asp.net mvc源码分析ActionResult篇 FindView

接着上篇asp.net mvc源码分析-ActionResult篇 ViewResult 中有ViewEngineResult result = ViewEngineCollection.FindView(context, ViewName, MasterName)这么一句，它究竟是怎么找到View的了？首先放我们看看你ViewEngineCollection中的FindView方法吧，其实就一句

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

不过这句干的事情可不少啊，调用内部的一个Find方法，

    private ViewEngineResult 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);
        }

这里的cacheLocator=e.FindView(controllerContext, viewName, masterName, true)，locator=e.FindView(controllerContext, viewName, masterName, false)，它也是在调用一个内部的find方法，

   private ViewEngineResult 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 = new List<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
                return new ViewEngineResult(searched.Distinct().ToList());
            }
            else {
                return null;
            }
        }

　其中 trackSearchedPaths表示是否记录收索路径，首先不记录收索路径看能否找到view，如果找不到就记录收索路径在此查找view。顺便提一下ViewEngineCollection中有一个FindPartialView方法和FindView方法逻辑一致。

这里 的CombinedItems其实就是ViewEngines中的Engines属性，默认只有WebFormViewEngine、RazorViewEngine这2个实例。所以它会遍历所有的IViewEngine来查找view。这里为了提高性能，我们可以移除一个我们不需要的IViewEngine。，例如我在用Razor开发mvc时可以把WebFormViewEngine给移除以提高性。Application_Start方法中添加  ViewEngines.Engines.RemoveAt(0);

trackSearchedPaths这个参数是记录查找路径的，究竟有什么效果了？如果我们在IViewEngine中没有找到相应的view则记录查找路径。 

   if (trackSearchedPaths) {
                        searched.AddRange(result.SearchedLocations);
                    }

如果 所有的IViewEngine都找完了，都没有找到查找到view，trackSearchedPaths为false值直接返回null，否者返回一个没有view的ViewEngineResult。

给我们带来效果如下：

现在 我们来看看你真正查找view的方法是在VirtualPathProviderViewEngine类的FindView方法：

        public virtual ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache) {
            if (controllerContext == null) {
                throw new ArgumentNullException("controllerContext");
            }
            if (String.IsNullOrEmpty(viewName)) {
                throw new ArgumentException(MvcResources.Common_NullOrEmpty, "viewName");
            }

            string[] viewLocationsSearched;
            string[] masterLocationsSearched;

            string controllerName = controllerContext.RouteData.GetRequiredString("controller");
            string viewPath = GetPath(controllerContext, ViewLocationFormats, AreaViewLocationFormats, "ViewLocationFormats", viewName, controllerName, _cacheKeyPrefix_View, useCache, out viewLocationsSearched);
            string masterPath = GetPath(controllerContext, MasterLocationFormats, AreaMasterLocationFormats, "MasterLocationFormats", masterName, controllerName, _cacheKeyPrefix_Master, useCache, out masterLocationsSearched);

            if (String.IsNullOrEmpty(viewPath) || (String.IsNullOrEmpty(masterPath) && !String.IsNullOrEmpty(masterName))) {
                return new ViewEngineResult(viewLocationsSearched.Union(masterLocationsSearched));
            }

            return new ViewEngineResult(CreateView(controllerContext, viewPath, masterPath), this);
        }

　这里面有一句很重要  string viewPath = GetPath(controllerContext, ViewLocationFormats, AreaViewLocationFormats, "ViewLocationFormats", viewName, controllerName, _cacheKeyPrefix_View, useCache, out viewLocationsSearched);，用GetPath来找viewpath、masterPath。

 private string GetPath(ControllerContext controllerContext, string[] locations, string[] areaLocations, string locationsPropertyName, string name, string controllerName, string cacheKeyPrefix, bool useCache, out string[] searchedLocations) {
            searchedLocations = _emptyLocations;

            if (String.IsNullOrEmpty(name)) {
                return String.Empty;
            }

            string areaName = AreaHelpers.GetAreaName(controllerContext.RouteData);
            bool usingAreas = !String.IsNullOrEmpty(areaName);
            List<ViewLocation> viewLocations = GetViewLocations(locations, (usingAreas) ? areaLocations : null);

            if (viewLocations.Count == 0) {
                throw new InvalidOperationException(String.Format(CultureInfo.CurrentCulture,
                    MvcResources.Common_PropertyCannotBeNullOrEmpty, locationsPropertyName));
            }

            bool nameRepresentsPath = IsSpecificPath(name);
            string cacheKey = CreateCacheKey(cacheKeyPrefix, name, (nameRepresentsPath) ? String.Empty : controllerName, areaName);

            if (useCache) {
                return ViewLocationCache.GetViewLocation(controllerContext.HttpContext, cacheKey);
            }

            return (nameRepresentsPath) ?
                GetPathFromSpecificName(controllerContext, name, cacheKey, ref searchedLocations) :
                GetPathFromGeneralName(controllerContext, viewLocations, name, controllerName, areaName, cacheKey, ref searchedLocations);
        }

　　

 string areaName = AreaHelpers.GetAreaName(controllerContext.RouteData);
            bool usingAreas = !String.IsNullOrEmpty(areaName);
默认情况下usingAreas 为false。GetViewLocations方法就是返回一个ViewLocation集合，里面的代码简单，举个例子吧，在RazorViewEngine中重置了

  ViewLocationFormats = new[] {
                "~/Views/{1}/{0}.cshtml",
                "~/Views/{1}/{0}.vbhtml",
                "~/Views/Shared/{0}.cshtml",
                "~/Views/Shared/{0}.vbhtml"
            };

加入当前的Controller为Home，Action为Index，

那么 后面实际 查找路径就会一次是

"~/Views/Home/Index.cshtml",
"~/Views/Home/Index.vbhtml",
"~/Views/Shared/Index.cshtml",
"~/Views/Shared/Index.vbhtml"

其实我们一般的项目要么就是C#和VB一种语言开发，所以ViewLocationFormats 可以移除2个元素以提高性能。

 bool nameRepresentsPath = IsSpecificPath(name)这句是看我们的viewname是否以~和/开头.

   string cacheKey = CreateCacheKey(cacheKeyPrefix, name, (nameRepresentsPath) ? String.Empty : controllerName, areaName); 创建一个缓存key。

 if (useCache) {
                return ViewLocationCache.GetViewLocation(controllerContext.HttpContext, cacheKey);
            }

从 缓存中返回viewpath， 默认情况相爱ViewLocationCache = new DefaultViewLocationCache()

DefaultViewLocationCache的主要方法如下：

 public string GetViewLocation(HttpContextBase httpContext, string key) {
            if (httpContext == null) {
                throw new ArgumentNullException("httpContext");
            }
            return (string)httpContext.Cache[key];
        }
        public void InsertViewLocation(HttpContextBase httpContext, string key, string virtualPath) {
            if (httpContext == null) {
                throw new ArgumentNullException("httpContext");
            }
            httpContext.Cache.Insert(key, virtualPath, null /* dependencies */, Cache.NoAbsoluteExpiration, TimeSpan);
        }

很简单的。

现在 我们回到GetPath方法中，如果不用缓存就 只剩下最后一句了

 return (nameRepresentsPath) ?
                GetPathFromSpecificName(controllerContext, name, cacheKey, ref searchedLocations) :
                GetPathFromGeneralName(controllerContext, viewLocations, name, controllerName, areaName, cacheKey, ref searchedLocations);

这 2个方法实现如下：

    private string GetPathFromGeneralName(ControllerContext controllerContext, List<ViewLocation> locations, string name, string controllerName, string areaName, string cacheKey, ref string[] searchedLocations) {
            string result = String.Empty;
            searchedLocations = new string[locations.Count];

            for (int i = 0; i < locations.Count; i++) {
                ViewLocation location = locations[i];
                string virtualPath = location.Format(name, controllerName, areaName);

                if (FileExists(controllerContext, virtualPath)) {
                    searchedLocations = _emptyLocations;
                    result = virtualPath;
                    ViewLocationCache.InsertViewLocation(controllerContext.HttpContext, cacheKey, result);
                    break;
                }

                searchedLocations[i] = virtualPath;
            }

            return result;
        }

        private string GetPathFromSpecificName(ControllerContext controllerContext, string name, string cacheKey, ref string[] searchedLocations) {
            string result = name;

            if (!(FilePathIsSupported(name) && FileExists(controllerContext, name))) {
                result = String.Empty;
                searchedLocations = new[] { name };
            }

            ViewLocationCache.InsertViewLocation(controllerContext.HttpContext, cacheKey, result);
            return result;
        }

　　这2个方法都调用一个共同的方法FileExists，FileExists是个内联函数直接调用VirtualPathProvider.FileExists(virtualPath)，默认情况下VirtualPathProvider=HostingEnvironment.VirtualPathProvider。但是实际上调用的是 BuildManager.GetObjectFactory(virtualPath, false) != null它是如何检查路径的我们这里就忽略它吧。在GetPathFromSpecificName方法中调用一个FilePathIsSupported方法，其实就是检查viewname的扩展名。
这里我们就完成可viewpath的查找。

现在 我们回到FindView中来，

  if (String.IsNullOrEmpty(viewPath) || (String.IsNullOrEmpty(masterPath) && !String.IsNullOrEmpty(masterName))) {
                return new ViewEngineResult(viewLocationsSearched.Union(masterLocationsSearched));
            }

这句 没什么说的了简单， return new ViewEngineResult(CreateView(controllerContext, viewPath, masterPath), this);这句返回一个ViewEngineResult，ViewEngineResult的构造函数也很简单，多说了。现在我们来看看这个CreateView方法。RazorViewEngine和WebFormViewEngine的具体实现如下：

 protected override IView CreateView(ControllerContext controllerContext, string viewPath, string masterPath) {
            var view = new RazorView(controllerContext, viewPath,
                                     layoutPath: masterPath, runViewStartPages: true, viewStartFileExtensions: FileExtensions, viewPageActivator: ViewPageActivator);
            return view;
        }
  protected override IView CreateView(ControllerContext controllerContext, string viewPath, string masterPath) {
            return new WebFormView(controllerContext, viewPath, masterPath, ViewPageActivator);
        }

至此 我们已经成功找到了View。