带你读开源—ASP.NET_MVC（五）
书接上文，我们继续跟踪CreateView方法，发现它是一个抽象方法，好吧，我们发现BuildManagerViewEngine继承了VirtualPathProviderViewEngine，但是它并没有重写了CreateView方法，不要慌，我们继续摸索，发现RazorViewEngine和WebFormViewEngine（先不管他）又继承了BuildManagerViewEngine，在这里我们只看RazorViewEngine的CreateView方法重写（代码段１）。
protected override IViewCreateView(ControllerContext controllerContext, string viewPath, stringmasterPath)
{
var view = newRazorView(controllerContext, viewPath,
layoutPath: masterPath, runViewStartPages: true,viewStartFileExtensions: FileExtensions, viewPageActivator: ViewPageActivator)
{
DisplayModeProvider =DisplayModeProvider
};
return view;
}
代码段 １
代码段1得到了一个IView对象，进而使得“带你读开源—ASP.NET_MVC（四）”代码段10得到一个ViewEngineResult，我们再转到“带你读开源—ASP.NET_MVC（四）”代码段2中的语句【result = FindView(context);View = result.View;】，到此我们便得到了一个View对象，进而调用View.Render(viewContext, writer)方法，而其中的writer 又是什么东东？我们找到语句【TextWriter writer =context.HttpContext.Response.Output;】，豁然开朗不？我们久违的、日思夜想的Response终于出现了，有了它，我们就可以把HTML响应渲染回送到浏览器，至此，从浏览器发出HTTP请求，到服务器发回HTML响应，构成完整的回路。我靠，累死我了！
我们再看代码段2，这是一个IView接口的定义，该接口仅有一个方法Render，用以向浏览器渲染HTML页面。
<pre name="code" class="csharp"> public interface IView
{
void Render(ViewContext viewContext, TextWriterwriter);
}
代码段 2
只有一个类实现了IView接口，这个类就是BuildManagerCompiledView，我们看Render方法的具体实现，见代码段3。
public virtual void Render(ViewContextviewContext, TextWriter writer)
{
if (viewContext == null)
{
throw newArgumentNullException("viewContext");
}
object instance = null;
Type type =BuildManager.GetCompiledType(ViewPath);
if (type != null)
{
instance =ViewPageActivator.Create(_controllerContext, type);
}
if (instance == null)
{
throw newInvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.CshtmlView_ViewCouldNotBeCreated,
ViewPath));
}
RenderView(viewContext, writer,instance);
}
代码段 3
代码段３中有两个地方需要我们注意：
①【instance= ViewPageActivator.Create(_controllerContext, type);】
②【RenderView(viewContext,writer, instance);】
其中，①是实例化视图类，②是渲染视图。为了突出主线，我们先分析②。我们发现RenderView是一个抽象方法，凭经验BuildManagerCompiledView类应该有子类重写了RenderView方法，在源码中寻找，发现它有两个子类，即WebFormView和RazorView，见到这两个类开心吧？呵呵！我们在这里先只分析RazorView的RenderView方法重写，见代码段4。
protected override voidRenderView(ViewContext viewContext, TextWriter writer, object instance)
{
if (writer == null)
{
throw newArgumentNullException("writer");
}
WebViewPage webViewPage = instanceas WebViewPage;
if (webViewPage == null)
{
throw newInvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.CshtmlView_WrongViewBase,
ViewPath));
}
// An overriden master layout mighthave been specified when the ViewActionResult got returned.
// We need to hold on to it so thatwe can set it on the inner page once it has executed.
webViewPage.OverridenLayoutPath =LayoutPath;
webViewPage.VirtualPath = ViewPath;
webViewPage.ViewContext =viewContext;
webViewPage.ViewData =viewContext.ViewData;
webViewPage.InitHelpers();
if (VirtualPathFactory != null)
{
webViewPage.VirtualPathFactory= VirtualPathFactory;
}
if (DisplayModeProvider != null)
{
webViewPage.DisplayModeProvider= DisplayModeProvider;
}
WebPageRenderingBase startPage =null;
if (RunViewStartPages)
{
startPage =StartPageLookup(webViewPage, RazorViewEngine.ViewStartFileName, ViewStartFileExtensions);
}
webViewPage.ExecutePageHierarchy(new WebPageContext(context:viewContext.HttpContext, page: null, model: null), writer, startPage);
}
代码段 4
在代码段４中找到语句【webViewPage.ExecutePageHierarchy(new WebPageContext(context:viewContext.HttpContext, page: null, model: null), writer, startPage);】，这句是执行渲染的核心语句。继续跟踪进入webViewPage.ExecutePageHierarchy，见代码段5。
// This method is only used byWebPageBase to allow passing in the view context and writer.
public voidExecutePageHierarchy(WebPageContext pageContext, TextWriter writer,WebPageRenderingBase startPage)
{
PushContext(pageContext, writer);
if (startPage != null)
{
if (startPage != this)
{
var startPageContext =WebPageContext.CreateNestedPageContext<object>(parentContext:pageContext, pageData: null, model: null, isLayoutPage: false);
startPageContext.Page =startPage;
startPage.PageContext = startPageContext;
}
startPage.ExecutePageHierarchy();
}
else
{
ExecutePageHierarchy();
}
PopContext();
}
代码段 5
在代码段５中找到ExecutePageHierarchy方法，进入其定义，见代码段6。
[SuppressMessage("Microsoft.Design","CA1031:DoNotCatchGeneralExceptionTypes", Justification = "Wereally don't care if SourceHeader fails, and we don't want it to fail any realrequests ever")]
public override voidExecutePageHierarchy()
{
// Unlike InitPages, for a WebPagethere is no hierarchy - it is always
// the last file to execute in thechain. There can still be layout pages
// and partial pages, but they arenever part of the hierarchy.
// (add server header for falcondebugging)
// call to MapPath() is expensive.If we are not emiting source files to header,
// don't bother to populate theSourceFiles collection. This saves perf significantly.
if(WebPageHttpHandler.ShouldGenerateSourceHeader(Context))
{
try
{
string vp = VirtualPath;
if (vp != null)
{
string path =Context.Request.MapPath(vp);
if (!path.IsEmpty())
{
PageContext.SourceFiles.Add(path);
}
}
}
catch
{
// we really don't care ifthis ever fails, so we swallow all exceptions
}
}
TemplateStack.Push(Context, this);
try
{
// Execute thedeveloper-written code of the WebPage
Execute();
}
finally
{
TemplateStack.Pop(Context);
}
}
代码段 6
在代码段６中找到语句Execute()方法，在这个方法上方有一条注释“//Execute thedeveloper-written code of the WebPage”，大概意思是“执行开发者编写的页面代码”，明白了吧？这句的功能就是执行你编写的视图代码，即.cshtml文件。那么肯定有人要提出疑问了，.cshtml文件怎么执行？它不是一段html脚本么？不错，.cshtml文件确实是一段html脚本，不能被直接执行，在此留一个悬念[1]。
未完待续。。。