[ASP.NET MVC]视图是如何呈现的 (续)

在上一篇文章中，我们知道了通过Controller执行ActionResult的Execute可以找到对应Controler对应的ViewEngine，然后在View中把Action的结果显示出来。那么ViewEngine到底是如何工作的？

我们首先从ViewReult的FindView方法开始

protectedoverrideViewEngineResult FindView(ControllerContext context)

{

ViewEngineResult result = ViewEngineCollection.FindView(context, ViewName, MasterName);

if (result.View != null)

{

return result;

}

// we need to generate an exception containing all the locations we searched

…

}

根据ControllerContext和ViewName，以及MasterName找到对应的ViewEngineResult对象。我们还是以HelloController和Index为例。那么这里result将返回Views/Hello/Index.cshtml编译后的实例。我们进入FindView方法，看看其具体的实现。

上述方法调用的是ViewEngineCollection类的虚方法FindView

publicvirtualViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName)

{

if (controllerContext == null)

{

thrownewArgumentNullException("controllerContext");

}

if (String.IsNullOrEmpty(viewName))

{

thrownewArgumentException(MvcResources.Common_NullOrEmpty, "viewName");

}

return Find(e => e.FindView(controllerContext, viewName, masterName, true),

e => e.FindView(controllerContext, viewName, masterName, false));

}

它又调用其私有的方法

privateViewEngineResult Find(Func<IViewEngine, ViewEngineResult> cacheLocator, Func<IViewEngine, ViewEngineResult> locator)

{

// First, look up using the cacheLocator and do not track the searched paths in non-matching view engines

// Then, look up using the normal locator and track the searched paths so that an error view engine can be returned

return Find(cacheLocator, trackSearchedPaths: false)

?? Find(locator, trackSearchedPaths: true);

}

然后又调用

privateViewEngineResult Find(Func<IViewEngine, ViewEngineResult> lookup, bool trackSearchedPaths)

{

// Returns

// 1st result

// OR list of searched paths (if trackSearchedPaths == true)

// OR null

ViewEngineResult result;

List<string> searched = null;

if (trackSearchedPaths)

{

searched = newList<string>();

}

foreach (IViewEngine engine in CombinedItems)

{

if (engine != null)

{

result = lookup(engine);

if (result.View != null)

{

return result;

}

if (trackSearchedPaths)

{

searched.AddRange(result.SearchedLocations);

}

}

}

if (trackSearchedPaths)

{

// Remove duplicate search paths since multiple view engines could have potentially looked at the same path

returnnewViewEngineResult(searched.Distinct().ToList());

}

else

{

returnnull;

}

}

注意，这里传入了一个Func<IViewEngine, ViewEngineResult>。输入一个IViewEngine类型，请注意ViewEngine是实际来自ViewEnglieCollection的属性CombinedItems。该对象来自IResolver<IEnumerable<IViewEngine>>接口的Current属性，其类型为IEnuerable<IViewEngine>。其实就是传入一个IViewEngine，然后一个ViewEngileResult。比如下面的例子：

// details of ViewEngineCollection.Find(*)

Func<IViewEngine, System.Web.Mvc.ViewEngineResult> cacheLocator = e => e.FindView(ControllerContext, "ViewInstance", "", false);

IViewEngine razorViewEngine = newRazorViewEngine();

System.Web.Mvc.ViewEngineResult result = cacheLocator(razorViewEngine);

请注意，IResolver<IEnumerable<IViewEngine>>的默认实例是newMultiServiceResolver<IViewEngine>(() => Items);
【有待于确认】。我们查看其构造函数，可以发现

public MultiServiceResolver(Func<IEnumerable<TService>> itemsThunk)

{

if (itemsThunk == null)

{

thrownewArgumentNullException("itemsThunk");

}

_itemsThunk = itemsThunk;

_resolverThunk = () => DependencyResolver.Current;

_itemsFromService = newLazy<IEnumerable<TService>>(() => _resolverThunk().GetServices<TService>());

}

也就是说IResolver的实例来自DependencyResolver.Current。通过DependencyResolver的定义，我们得知Current属性返回的单列new DenpendencyResolver.InnerCurrent，InnerCurrent其实就是newDefaultDependencyResolver()。DedendencyResolver在创建，创建单列的DefaultDependencyResolver,并将其赋值给_current和_currentCache。_currentCache对应的是InnerCurrentCache，该属性在创建View使没有使用，在创建Controller的时候会使用。

然后，调用ViewResult对象View属性的Render方法，次方法首先会创建View实例。

object instance = null;

Type type = BuildManager.GetCompiledType(ViewPath);

if (type != null)

{

instance = ViewPageActivator.Create(_controllerContext, type);

}

而实际上，现在的ViewPageActivator实际是newBuildManagerViewEngine.DefaultViewPageActivator(dependencyResolver)，而这里的denpendencyResolver就是DefaultDependencyResolver。

OK,我们最后来看一下instance是如何创建的：

_resolverThunk().GetService(type) ?? Activator.CreateInstance(type);

请注意，_resolverThunk = () => DependencyResolver.Current;由此可见在默认的PageActivator内部的私有Fun变量_resolverThunk同样来自DefaultDependencyResolver.Current。

而instance要么DefaultDependencyResolver的GetService()创建，要么通过Activator.CreateInstance来创建，其实它们内部根本没有多少差别。

publicobject GetService(Type serviceType)

{

// Since attempting to create an instance of an interface or an abstract type results in an exception, immediately return null

// to improve performance and the debugging experience with first-chance exceptions enabled.

if (serviceType.IsInterface || serviceType.IsAbstract)

{

returnnull;

}

try

{

returnActivator.CreateInstance(serviceType);

}

catch

{

returnnull;

}

}

由此可见，GetService内部还是调用了Activator.CreateInstance方法创建实例。

剖析如何获取View的具体信息

1. 获取View的Path

// show the details of retrieving view path

publicActionResult List()

{

string[] ViewLocationFormats = new[]

{

"~/Views/{1}/{0}.cshtml",

"~/Views/{1}/{0}.vbhtml",

"~/Views/Shared/{0}.cshtml",

"~/Views/Shared/{0}.vbhtml"

};

string name = "List";

string controllerName = "InnerView";

string areaName = "";

List<ViewLocation> allLocations = newList<ViewLocation>();

foreach (string viewLocationFormat in ViewLocationFormats)

allLocations.Add(newViewLocation(viewLocationFormat));

DisplayModeProvider instance = DisplayModeProvider.Instance;

for (int i = 0; i < allLocations.Count; i++)

{

ViewLocation location = allLocations[i];

string virtualPath = location.Format(name, controllerName, areaName);

DisplayInfo virtualPathDisplayInfo = instance.GetDisplayInfoForVirtualPath(virtualPath, ControllerContext.HttpContext,

path => FileExists(ControllerContext, path), null);

if (virtualPathDisplayInfo == null)

continue;

var viewPath = virtualPathDisplayInfo.FilePath;

Response.Write(viewPath);

}

return View();

}

返回结果为：

1. 获取View的对象

publicActionResult Activator()

{

// remove the dependency

IDependencyResolver dependencyResolver = DependencyResolver.Current;

IResolver<IViewPageActivator> activatorResolver = newSingleServiceResolver<IViewPageActivator>(

() => null, newDefaultViewPageActivator(dependencyResolver), "BuildManagerViewEngine constructor");

IViewPageActivator pageActivator = activatorResolver.Current;

// HelloMVC.Controllers.InnerViewController

Object controllerInstance = pageActivator.Create(ControllerContext, this.GetType());

// Page instance

IBuildManager buildManager = newBuildManagerWrapper();

Object viewpageInstance = pageActivator.Create(ControllerContext, buildManager.GetCompiledType("~/Views/InnerView/List.cshtml "));

Response.Write(string.Format("controllerInstance is {0} <br /> viewpageInstance is {1}", controllerInstance.GetType(), viewpageInstance.GetType()));

return View();

}

http://www.professionals-helpdesk.com/2012/08/exploring-mvc-framwwork-in-deep_10.html exploring MVC framework in deep – DependencyResolver Class