asp.net mvc源码分析RenderAction和RenderPartial

截止上篇文章asp.net mvc源码分析-ActionResult篇 RazorView.RenderView 相信大家对mvc的大致流程应该有所了解。现在我们来看看我们在mvc开发中用的最多的几个方法，我想排在第一的应该是Html.RenderAction和Html.RenderPartial吧。先说简单的吧：RenderPartial和Partial

 public static void RenderPartial(this HtmlHelper htmlHelper, string partialViewName, object model, ViewDataDictionary viewData) {
            htmlHelper.RenderPartialInternal(partialViewName, viewData, model, htmlHelper.ViewContext.Writer, ViewEngines.Engines);
        }

  public static MvcHtmlString Partial(this HtmlHelper htmlHelper, string partialViewName, object model, ViewDataDictionary viewData) {
            using (StringWriter writer = new StringWriter(CultureInfo.CurrentCulture)) {
                htmlHelper.RenderPartialInternal(partialViewName, viewData, model, writer, ViewEngines.Engines);
                return MvcHtmlString.Create(writer.ToString());
            }
        }

从这里我们可以知道RenderPartial和Partial它们返回的东西写到的流不一致，一个是当前的writer，一个是新建的writer，当然新建的writer便于返回字符文本。

RenderPartialInternal的定义和核心代码如下：

  internal virtual void RenderPartialInternal(string partialViewName, ViewDataDictionary viewData, object model, TextWriter writer, ViewEngineCollection viewEngineCollection) {

            ViewContext newViewContext = new ViewContext(ViewContext, ViewContext.View, newViewData, ViewContext.TempData, writer);
            IView view = FindPartialView(newViewContext, partialViewName, viewEngineCollection);
            view.Render(newViewContext, writer);
        }

看看 是不是很简单，而这里的关键FindPartialView，找到view后然后调用其Render方法，FindPartialView核心代码就一句，ViewEngineResult result = viewEngineCollection.FindPartialView(viewContext, partialViewName);我们从前面的文章中知道FindView 和FindPartialView的逻辑是一致的。

现在 我们来看看RenderAction和 Action方法

  public static MvcHtmlString Action(this HtmlHelper htmlHelper, string actionName, string controllerName, RouteValueDictionary routeValues) {
            using (StringWriter writer = new StringWriter(CultureInfo.CurrentCulture)) {
                ActionHelper(htmlHelper, actionName, controllerName, routeValues, writer);
                return MvcHtmlString.Create(writer.ToString());
            }
        }

  public static void RenderAction(this HtmlHelper htmlHelper, string actionName, string controllerName, RouteValueDictionary routeValues) {
            ActionHelper(htmlHelper, actionName, controllerName, routeValues, htmlHelper.ViewContext.Writer);
        }

internal static void ActionHelper(HtmlHelper htmlHelper, string actionName, string controllerName, RouteValueDictionary routeValues, TextWriter textWriter) {
            if (htmlHelper == null) {
                throw new ArgumentNullException("htmlHelper");
            }
            if (String.IsNullOrEmpty(actionName)) {
                throw new ArgumentException(MvcResources.Common_NullOrEmpty, "actionName");
            }

            RouteValueDictionary additionalRouteValues = routeValues;
            routeValues = MergeDictionaries(routeValues, htmlHelper.ViewContext.RouteData.Values);
                     
            routeValues["action"] = actionName;
            if (!String.IsNullOrEmpty(controllerName)) {
                routeValues["controller"] = controllerName;
            }

            bool usingAreas;
            VirtualPathData vpd = htmlHelper.RouteCollection.GetVirtualPathForArea(htmlHelper.ViewContext.RequestContext, null /* name */, routeValues, out usingAreas);
            if (vpd == null) {
                throw new InvalidOperationException(MvcResources.Common_NoRouteMatched);
            }
   
            if (usingAreas) {
                routeValues.Remove("area");
                if (additionalRouteValues != null) {
                    additionalRouteValues.Remove("area");
                }
            }

            if (additionalRouteValues != null) {
                routeValues[ChildActionValueProvider.ChildActionValuesKey] = new DictionaryValueProvider<object>(additionalRouteValues, CultureInfo.InvariantCulture);
            }

            RouteData routeData = CreateRouteData(vpd.Route, routeValues, vpd.DataTokens, htmlHelper.ViewContext);
            HttpContextBase httpContext = htmlHelper.ViewContext.HttpContext;
            RequestContext requestContext = new RequestContext(httpContext, routeData);
            ChildActionMvcHandler handler = new ChildActionMvcHandler(requestContext);
            httpContext.Server.Execute(HttpHandlerUtil.WrapForServerExecute(handler), textWriter, true /* preserveForm */);
        }

        private static RouteData CreateRouteData(RouteBase route, RouteValueDictionary routeValues, RouteValueDictionary dataTokens, ViewContext parentViewContext) {
            RouteData routeData = new RouteData();

            foreach (KeyValuePair<string, object> kvp in routeValues) {
                routeData.Values.Add(kvp.Key, kvp.Value);
            }

            foreach (KeyValuePair<string, object> kvp in dataTokens) {
                routeData.DataTokens.Add(kvp.Key, kvp.Value);
            }

            routeData.Route = route;
            routeData.DataTokens[ControllerContext.PARENT_ACTION_VIEWCONTEXT] = parentViewContext;
            return routeData;
        }

        private static RouteValueDictionary MergeDictionaries(params RouteValueDictionary[] dictionaries) {
            // Merge existing route values with the user provided values
            var result = new RouteValueDictionary();

            foreach (RouteValueDictionary dictionary in dictionaries.Where(d => d != null)) {
                foreach (KeyValuePair<string, object> kvp in dictionary) {
                    if (!result.ContainsKey(kvp.Key)) {
                        result.Add(kvp.Key, kvp.Value);
                    }
                }
            }

            return result;
        }

RenderAction和 Action方法的区别和RenderPartial和Partial的区别一样，一个是把内容返回到当前流一个是返回到一个字符串。

htmlHelper.ViewContext.RouteData.Values这个Values里面包括我们所有的路由信息，典型的就是action、controller，这里面有这么一句 if (!String.IsNullOrEmpty(controllerName)) {routeValues["controller"] = controllerName;}，这句就解释了如果我们调用RenderPartial不传controller就会默认为当前的controller。现在让我们来看看GetVirtualPathForArea这个方法是如何获取VirtualPathData的。

   internal static VirtualPathData GetVirtualPathForArea(this RouteCollection routes, RequestContext requestContext, string name, RouteValueDictionary values, out bool usingAreas) {
            if (routes == null) {
                throw new ArgumentNullException("routes");
            }

            if (!String.IsNullOrEmpty(name)) {
                // the route name is a stronger qualifier than the area name, so just pipe it through
                usingAreas = false;
                return routes.GetVirtualPath(requestContext, name, values);
            }

            string targetArea = null;
            if (values != null) {
                object targetAreaRawValue;
                if (values.TryGetValue("area", out targetAreaRawValue)) {
                    targetArea = targetAreaRawValue as string;
                }
                else {
                    // set target area to current area
                    if (requestContext != null) {
                        targetArea = AreaHelpers.GetAreaName(requestContext.RouteData);
                    }
                }
            }

            // need to apply a correction to the RVD if areas are in use
            RouteValueDictionary correctedValues = values;
            RouteCollection filteredRoutes = FilterRouteCollectionByArea(routes, targetArea, out usingAreas);
            if (usingAreas) {
                correctedValues = new RouteValueDictionary(values);
                correctedValues.Remove("area");
            }

            VirtualPathData vpd = filteredRoutes.GetVirtualPath(requestContext, correctedValues);
            return vpd;
        }

　　我们默认传进来的name=null，usingAreas=false；这里首先获取area，获取的优先级是：（1）当前的RouteValueDictionary是否含有area，（2）当前请求requestContext.RouteData.DataTokens是否含有area，（3）requestContext.RouteData.Route.DataTokens是否含有area。这个我AreaHelpers的代码如下：

 internal static class AreaHelpers {

        public static string GetAreaName(RouteBase route) {
            IRouteWithArea routeWithArea = route as IRouteWithArea;
            if (routeWithArea != null) {
                return routeWithArea.Area;
            }

            Route castRoute = route as Route;
            if (castRoute != null && castRoute.DataTokens != null) {
                return castRoute.DataTokens["area"] as string;
            }

            return null;
        }

        public static string GetAreaName(RouteData routeData) {
            object area;
            if (routeData.DataTokens.TryGetValue("area", out area)) {
                return area as string;
            }

            return GetAreaName(routeData.Route);
        }

    }

FilterRouteCollectionByArea这个方法就是去掉与当前路由信息中area不同的所有路由信息，构建新的路由信息。
 private static RouteCollection FilterRouteCollectionByArea(RouteCollection routes, string areaName, out bool usingAreas) {
            if (areaName == null) {
                areaName = String.Empty;
            }
            usingAreas = false;
            RouteCollection filteredRoutes = new RouteCollection();
            using (routes.GetReadLock()) {
                foreach (RouteBase route in routes) {
                    string thisAreaName = AreaHelpers.GetAreaName(route) ?? String.Empty;
                    usingAreas |= (thisAreaName.Length > 0);
                    if (String.Equals(thisAreaName, areaName, StringComparison.OrdinalIgnoreCase)) {
                        filteredRoutes.Add(route);
                    }
                }
            }
            // if areas are not in use, the filtered route collection might be incorrect
            return (usingAreas) ? filteredRoutes : routes;
        }
最后调用RouteCollection的GetVirtualPath方法。RouteCollection的GetVirtualPath方法其实就是循环调用里面每个RouteBase 的base2.GetVirtualPath方法，里面有这么一句

 foreach (RouteBase base2 in this)
        {
            VirtualPathData virtualPath = base2.GetVirtualPath(requestContext, values);
            if (virtualPath != null)
            {
                virtualPath.VirtualPath = this.NormalizeVirtualPath(requestContext, virtualPath.VirtualPath);
                return virtualPath;
            }
        }

我们知道这里的RouteBase实际上是一个Route实例，我们来看看它的GetVirtualPath方法，里面有这么一句

    BoundUrl url = this._parsedRoute.Bind(requestContext.RouteData.Values, values, this.Defaults, this.Constraints)实际上是调用的ParsedRoute的Bind方法，

这个Bind方法返回的BoundUrl 有一个很特殊的属性 return new BoundUrl { Url = builder.ToString(), Values = acceptedValues };

结合整过方法我们就知道它把我们调用RenderAction时传入的routeValues拼接成url字符串。例如：

可惜的是在mvc中没有使用这个VirtualPath属性，因为我们不需要从这里取值。真正取值是靠下面这句

 routeValues[ChildActionValueProvider.ChildActionValuesKey] = new DictionaryValueProvider<object>(additionalRouteValues, CultureInfo.InvariantCulture);

这句 就是把我们传进来的routeValues给保存起来，便于后面调用。

那么这里我们顺便看看ChildActionValueProvider的关键实现代码：

        public override ValueProviderResult GetValue(string key) {
            ValueProviderResult explicitValues = base.GetValue(ChildActionValuesKey);
            if (explicitValues != null) {
                DictionaryValueProvider<object> rawExplicitValues = explicitValues.RawValue as DictionaryValueProvider<object>;
                if (rawExplicitValues != null) {
                    return rawExplicitValues.GetValue(key);
                }
            }
            return null;
        }

先通过ChildActionValuesKey取得routeValues，再在routeValues中根据key来取值。我想大家看到这里就应该明白为什么RenderAction时DefaultModelBinder会走BindSimpleModel方法了吧，我想大家看到这里就应该明白为什么RenderAction时DefaultModelBinder会走BindSimpleModel方法了吧，但是如果routeValues中并没有传递我们需要的参数，而我们的参数又是一个复杂类型那么就会走BindComplexModel方法，是简单类型就直接返回一个null，是否是简单类型是看起能否转换成string。

CreateRouteData这个方法没什么特别的，只是 routeData.DataTokens[ControllerContext.PARENT_ACTION_VIEWCONTEXT] = parentViewContext;把新的Action作为子Action。

HttpHandlerUtil.WrapForServerExecute这个方法没什么好说的，把当前ChildActionMvcHandler包装成一个ServerExecuteHttpHandlerWrapper，不过ServerExecuteHttpHandlerWrapper继承于Page类。

从 这里我们知道RenderAction是发起一个handler请求处理和RenderPartial只是呈现试图，所以RenderPartial的性能要高出很多。