refactor the runtime code

重构了运行时代码，支持全新的文件系统。

Assets/YooAsset/Runtime/DownloadSystem/Operation/Internal/UnityWebTextRequestOperation.cs

@@ -0,0 +1,77 @@
+using UnityEngine.Networking;
+using UnityEngine;
+
+namespace YooAsset
+{
+    internal class UnityWebTextRequestOperation : UnityWebRequestOperation
+    {
+        private UnityWebRequestAsyncOperation _requestOperation;
+
+        /// <summary>
+        /// 请求结果
+        /// </summary>
+        public string Result { private set; get; }
+
+
+        internal UnityWebTextRequestOperation(string url, int timeout = 60) : base(url, timeout)
+        {
+        }
+        internal override void InternalOnStart()
+        {
+            _steps = ESteps.CreateRequest;
+        }
+        internal override void InternalOnUpdate()
+        {
+            if (_steps == ESteps.None || _steps == ESteps.Done)
+                return;
+
+            if (_steps == ESteps.CreateRequest)
+            {
+                _latestDownloadBytes = 0;
+                _latestDownloadRealtime = Time.realtimeSinceStartup;
+
+                CreateWebRequest();
+                _steps = ESteps.Download;
+            }
+
+            if (_steps == ESteps.Download)
+            {
+                Progress = _requestOperation.progress;
+                if (_requestOperation.isDone == false)
+                {
+                    CheckRequestTimeout();
+                    return;
+                }
+
+                if (CheckRequestResult())
+                {
+                    _steps = ESteps.Done;
+                    Result = _webRequest.downloadHandler.text;
+                    Status = EOperationStatus.Succeed;
+                }
+                else
+                {
+                    _steps = ESteps.Done;
+                    Status = EOperationStatus.Failed;
+                }
+
+                // 注意：最终释放请求器
+                DisposeRequest();
+            }
+        }
+        internal override void InternalOnAbort()
+        {
+            _steps = ESteps.Done;
+            DisposeRequest();
+        }
+
+        private void CreateWebRequest()
+        {
+            _webRequest = DownloadSystemHelper.NewUnityWebRequestGet(_requestURL);
+            DownloadHandlerBuffer handler = new DownloadHandlerBuffer();
+            _webRequest.downloadHandler = handler;
+            _webRequest.disposeDownloadHandlerOnDispose = true;
+            _requestOperation = _webRequest.SendWebRequest();
+        }
+    }
+}