Fixed operation system error

修复异步操作系统的Task二次等待失败的问题。

Assets/YooAsset/Runtime/OperationSystem/AsyncOperationBase.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Threading.Tasks;
 
 namespace YooAsset
 {
@@ -48,12 +49,31 @@ namespace YooAsset
 			}
 		}
 
+		/// <summary>
+		/// 异步操作任务
+		/// </summary>
+		public Task Task
+		{
+			get
+			{
+				if (_taskCompletionSource == null)
+				{
+					_taskCompletionSource = new TaskCompletionSource<object>();
+					if (IsDone)
+						_taskCompletionSource.SetResult(null);
+				}
+				return _taskCompletionSource.Task;
+			}
+		}
+
 		internal abstract void Start();
 		internal abstract void Update();
 		internal void Finish()
 		{
 			_callback?.Invoke(this);
-			_waitHandle?.Set();
+
+			if (_taskCompletionSource != null)
+				_taskCompletionSource.TrySetResult(null);
 		}
 
 		#region 异步编程相关
@@ -66,28 +86,7 @@ namespace YooAsset
 		}
 		public object Current => null;
 
-		private System.Threading.EventWaitHandle _waitHandle;
-		private System.Threading.WaitHandle WaitHandle
-		{
-			get
-			{
-				if (_waitHandle == null)
-					_waitHandle = new System.Threading.EventWaitHandle(false, System.Threading.EventResetMode.ManualReset);
-				_waitHandle.Reset();
-				return _waitHandle;
-			}
-		}
-		public System.Threading.Tasks.Task Task
-		{
-			get
-			{
-				var handle = WaitHandle;
-				return System.Threading.Tasks.Task.Factory.StartNew(o =>
-				{
-					handle.WaitOne();
-				}, this);
-			}
-		}
+		private TaskCompletionSource<object> _taskCompletionSource;
 		#endregion
 	}
 }