[源码学习]调试Razor从哪里开始

使用ASP.NET MVC时，我们知道，要使用Views中的视图，需要在Action中写

return View();

这个方法返回的返回值是一个 ViewResult，进入这个类，继承了父类ViewResultBase后只写了MasterName属性和FindView方法。

不过已经开始看到到ViewEngine的踪影了。

        protected override ViewEngineResult FindView(ControllerContext context) {
            ViewEngineResult result = ViewEngineCollection.FindView(context, ViewName, MasterName);
            ...;
        }

跟进ViewEngineCollection.FindView去看看。

来到了ViewEngineCollection类，这是一个实现了Collection<IViewEngine>的类。

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

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

很简单的一个方法，跟进去

      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);
        }

        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;
            }
        }

乍一看，又是for又是if、else的有点不知所措，其实仔细一看结合上面的Find参数就能找到黄色加亮的几句代码关键代码。

是遍历了注册ViewEngine集合，调用ViewEngine各自的FindView，谁能找到View，就用谁。

那么遍历的ViewEngine集合怎么来的呢？要回到ViewResult的父类ViewResultBase中去看。

        public ViewEngineCollection ViewEngineCollection {
            get {
                return _viewEngineCollection ?? ViewEngines.Engines;
            }
            set {
                _viewEngineCollection = value;
            }
        }

如果没有定义那么就调用ViewEngines.Engines，这是一个很简单的静态类属性

    public static class ViewEngines {

        private readonly static ViewEngineCollection _engines = new ViewEngineCollection {
            new WebFormViewEngine(),
            new RazorViewEngine(),
        };

        public static ViewEngineCollection Engines {
            get {
                return _engines;
            }
        }
    }

回到刚才的遍历，由于RazorViewEngine的构造函数中定义了以下格式，在Views文件夹中也创建了相应的文件，所以选择了RazorViewEngine。

 AreaViewLocationFormats = new[] {
                "~/Areas/{2}/Views/{1}/{0}.cshtml",
                "~/Areas/{2}/Views/{1}/{0}.vbhtml",
                "~/Areas/{2}/Views/Shared/{0}.cshtml",
                "~/Areas/{2}/Views/Shared/{0}.vbhtml"
            };

好了，终于找到了Razor。