Allow retrieving count of available stable imports

osu.Game/Collections/CollectionManager.cs

@@ -111,6 +111,18 @@ private void collectionsChanged(object sender, NotifyCollectionChangedEventArgs
 
         public Action<Notification> PostNotification { protected get; set; }
 
+        public Task<int> GetAvailableCount(StableStorage stableStorage)
+        {
+            if (!stableStorage.Exists(database_name))
+                return Task.FromResult(0);
+
+            return Task.Run(() =>
+            {
+                using (var stream = stableStorage.GetStream(database_name))
+                    return readCollections(stream).Count;
+            });
+        }
+
         /// <summary>
         /// This is a temporary method and will likely be replaced by a full-fledged (and more correctly placed) migration process in the future.
         /// </summary>

osu.Game/Database/LegacyImportManager.cs

@@ -49,6 +49,29 @@ public class LegacyImportManager : Component
 
         public bool SupportsImportFromStable => RuntimeInfo.IsDesktop;
 
+        public async Task<int> GetImportCount(StableContent content)
+        {
+            var stableStorage = await getStableStorage().ConfigureAwait(false);
+
+            switch (content)
+            {
+                case StableContent.Beatmaps:
+                    return await new LegacyBeatmapImporter(beatmaps).GetAvailableCount(stableStorage);
+
+                case StableContent.Skins:
+                    return await new LegacySkinImporter(skins).GetAvailableCount(stableStorage);
+
+                case StableContent.Collections:
+                    return await collections.GetAvailableCount(stableStorage);
+
+                case StableContent.Scores:
+                    return await new LegacyScoreImporter(scores).GetAvailableCount(stableStorage);
+
+                default:
+                    throw new ArgumentException($"Only one {nameof(StableContent)} flag should be specified.");
+            }
+        }
+
         public async Task ImportFromStableAsync(StableContent content)
         {
             var stableStorage = await getStableStorage().ConfigureAwait(false);

osu.Game/Database/LegacyModelImporter.cs

@@ -34,6 +34,8 @@ protected LegacyModelImporter(IModelImporter<TModel> importer)
             Importer = importer;
         }
 
+        public Task<int> GetAvailableCount(StableStorage stableStorage) => Task.Run(() => GetStableImportPaths(stableStorage).Count());
+
         public Task ImportFromStableAsync(StableStorage stableStorage)
         {
             var storage = PrepareStableStorage(stableStorage);