定制属于你自己的ViewEngine(一套逻辑多套UI)

ASP.NET MVC出来这么久了，心中却又很多的疑惑：为什么所有的View都要放在Views目录下? 为什么Shared文件夹下面的页面可以被共享? 为什么Page既可以是*.cshtml，也可以是*.aspx?

其实上面的几个问题归结起来都是视图引擎的功效。

在传统的ASP.NET中，可能还没有ViewEngine的概念。因为在Web From里面，实现Page实现了IHttpHanlder的接口，所以Page既是响应的处理类，也是视图的渲染类。在ASP.NET MVC中，视图的概念被抽象了出来，试图引擎的概念也被抽象成了一个接口。

首先来看一下IViewEngine接口的定义

 namespace System.Web.Mvc
 {
     public interface IViewEngine
     {
         ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache);
         ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache);
         void ReleaseView(ControllerContext controllerContext, IView view);
     }
 }

总共3个函数，总结起来大概就是两个功能：Find & Release。

默认情况下，ASP.NET MVC提供了两个视图引擎：WebFormViewEngine和RazorViewEngine

 namespace System.Web.Mvc
 {
     public static class ViewEngines
     {
         private static readonly ViewEngineCollection _engines = new ViewEngineCollection
         {
             new WebFormViewEngine(),
             new RazorViewEngine(),
         };

         public static ViewEngineCollection Engines
         {
             get { return _engines; }
         }
     }
 }

这就是为什么ASP.NET MVC既支持*.aspx，又支持*.cshtml的原因了（个人觉得如果已经确定要使用RazorView的话，不如把WebFormViewEngine给移除，可能对性能会有所帮助）。

那为什么所有的视图都要放在Views目录下呢，这个就要拜RazorViewngines所赐了。

下面是RazorViewEngine的构造函数：

         public RazorViewEngine(IViewPageActivator viewPageActivator)
             : base(viewPageActivator)
         {
             AreaViewLocationFormats = new[]
             {
                 "~/Areas/{2}/Views/{1}/{0}.cshtml",
                 "~/Areas/{2}/Views/{1}/{0}.vbhtml",
                 "~/Areas/{2}/Views/Shared/{0}.cshtml",
                 "~/Areas/{2}/Views/Shared/{0}.vbhtml"
             };
             AreaMasterLocationFormats = new[]
             {
                 "~/Areas/{2}/Views/{1}/{0}.cshtml",
                 "~/Areas/{2}/Views/{1}/{0}.vbhtml",
                 "~/Areas/{2}/Views/Shared/{0}.cshtml",
                 "~/Areas/{2}/Views/Shared/{0}.vbhtml"
             };
             AreaPartialViewLocationFormats = new[]
             {
                 "~/Areas/{2}/Views/{1}/{0}.cshtml",
                 "~/Areas/{2}/Views/{1}/{0}.vbhtml",
                 "~/Areas/{2}/Views/Shared/{0}.cshtml",
                 "~/Areas/{2}/Views/Shared/{0}.vbhtml"
             };

             ViewLocationFormats = new[]
             {
                 "~/Views/{1}/{0}.cshtml",
                 "~/Views/{1}/{0}.vbhtml",
                 "~/Views/Shared/{0}.cshtml",
                 "~/Views/Shared/{0}.vbhtml"
             };
             MasterLocationFormats = new[]
             {
                 "~/Views/{1}/{0}.cshtml",
                 "~/Views/{1}/{0}.vbhtml",
                 "~/Views/Shared/{0}.cshtml",
                 "~/Views/Shared/{0}.vbhtml"
             };
             PartialViewLocationFormats = new[]
             {
                 "~/Views/{1}/{0}.cshtml",
                 "~/Views/{1}/{0}.vbhtml",
                 "~/Views/Shared/{0}.cshtml",
                 "~/Views/Shared/{0}.vbhtml"
             };

             FileExtensions = new[]
             {
                 "cshtml",
                 "vbhtml",
             };
         }

所有的寻址路径都被格式化了，是不是很眼熟呢，关于这里为啥用数组而不用List，个人觉得，数组的寻址效率要更高些，遍历速度更快。

好了，找了“罪魁祸首”，就好好地调教一个，让它乖乖听话，小样让去哪就去哪里。

  /// <summary>
     /// razor视图引擎扩展
     /// </summary>
     public class CustomerViewEngine : RazorViewEngine
     {
         /// <summary>
         /// 可以分开部署不同语种
         /// </summary>
         /// <param name="engineName"></param>
         public CustomerViewEngine(string engineName)
         {
             base.ViewLocationFormats = new[]
                 {
                     "~/Views" + engineName + "/{1}/{0}.cshtml",
                     "~/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.PartialViewLocationFormats = new[]
                 {
                     "~/Views" + engineName + "/{1}/{0}.cshtml",
                     "~/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.AreaViewLocationFormats = new[]
                 {
                     "~Areas/{2}/Views" + engineName + "/{1}/{0}.cshtml",
                     "~Areas/{2}/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.AreaPartialViewLocationFormats = new[]
                 {
                     "~Areas/{2}/Views" + engineName + "/{1}/{0}.cshtml",
                     "~Areas/{2}/Views" + engineName + "/Shared/{0}.cshtml"
                 };
         }

         public override ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
         {
             this.SetEngine(controllerContext);
             return base.FindView(controllerContext, viewName, masterName, useCache);
         }

         public override ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache)
         {
             this.SetEngine(controllerContext);
             return base.FindPartialView(controllerContext, partialViewName, useCache);
         }

         /// <summary>
         /// 根据条件自行设置,目前是chrome浏览器就展示默认的
         /// 不是chrome浏览器的话就展示/Themes/Eleven下的
         /// 可以直接测试是移动端还是pc端
         /// 然后写入cookie
         /// </summary>
         private void SetEngine(ControllerContext controllerContext)
         {
             string engineName = "/Themes/Eleven";
             if (controllerContext.HttpContext.Request.UserAgent.IndexOf("Chrome/65") >= )
             {
                 engineName = null;
             }

             //if (controllerContext.HttpContext.Request.IsMobile())//检测是不是移动端
             //{
             //    engineName = null;
             //}

             base.ViewLocationFormats = new[]
                {
                     "~/Views" + engineName + "/{1}/{0}.cshtml",
                     "~/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.PartialViewLocationFormats = new[]
                 {
                     "~/Views" + engineName + "/{1}/{0}.cshtml",
                     "~/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.AreaViewLocationFormats = new[]
                 {
                     "~Areas/{2}/Views" + engineName + "/{1}/{0}.cshtml",
                     "~Areas/{2}/Views" + engineName + "/Shared/{0}.cshtml"
                 };

             base.AreaPartialViewLocationFormats = new[]
                 {
                     "~Areas/{2}/Views" + engineName + "/{1}/{0}.cshtml",
                     "~Areas/{2}/Views" + engineName + "/Shared/{0}.cshtml"
                 };
         }

接下去就很简单了，只需要把原来的视图引擎清空，加载自己的视图引擎就可以了

         protected void Application_Start()
         {
             AreaRegistration.RegisterAllAreas();

             ViewEngines.Engines.Clear();

             ViewEngines.Engines.Add(new CustomViewEngine());

             RegisterGlobalFilters(GlobalFilters.Filters);
             RegisterRoutes(RouteTable.Routes);
         }