Optimize the scene unload logic.

优化场景卸载逻辑，在加载新的主场景的时候自动卸载已经加载的所有场景。

Assets/YooAsset/Runtime/AssetSystem/AssetSystem.cs

@@ -10,6 +10,7 @@ namespace YooAsset
 	{
 		private static readonly List<AssetBundleLoaderBase> _loaders = new List<AssetBundleLoaderBase>(1000);
 		private static readonly List<ProviderBase> _providers = new List<ProviderBase>(1000);
+		private static readonly Dictionary<string, SceneOperationHandle> _sceneHandles = new Dictionary<string, SceneOperationHandle>(100);
 
 		/// <summary>
 		/// 在编辑器下模拟运行
@@ -132,6 +133,14 @@ namespace YooAsset
 		/// </summary>
 		public static SceneOperationHandle LoadSceneAsync(string scenePath, LoadSceneMode sceneMode, bool activateOnLoad, int priority)
 		{
+			// 注意：场景句柄永远保持唯一
+			if (_sceneHandles.ContainsKey(scenePath))
+				return _sceneHandles[scenePath];
+
+			// 如果加载的是主场景，则卸载所有缓存的场景
+			if (sceneMode == LoadSceneMode.Single)
+				UnloadAllScene();
+
 			ProviderBase provider = TryGetProvider(scenePath);
 			if (provider == null)
 			{
@@ -141,7 +150,9 @@ namespace YooAsset
 					provider = new BundledSceneProvider(scenePath, sceneMode, activateOnLoad, priority);
 				_providers.Add(provider);
 			}
-			return provider.CreateHandle() as SceneOperationHandle;
+			var handle = provider.CreateHandle() as SceneOperationHandle;
+			_sceneHandles.Add(scenePath, handle);
+			return handle;
 		}
 
 		/// <summary>
@@ -179,6 +190,48 @@ namespace YooAsset
 		}
 
 
+		internal static void UnloadSubScene(ProviderBase provider)
+		{
+			string scenePath = provider.AssetPath;
+			if (_sceneHandles.ContainsKey(scenePath) == false)
+				throw new Exception("Should never get here !");
+
+			// 释放子场景句柄
+			_sceneHandles[scenePath].ReleaseInternal();
+			_sceneHandles.Remove(scenePath);
+
+			// 卸载未被使用的资源（包括场景）
+			AssetSystem.UnloadUnusedAssets();
+
+			// 检验子场景是否销毁
+			if (provider.IsDestroyed == false)
+			{
+				throw new Exception("Should never get here !");
+			}
+		}
+		internal static void UnloadAllScene()
+		{
+			// 释放所有场景句柄
+			foreach (var valuePair in _sceneHandles)
+			{
+				valuePair.Value.ReleaseInternal();
+			}
+			_sceneHandles.Clear();
+
+			// 卸载未被使用的资源（包括场景）
+			AssetSystem.UnloadUnusedAssets();
+
+			// 检验所有场景是否销毁
+			foreach (var provider in _providers)
+			{
+				if (provider.IsSceneProvider())
+				{
+					if (provider.IsDestroyed == false)
+						throw new Exception("Should never get here !");
+				}
+			}
+		}
+
 		internal static AssetBundleLoaderBase CreateOwnerAssetBundleLoader(string assetPath)
 		{
 			string bundleName = BundleServices.GetBundleName(assetPath);
@@ -254,6 +307,7 @@ namespace YooAsset
 			return provider;
 		}
 
+
 		#region 调试专属方法
 		internal static void GetDebugReport(DebugReport report)
 		{