ASP.NET MVC的Razor引擎：RazorViewEngine

基於Web Form引擎的WebFormViewEngine和針對Razor引擎的RazorViewEngine都是抽象類型BuildManagerViewEngine的子類，而後者又繼承自VirtualPathProviderViewEngine。在這裡我們僅僅對實現在RazorViewEngine中View獲取的邏輯進行簡單介紹。由於Razor引擎下的View通過RazorView對象來表示，而RazorView通過View文件的虛擬路徑來構建，所以RazorViewEngine的View獲取機制在於根據當前上下文找到與指定View名稱相匹配的View文件（.cshtml或者.vbhtml文件），然後根據該 View文件的虛擬路徑創建一個RazorView對象並最終封裝成ViewEngineResult對象返回。

實現在RazorViewEngine中的目標View文件的搜索是根據一個預定義順序進行的。如果當前請求不是針對某個Area的，下面的列表代表了View的搜索順序：

~/Views/{ControllerName}/{ViewName}.cshtml

~/Views/{ControllerName}/{ViewName}.vbhtml

~/Views/Shared/{ViewName}.cshtml

~/Views/Shared/{ViewName}.vbhtml

對於針對某個Area的請求，RazorViewEngine會先按照如下的順序對目標View進行搜索。如果在這個列表中沒有成功找到目標View文件，會繼續按照上面的屬性進行搜索。

~/Areas/{AreaName}/Views/{ControllerName}/{ViewName}.cshtml

~/Areas/{AreaName}/Views/{ControllerName}/{ViewName}.vbhtml

~/Areas/{AreaName}/Views/ Shared /{ViewName}.cshtml

~/Areas/{AreaName}/Views/ Shared /{ViewName}.vbhtml

如果按照上面的搜索順序依然找不目標View文件，RazorViewEngine會根據這個列表創建並返回一個ViewEngineResult對象。這裡介紹的View搜索機制不僅僅應用於普通的View文件，還應用於Partial View和布局文件的搜索。

ViewEngine不僅僅通過FindView/FindPartialView根據當前上下文獲取指定的View，還通過ReleaseView對指定的View進行釋放回收操作。ReleaseView方法在RazorViewEngine的實現很簡單，如果指定的View對象的類型實現IDispose接口，它會直接調用其Dispose方法。下圖所示的UML體現了Razor引擎涉及的相關類型/接口以及它們之間的相互關系。

在《ASP.NET MVC的Razor引擎：RazorView》一文中我們創建了一個用於模擬RazorView的SimpleRazorView，現在我們為它創建一個對應的RazorViewEngine，我們直接在該實例項目中添加如下一個SimpleRazorViewEngine。

   1: public class SimpleRazorViewEngine: IViewEngine
   2: {
   3:     private string[] viewLocationFormats = new string[] {
   4:         "~/Views/{1}/{0}.cshtml",
   5:         "~/Views/{1}/{0}.vbhtml",
   6:         "~/Views/Shared/{0}.cshtml",
   7:         "~/Views/Shared/{0}.vbhtml" };
   8:     private string[] areaViewLocationFormats = new string[] {
   9:         "~/Areas/{2}/Views/{1}/{0}.cshtml",
  10:         "~/Areas/{2}/Views/{1}/{0}.vbhtml",
  11:         "~/Areas/{2}/Views/Shared/{0}.cshtml",
  12:         "~/Areas/{2}/Views/Shared/{0}.vbhtml" };
  13:
  14:     public ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)
  15:     {
  16:         return FindView(controllerContext, partialViewName, null, useCache);
  17:     }
  18: 
  19:     public ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
  20:     {
  21:         string controllerName = controllerContext.RouteData.GetRequiredString("controller");
  22:         object areaName;
  23:         List<string> viewLocations = new List<string>();
  24:         Array.ForEach(viewLocationFormats, format => viewLocations.Add(string.Format(format, viewName, controllerName)));
  25:         if (controllerContext.RouteData.Values.TryGetValue("area", out areaName))
  26:         {
  27:             Array.ForEach(areaViewLocationFormats, format=>viewLocations.Add(string.Format(format,viewName,controllerName, areaName)));
  28:         }
  29:         foreach (string viewLocation in viewLocations)
  30:         {
  31:             string filePath = controllerContext.HttpContext.Request.MapPath(viewLocation);
  32:             if (File.Exists(filePath))
  33:             {
  34:                 return new ViewEngineResult(new SimpleRazorView(viewLocation),
  35:                     this);
  36:             }
  37:         }
  38:         return new ViewEngineResult(viewLocations);
  39:     }
  40: 
  41:     public void ReleaseView(ControllerContext controllerContext, IView view)
  42:     {
  43:         IDisposable disposable = view as IDisposable;
  44:         if (null != disposable)
  45:         {
  46:             disposable.Dispose();
  47:         }
  48:     }
  49: }