ASP.NET MVC的View是如何呈现出来的[实例篇]

在《[设计篇]》篇中我们通过对View引擎的总体介绍讲述了从ViewResult的创建到View呈现的原理，为了让读者对View引擎及其View呈现机制具有一个深刻的认识，我们自定义一个简单的用于呈现静态HTML的StaticFileViewEngine。在一个通过Visual Studio的ASP.NET MVC项目模板创建的空Web应用中，我们定义了如下一个针对于静态HTML内容呈现的自定义StaticFileView。StaticFileView实现了IView接口，在实现的Render方法中读取制定文件的内容写入作为参数的TextWriter。

   1: public class StaticFileView:IView   2: {   3:     public string FileName { get; private set; }   4:     public StaticFileView(string fileName)   5:     {   6:         this.FileName = fileName;   7:     }   8:     public void Render(ViewContext viewContext, TextWriter writer)   9:     {  10:         byte[] buffer;  11:         using (FileStream fs = new FileStream(this.FileName, FileMode.Open))  12:         {  13:             buffer = new byte[fs.Length];  14:             fs.Read(buffer, 0, buffer.Length);  15:         }  16:         writer.Write(Encoding.UTF8.GetString(buffer));  17:     }  18: }

由于StaticFileView中定义的内容完全是静态的，所以缓存显得很有必要。我们只需要基于Controller和View名称对View实施缓存，为此我们定义了如下一个作为Key的数据类型ViewEngineResultCacheKey。

   1: internal class ViewEngineResultCacheKey   2: {   3:     public string ControllerName { get; private set; }   4:     public string ViewName { get; private set; }   5:   6:     public ViewEngineResultCacheKey(string controllerName, string viewName)   7:     {   8:         this.ControllerName = controllerName ?? string.Empty;   9:         this.ViewName = viewName ?? string.Empty;  10:     }  11:     public override int GetHashCode()  12:     {  13:         return this.ControllerName.ToLower().GetHashCode() ^ this.ViewName.ToLower().GetHashCode();  14:     }  15:  16:     public override bool Equals(object obj)  17:     {  18:         ViewEngineResultCacheKey key = obj as ViewEngineResultCacheKey;  19:         if (null == key)  20:         {  21:             return false;  22:         }  23:         return key.GetHashCode() == this.GetHashCode();  24:     }  25: }

具有如下定义的StaticFileViewEngine代表StaticFileView对应的ViewEngine。我们通过一个字典类型的字段viewEngineResults作为对ViewEngineResult的缓存，而View的获取操作最终实现在InternalFindView方法中。通过StaticFileView表示的View定义在一个以View名称作为文件名的文本文件中，该文件的扩展名为.shtml（Static HTML）。

   1: public class StaticFileViewEngine : IViewEngine   2: {   3:     private Dictionary<ViewEngineResultCacheKey, ViewEngineResult> viewEngineResults = new Dictionary<ViewEngineResultCacheKey, ViewEngineResult>();   4:     private object syncHelper = new object();   5:     public ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)   6:     {   7:         return this.FindView(controllerContext, partialViewName, null, useCache);   8:     }   9:  10:     public ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)  11:     {  12:         string controllerName = controllerContext.RouteData.GetRequiredString("controller");  13:         ViewEngineResultCacheKey key = new ViewEngineResultCacheKey(controllerName, viewName);  14:         ViewEngineResult result;  15:         if (!useCache)  16:         {  17:             result = InternalFindView(controllerContext, viewName, controllerName);  18:             viewEngineResults[key] = result;  19:             return result;  20:         }  21:         if(viewEngineResults.TryGetValue(key, out result))  22:         {  23:             return result;  24:         }  25:         lock (syncHelper)  26:         {  27:             if (viewEngineResults.TryGetValue(key, out result))  28:             {  29:                 return result;  30:             }  31:  32:             result = InternalFindView(controllerContext, viewName, controllerName);  33:             viewEngineResults[key] = result;  34:             return result;  35:         }  36:     }  37:  38:     private ViewEngineResult InternalFindView(ControllerContext controllerContext, string viewName, string controllerName)  39:     {  40:         string[] searchLocations = new string[]  41:         {  42:             string.Format( "~/views/{0}/{1}.shtml", controllerName, viewName),  43:             string.Format( "~/views/Shared/{0}.shtml", viewName)  44:         };  45:  46:         string fileName = controllerContext.HttpContext.Request.MapPath(searchLocations[0]);  47:         if (File.Exists(fileName))  48:         {  49:             return new ViewEngineResult(new StaticFileView(fileName), this);  50:         }  51:         fileName = string.Format(@"\views\Shared\{0}.shtml", viewName);  52:         if (File.Exists(fileName))  53:         {  54:             return new ViewEngineResult(new StaticFileView(fileName), this);  55:         }  56:         return new ViewEngineResult(searchLocations);  57:     }  58:  59:     public void ReleaseView(ControllerContext controllerContext, IView view)  60:     { }  61: }

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