前言
目前开源的CMS、Blog或者电子商务站点，他们都有一个共同的亮点，无疑就是可任意切换皮肤，并且定制和扩展能力都非常强。在这方面PHP可以说做的是最好的。那么我们如何能够在我们的ASP.NET MVC站点下面实现任意切换皮肤呢？我立马想到最近流行的NopCommerce—开源的 ASP.NET MVC 电子商务站点。它提供了强大的换肤功能，可通过一键切换皮肤。那接下来，我们就一起去寻找换肤的秘诀，让我们的ASP.NET MVC站点也具有一键换肤的功能吧。让我们的ASP.NET MVC 站点可以随意 变 变 变!
换肤试用
先试用下Nop站点的换肤效果吧，打开Nop的源码，下载地址：http://nopcommerce.codeplex.com, 按照官方的Theme制作方法：http://www.nopcommerce.com/docs/72/designers-guide.aspx，拷贝默认的皮肤DarkOrange，并做相应处理。此处略去500字…
运行站点，首先呈现的是默认皮肤：
切换成我们刚才制作的皮肤：
换肤后的思考？
我们刚才制作皮肤的时候，将默认的皮肤文件夹下所有的文件拷贝到新的皮肤文件夹下面，并做了样式和HTML结构的修改。Nop应该是根据客户选择的皮肤定位到相应的皮肤文件夹下面，去找到View并加载出来。那实现换肤功能的关键就是： 根据用户选择的皮肤，ASP.NET MVC动态定位到皮肤文件夹下的View，并呈现出来。
做过ASP.NET MVC开发的朋友都知道，如果在Controller里面新建一个Action，但View不存在，页面肯定会报如下错误：
从异常信息可以看出，ASP.NET MVC内部有一种加载View的机制。如果我们能够扩展这种内部的加载View的机制，去按照我们的自定义逻辑根据不同的皮肤加载不同的View，那我们的站点就能够实现换肤功能了。实现这个功能的核心就是IViewEngine，资料介绍：http://www.cnblogs.com/answercard/archive/2011/05/07/2039809.html。该接口定义如下：
/// <summary>
/// Defines the methods that are required for a view engine.
/// </summary>
public interface IViewEngine
{
/// <summary>
/// Finds the specified partial view by using the specified controller context.
/// </summary>
ViewEngineResult FindPartialView(ControllerContext controllerContext, string partialViewName, bool useCache);
/// <summary>
/// Finds the specified view by using the specified controller context.
/// </summary>
ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache);
/// <summary>
/// Releases the specified view by using the specified controller context.
/// </summary>
/// <param name="controllerContext">The controller context.</param><param name="view">The view.</param>
void ReleaseView(ControllerContext controllerContext, IView view);
}
深入Nop，找到幕后黑手
那我们就到Nop的源代码中去寻找 IViewEngine 的实现类，看看运气如何？ 运气不错，找到了3个Themeable****ViewEngine. 从名字就可以断定该类是用来实现Theme的。
（Tips: 借助Reshareper可轻松的查找某个接口的实现类，此外Reshareper还有其它的高级功能，谁用谁知道…）
先看看离接口IViewEngine最近的类—ThemeableVirtualPathProviderViewEngine，该类重写了FindView 和 FindPartialView 2个方法。我们以FindView为例进行研究吧，实际上FindPartialView跟FindView都差不多。
public override ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
{
var mobileDeviceHelper = EngineContext.Current.Resolve<IMobileDeviceHelper>();
bool useMobileDevice = mobileDeviceHelper.IsMobileDevice(controllerContext.HttpContext)
&& mobileDeviceHelper.MobileDevicesSupported()
&& !mobileDeviceHelper.CustomerDontUseMobileVersion(); string overrideViewName = useMobileDevice ?
string.Format("{0}.{1}", viewName, _mobileViewModifier)
: viewName; ViewEngineResult result = FindThemeableView(controllerContext, overrideViewName, masterName, useCache, useMobileDevice);
// If we're looking for a Mobile view and couldn't find it try again without modifying the viewname
if (useMobileDevice && (result == null || result.View == null))
result = FindThemeableView(controllerContext, viewName, masterName, useCache, false);
return result;
}
查找View的重担放到了内部方法FindThemeableView中完成的，看看该方法的实现吧：
protected virtual ViewEngineResult FindThemeableView(ControllerContext controllerContext, string viewName, string masterName, bool useCache, bool mobile)
{
string[] strArray;
string[] strArray2;
if (controllerContext == null)
{
throw new ArgumentNullException("controllerContext");
}
if (string.IsNullOrEmpty(viewName))
{
throw new ArgumentException("View name cannot be null or empty.", "viewName");
}
var theme = GetCurrentTheme(mobile);
string requiredString = controllerContext.RouteData.GetRequiredString("controller");
string str2 = this.GetPath(controllerContext, this.ViewLocationFormats, this.AreaViewLocationFormats, "ViewLocationFormats", viewName, requiredString, theme, "View", useCache, mobile, out strArray);
string str3 = this.GetPath(controllerContext, this.MasterLocationFormats, this.AreaMasterLocationFormats, "MasterLocationFormats", masterName, requiredString, theme, "Master", useCache, mobile, out strArray2);
if (!string.IsNullOrEmpty(str2) && (!string.IsNullOrEmpty(str3) || string.IsNullOrEmpty(masterName)))
{
return new ViewEngineResult(this.CreateView(controllerContext, str2, str3), this);
}
if (strArray2 == null)
{
strArray2 = new string[0];
}
return new ViewEngineResult(strArray.Union<string>(strArray2));
}
这段代码读起来有点费力，又一次告诉大家命名的重要性，当然你不想让别人看懂你的代码那就是另外一回事哈。str2实际上是ViewPath，str3是MasterPagePath。其中内部方法GetPath是用来获取View的实际路径。我们来研究下GetPath的参数吧，其中最关键的是属性ViewLocationFormats和AreaViewLocationFormats。由于ThemeableVirtualPathProviderViewEngine是抽象类，我们看看派生自该类的ThemeableRazorViewEngine吧：
public ThemeableRazorViewEngine()
{
AreaViewLocationFormats = new[]
{
//themes
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.vbhtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.vbhtml", //default
"~/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[]
{
//themes
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.vbhtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.vbhtml", //default
"~/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[]
{
//themes
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/{1}/{0}.vbhtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.cshtml",
"~/Areas/{2}/Themes/{3}/Views/Shared/{0}.vbhtml", //default
"~/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[]
{
//themes
"~/Themes/{2}/Views/{1}/{0}.cshtml",
"~/Themes/{2}/Views/{1}/{0}.vbhtml",
"~/Themes/{2}/Views/Shared/{0}.cshtml",
"~/Themes/{2}/Views/Shared/{0}.vbhtml", //default
"~/Views/{1}/{0}.cshtml",
"~/Views/{1}/{0}.vbhtml",
"~/Views/Shared/{0}.cshtml",
"~/Views/Shared/{0}.vbhtml", //Admin
"~/Administration/Views/{1}/{0}.cshtml",
"~/Administration/Views/{1}/{0}.vbhtml",
"~/Administration/Views/Shared/{0}.cshtml",
"~/Administration/Views/Shared/{0}.vbhtml",
}; MasterLocationFormats = new[]
{
//themes
"~/Themes/{2}/Views/{1}/{0}.cshtml",
"~/Themes/{2}/Views/{1}/{0}.vbhtml",
"~/Themes/{2}/Views/Shared/{0}.cshtml",
"~/Themes/{2}/Views/Shared/{0}.vbhtml", //default
"~/Views/{1}/{0}.cshtml",
"~/Views/{1}/{0}.vbhtml",
"~/Views/Shared/{0}.cshtml",
"~/Views/Shared/{0}.vbhtml"
}; PartialViewLocationFormats = new[]
{
//themes
"~/Themes/{2}/Views/{1}/{0}.cshtml",
"~/Themes/{2}/Views/{1}/{0}.vbhtml",
"~/Themes/{2}/Views/Shared/{0}.cshtml",
"~/Themes/{2}/Views/Shared/{0}.vbhtml", //default
"~/Views/{1}/{0}.cshtml",
"~/Views/{1}/{0}.vbhtml",
"~/Views/Shared/{0}.cshtml",
"~/Views/Shared/{0}.vbhtml", //Admin
"~/Administration/Views/{1}/{0}.cshtml",
"~/Administration/Views/{1}/{0}.vbhtml",
"~/Administration/Views/Shared/{0}.cshtml",
"~/Administration/Views/Shared/{0}.vbhtml",
}; FileExtensions = new[] { "cshtml", "vbhtml" };
}
看到这里你是否了解有些明白了？这里就是定义的查找View的路径的模版，程序（Nop和MVC默认实现都是相同的策略）会按照顺序依次查找View是否存在。
再来看看GetPath的实现吧：
protected virtual string GetPath(ControllerContext controllerContext, string[] locations,
string[] areaLocations, string locationsPropertyName, string name,
string controllerName, string theme, string cacheKeyPrefix,
bool useCache, bool mobile, out string[] searchedLocations)
{
searchedLocations = _emptyLocations;
if (string.IsNullOrEmpty(name))
{
return string.Empty;
}
string areaName = GetAreaName(controllerContext.RouteData); //little hack to get nop's admin area to be in /Administration/ instead of /Nop/Admin/ or Areas/Admin/
if (!string.IsNullOrEmpty(areaName) && areaName.Equals("admin", StringComparison.InvariantCultureIgnoreCase))
{
//admin area does not support mobile devices
if (mobile)
{
searchedLocations = new string[0];
return string.Empty;
}
var newLocations = areaLocations.ToList();
newLocations.Insert(0, "~/Administration/Views/{1}/{0}.cshtml");
newLocations.Insert(0, "~/Administration/Views/{1}/{0}.vbhtml");
newLocations.Insert(0, "~/Administration/Views/Shared/{0}.cshtml");
newLocations.Insert(0, "~/Administration/Views/Shared/{0}.vbhtml");
areaLocations = newLocations.ToArray();
} bool flag = !string.IsNullOrEmpty(areaName);
List<ViewLocation> viewLocations = GetViewLocations(locations, flag ? areaLocations : null);
if (viewLocations.Count == 0)
{
throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, "Properties cannot be null or empty.", new object[] { locationsPropertyName }));
}
bool flag2 = IsSpecificPath(name);
string key = this.CreateCacheKey(cacheKeyPrefix, name, flag2 ? string.Empty : controllerName, areaName, theme);
if (useCache)
{
var cached = this.ViewLocationCache.GetViewLocation(controllerContext.HttpContext, key);
if (cached != null)
{
return cached;
}
}
if (!flag2)
{
return this.GetPathFromGeneralName(controllerContext, viewLocations, name, controllerName, areaName, theme, key, ref searchedLocations);
}
return this.GetPathFromSpecificName(controllerContext, name, key, ref searchedLocations);
}
这里明显使用了缓存，所以大家不用担心每次读取View都要依次去进行IO操作去查找View引起的性能问题。
最后我们再来看看GetPathFromGeneralName的具体实现吧：
protected virtual string GetPathFromGeneralName(ControllerContext controllerContext,
List<ViewLocation> locations, string name, string controllerName, string areaName,
string theme, string cacheKey, ref string[] searchedLocations)
{
string virtualPath = string.Empty;
searchedLocations = new string[locations.Count];
for (int i = 0; i < locations.Count; i++)
{
string str2 = locations[i].Format(name, controllerName, areaName, theme);
if (this.FileExists(controllerContext, str2))
{
searchedLocations = _emptyLocations;
virtualPath = str2;
this.ViewLocationCache.InsertViewLocation(controllerContext.HttpContext, cacheKey, virtualPath);
return virtualPath;
}
searchedLocations[i] = str2;
}
return virtualPath;
}
该方法会将参数Theme、Controller和Action传入上文提到的View路径模版，生成实际的路径，如果文件不存在，继续尝试下一个View路径模版。直到找到View存在的实际路径。
偷梁换柱，让MVC使用自定义的ViewEngine
Nop是通过在Global文件的事件Application_Start中注入以下代码：
//remove all view engines
ViewEngines.Engines.Clear();
//except the themeable razor view engine we use
ViewEngines.Engines.Add(new ThemeableRazorViewEngine());
总结
到这里，你是否已经知道了Nop实现Theme的奥秘？但又觉得过于复杂？其实Nop实现Theme的同时，还为Mobile和Admin管理后台做了很多特殊处理，所以代码看起来有点乱。那我们就来自己动手打造一个轻量级的ThemeViewEngine吧。预知后事，且听下回分解。