asp.net mvc源码分析-ActionResult篇 RazorView.RenderView
接着上文asp.net mvc源码分析-ActionResult篇 FindView 我们已经创建好view了，大家还记得在BuildManagerCompiledView的Render方法中最后调用的是RenderView。可能是跟人喜好问题，还有就是我工作项目用到的多数是Razor，所以这里就讲讲RazorView吧。
想让我们可看看RazorView的构造函数有什么特别的地方
public RazorView(ControllerContext controllerContext, string viewPath, string layoutPath, bool runViewStartPages, IEnumerable<string> viewStartFileExtensions, IViewPageActivator viewPageActivator)
: base(controllerContext, viewPath, viewPageActivator) {
LayoutPath = layoutPath ?? String.Empty;
RunViewStartPages = runViewStartPages;
StartPageLookup = StartPage.GetStartPage;
ViewStartFileExtensions = viewStartFileExtensions ?? Enumerable.Empty<string>();
}
其中LayoutPath 就是我们的masterPath，RunViewStartPages =true，ViewStartFileExtensions =FileExtensions，viewPageActivator=DefaultViewPageActivator的一个实例，viewPageActivator的设置在父类BuildManagerCompiledView的构造函数中设置。现在让我们看看RenderView这个方法：
protected override void RenderView(ViewContext viewContext, TextWriter writer, object instance) {
if (writer == null) {
throw new ArgumentNullException("writer");
}
WebViewPage webViewPage = instance as WebViewPage;
if (webViewPage == null) {
throw new InvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.CshtmlView_WrongViewBase,
ViewPath));
}
// An overriden master layout might have been specified when the ViewActionResult got returned.
// We need to hold on to it so that we 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();
WebPageRenderingBase startPage = null;
if (RunViewStartPages) {
startPage = StartPageLookup(webViewPage, RazorViewEngine.ViewStartFileName, ViewStartFileExtensions);
}
webViewPage.ExecutePageHierarchy(new WebPageContext(context: viewContext.HttpContext, page: null, model: null), writer, startPage);
}
首先把当前view所对应的类型实例转化为WebViewPage，转换失败则抛出异常。WebViewPage的继承结构如下：WebViewPage-》WebPageBase-》WebPageRenderingBase-》WebPageExecutingBase。
记下来设置webViewPage的几个重要属性
webViewPage.OverridenLayoutPath = LayoutPath;
webViewPage.VirtualPath = ViewPath;
webViewPage.ViewContext = viewContext;
webViewPage.ViewData = viewContext.ViewData;
然后调用webViewPage.InitHelpers()
public virtual void InitHelpers() {
Ajax = new AjaxHelper<object>(ViewContext, this);
Html = new HtmlHelper<object>(ViewContext, this);
Url = new UrlHelper(ViewContext.RequestContext);
}
设置 Ajax，Html，Url3个属性
默认 情况下RunViewStartPages为true。
startPage = StartPageLookup(webViewPage, RazorViewEngine.ViewStartFileName, ViewStartFileExtensions);
其中 ViewStartFileName = "_ViewStart";FileExtensions = new[] { "cshtml","vbhtml",};
这里 多说明一下StartPage直接继承于WebPageRenderingBase,我们还是来看看它的GetStartPage是怎么实现的吧：
public static WebPageRenderingBase GetStartPage(WebPageRenderingBase page, string fileName, IEnumerable<string> supportedExtensions) {
if (page == null) {
throw new ArgumentNullException("page");
}
if (String.IsNullOrEmpty(fileName)) {
throw new ArgumentException(String.Format(CultureInfo.CurrentCulture, CommonResources.Argument_Cannot_Be_Null_Or_Empty, "fileName"), "fileName");
}
if (supportedExtensions == null) {
throw new ArgumentNullException("supportedExtensions");
}
// Build up a list of pages to execute, such as one of the following:
// ~/somepage.cshtml
// ~/_pageStart.cshtml --> ~/somepage.cshtml
// ~/_pageStart.cshtml --> ~/sub/_pageStart.cshtml --> ~/sub/somepage.cshtml
WebPageRenderingBase currentPage = page;
var pageDirectory = VirtualPathUtility.GetDirectory(page.VirtualPath);
// Start with the requested page's directory, find the init page,
// and then traverse up the hierarchy to find init pages all the
// way up to the root of the app.
while (!String.IsNullOrEmpty(pageDirectory) && pageDirectory != "/" && Util.IsWithinAppRoot(pageDirectory)) {
// Go through the list of support extensions
foreach (var extension in supportedExtensions) {
var path = VirtualPathUtility.Combine(pageDirectory, fileName + "." + extension);
if (currentPage.FileExists(path, useCache: true)) {
var factory = currentPage.GetObjectFactory(path);
var parentStartPage = (StartPage)factory();
parentStartPage.VirtualPath = path;
parentStartPage.ChildPage = currentPage;
currentPage = parentStartPage;
break;
}
}
pageDirectory = currentPage.GetDirectory(pageDirectory);
}
// At this point 'currentPage' is the root-most StartPage (if there were
// any StartPages at all) or it is the requested page itself.
return currentPage;
}
首先 WebPageRenderingBase currentPage = page;这句就不说了； var pageDirectory = VirtualPathUtility.GetDirectory(page.VirtualPath)是返回VirtualPath所对应的目录，举个例子吧，VirtualPath=~/Views/Home/Index.cshtml，那么pageDirectory=~/Views/Home/，那么现在就应该进入while循环了，
var path = VirtualPathUtility.Combine(pageDirectory, fileName + "." + extension);生成新的起始页的path，path=~/Views/Home/_ViewStart.cshtml，很显然这个文件不存在。内部的if语句无法执行。这个foreach是循环的文件扩展名，在实际的项目开发中可以考虑将FileExtensions = new[] { "cshtml","vbhtml",};中2个元素移除一个以提高性能。
第二次计入while时pageDirectory=~/Views/，那么现在对应的path=~/Views/_ViewStart.cshtml我们知道这个文件默认是存在的。
var factory = currentPage.GetObjectFactory(path);
var parentStartPage = (StartPage)factory();
parentStartPage.VirtualPath = path;
parentStartPage.ChildPage = currentPage;
currentPage = parentStartPage;
这2句也很好理解不过具体实现就很复杂了，根据当前的path新建的StartPage，并设置它的VirtualPath、ChildPage ，把它作为返回值。
直到pageDirectory=/才推出这个while循环。从这里我们知道_ViewStart可以嵌套另一个_ViewStar，有点像我们的view有自己的Layout，而Layout对应的view也有Layout层层递归。
最后调用 webViewPage.ExecutePageHierarchy(new WebPageContext(context: viewContext.HttpContext, page: null, model: null), writer, startPage);
这里创建了一个WebPageContext，WebPageContext也没什么特别的地方。ExecutePageHierarchy的具体定义是在WebPageBase中，
public void ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) {
PushContext(pageContext, writer);
if (startPage != null) {
if (startPage != this) {
var startPageContext = Util.CreateNestedPageContext<object>(parentContext: pageContext, pageData: null, model: null, isLayoutPage: false);
startPageContext.Page = startPage;
startPage.PageContext = startPageContext;
}
startPage.ExecutePageHierarchy();
}
else {
ExecutePageHierarchy();
}
PopContext();
}
首先在方法开始的地方先保存pageContext和writer，在方法结束前在弹出。默认情况下我们会新建一个WebPageContext作为page的PageContext属性，最后调用startpage的
ExecutePageHierarchy方法。该方法的实现在StartPage中
public override void ExecutePageHierarchy() {
// Push the current pagestart on the stack.
TemplateStack.Push(Context, this);
try {
// Execute the developer-written code of the InitPage
Execute();
// If the child page wasn't explicitly run by the developer of the InitPage, then run it now.
// The child page is either the next InitPage, or the final WebPage.
if (!RunPageCalled) {
RunPage();
}
}
finally {
TemplateStack.Pop(Context);
}
}
public void RunPage() {
RunPageCalled = true;
//ChildPage.PageContext = PageContext;
ChildPage.ExecutePageHierarchy();
}
public void RunPage() {
RunPageCalled = true;
//ChildPage.PageContext = PageContext;
ChildPage.ExecutePageHierarchy();
}
这里的 Execute();是真正调用_ViewStart.cshtml ，在 RunPage方法中有 ChildPage.ExecutePageHierarchy()， 实际是要调用WebViewPage的ExecutePageHierarchy方法。
public override void ExecutePageHierarchy() {
// Change the Writer so that things like Html.BeginForm work correctly
ViewContext.Writer = Output;
base.ExecutePageHierarchy();
// Overwrite LayoutPage so that returning a view with a custom master page works.
if (!String.IsNullOrEmpty(OverridenLayoutPath)) {
Layout = OverridenLayoutPath;
}
}
可以见render一个page是多么的复杂啊。具体实现我们就不关心了，我们只要知道在RenderView时是递归render相应的view。我们只要知道在RenderView时是递归render相应的view。 同时我们需要知道在一次完整的http请求过程中_ViewStart.cshtml是最先执行的，_Layout.cshtml是最后执行
