce loadingImages & add a lock around image load/unload state

Program.cs

@@ -571,6 +571,8 @@ public class Game : GameWindow {
   List<Photo> allPhotos = new();
   List<Photo> photos = new();
   HashSet<Photo> loadedImages = new();
+  HashSet<Photo> loadingImages = new();
+  readonly object loadedImagesLock = new();
   int photoIndex = 0;
   int ribbonIndex = 0;
   Shader shader = new();
@@ -777,10 +779,10 @@ public class Game : GameWindow {
 
     // Load photos from a directory.
     // string[] files = Directory.GetFiles(@"c:\users\colin\desktop\photos-test\");
-    string[] files = Directory.GetFiles(@"c:\users\colin\pictures\photos\2023\07\14\");
+    // string[] files = Directory.GetFiles(@"c:\users\colin\pictures\photos\2023\07\14\");
     // string[] files = Directory.GetFiles(@"G:\DCIM\100EOSR6\");
     // string[] files = Directory.GetFiles(@"C:\Users\colin\Pictures\photos\2018\06\23");
-    // string[] files = Directory.GetFiles(@"C:\Users\colin\Desktop\Germany all\104D7000");
+    string[] files = Directory.GetFiles(@"C:\Users\colin\Desktop\Germany all\104D7000");
     // string[] files = Directory.GetFiles(@"C:\Users\colin\Desktop\many-birds\");
 
     for (int i = 0; i < files.Count(); i++) {
@@ -809,43 +811,62 @@ public class Game : GameWindow {
     base.OnUnload();
   }
 
-  private async void LoadAndUnloadImagesAsync() {
-    int minLoadedImage = Math.Max(0, photoIndex - 20);
-    int maxLoadedImage = Math.Min(photoIndex + 20, photos.Count - 1);
-    // First, unload images that haven't been touched in a while.
-    // FIXME: also cancel any of these if they still have an in-progress loading task?
+  private void UnloadImages() {
+    // Unload images that haven't been touched in a while.
     // FIXME: keep around thumbnail-sized textures?
-    while (loadedImages.Count > 60) {
-      long earliestTime = long.MaxValue;
-      Photo? earliest = null;
-      foreach (Photo photo in loadedImages) {
-        if (photo.LastTouch < earliestTime) {
-          earliest = photo;
-          earliestTime = photo.LastTouch;
+    lock (loadedImagesLock) {
+      while (loadedImages.Count > 60) {
+        long earliestTime = long.MaxValue;
+        Photo? earliest = null;
+        foreach (Photo photo in loadedImages) {
+          if (photo.LastTouch < earliestTime) {
+            earliest = photo;
+            earliestTime = photo.LastTouch;
+          }
+        }
+        if (earliest != null) {
+          // Console.WriteLine($"loadedImages.Count: {loadedImages.Count}, evicting {earliest.Filename} @ {earliestTime}");
+          // TODO: we have to free textures on the GL thread, but could we do that async'ly to keep the UI responsive?
+          earliest.Unload();
+          loadedImages.Remove(earliest);
         }
       }
-      if (earliest != null) {
-        Console.WriteLine($"loadedImages.Count: {loadedImages.Count}, evicting {earliest.Filename} @ {earliestTime}");
-        // TODO: we have to free textures on the GL thread, but could we do that async'ly to keep the UI responsive?
-        earliest.Unload();
-        loadedImages.Remove(earliest);
+    }
+  }
+
+  private async void LoadImagesAsync() {
+    foreach (Photo p in loadingImages) {
+      if (p.Loaded) {
+        lock (loadedImagesLock) {
+          loadedImages.Add(p);
+          loadingImages.Remove(p);
+        }
       }
     }
-    // Then, start loading any images that are in our window but not yet loaded.
+    // Start loading any images that are in our window but not yet loaded.
+    int minLoadedImage = Math.Max(0, photoIndex - 20);
+    int maxLoadedImage = Math.Min(photoIndex + 20, photos.Count - 1);
+    List<Photo> toLoad = new();
     for (int i = minLoadedImage; i <= maxLoadedImage; i++) {
-      if (!loadedImages.Contains(photos[i])) {
-        Console.WriteLine("loading " + i);
-        loadedImages.Add(photos[i]);
-        await Task.Run( () => { photos[i].LoadAsync(); });
+      lock (loadedImagesLock) {
+        if (!loadedImages.Contains(photos[i]) && !loadingImages.Contains(photos[i])) {
+          Console.WriteLine("loading " + i);
+          loadingImages.Add(photos[i]);
+          toLoad.Add(photos[i]);
+        }
       }
     }
+    foreach (Photo p in toLoad) {
+      await Task.Run( () => { p.LoadAsync(); });
+    }
   }
 
   protected override void OnRenderFrame(FrameEventArgs e) {
     base.OnRenderFrame(e);
     fpsCounter.Update();
 
-    LoadAndUnloadImagesAsync();
+    UnloadImages();
+    LoadImagesAsync();
 
     GL.Clear(ClearBufferMask.ColorBufferBit);
     GL.BindBuffer(BufferTarget.ArrayBuffer, VertexBufferObject);