Asp.net web Api源码分析-HttpControllerDispatcher （Controller的创建）

紧接着上文Asp.net web Api源码分析-HttpServer的创建最后我们提到了一个HttpRoutingDispatcher，一看这个类的名字我想我们也就能猜到它是干什么的吧。查找路由信息，那么找到路由后干什么了，是不是就该调用handler了？

首先还是让我们来看看HttpRoutingDispatcher的SendAsync方法

  protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

        {

            // Lookup route data, or if not found as a request property then we look it up in the route table

            IHttpRouteData routeData;

            if (!request.Properties.TryGetValue(HttpPropertyKeys.HttpRouteDataKey, out routeData))

            {

                routeData = _configuration.Routes.GetRouteData(request);

                if (routeData != null)

                {

                    request.Properties.Add(HttpPropertyKeys.HttpRouteDataKey, routeData);

                }

                else

                {

                    return TaskHelpers.FromResult(request.CreateErrorResponse(

                        HttpStatusCode.NotFound,

                        Error.Format(SRResources.ResourceNotFound, request.RequestUri),

                        SRResources.NoRouteData));

                }

            }

            RemoveOptionalRoutingParameters(routeData.Values);

            var invoker = routeData.Route.Handler == null ? _defaultInvoker : new HttpMessageInvoker(routeData.Route.Handler, disposeHandler: false);

            return invoker.SendAsync(request, cancellationToken);

        }

首先这里先从HttpRequestMessage中获取路由信息，如果不能获取则调用_configuration.Routes.GetRouteData来获取路由信息，默认情况下这里是可以获取到路由信息（因为在HttpControllerHandler的BeginProcessRequest方法中有这么一句 request.Properties[HttpPropertyKeys.HttpRouteDataKey] = _routeData;）然后调用RemoveOptionalRoutingParameters方法来移除路由中的可选参数。

这里的routeData我们知道它是在HttpControllerHandler的构造函数中创建的一个HostedHttpRouteData实例，其Route属性是一个HostedHttpRoute实例，routeData.Route.Handler就是我们路由中注册的handler，默认情况下我们没有注册自己的handler，所以这里的invoker=_defaultInvoker，这里的_defaultInvoker=      new HttpMessageInvoker(new
HttpControllerDispatcher(configuration))，所以调用invoker的SendAsync方法，其实就是调用HttpControllerDispatcher的SendAsync方法。

HttpControllerDispatcher这个名称我们就可以猜测到它是把当前http请求交给特定的Controller来处理。HttpControllerDispatcher的SendAsync方法差不多就是一个内联函数，直接调用内部的SendAsyncInternal方法：

private Task<HttpResponseMessage> SendAsyncInternal(HttpRequestMessage request, CancellationToken cancellationToken)
{
if (request == null)
{
throw Error.ArgumentNull("request");
}

IHttpRouteData routeData = request.GetRouteData();
Contract.Assert(routeData != null);
HttpControllerDescriptor httpControllerDescriptor = ControllerSelector.SelectController(request);
if (httpControllerDescriptor == null)
{
return TaskHelpers.FromResult(request.CreateErrorResponse(
HttpStatusCode.NotFound,
Error.Format(SRResources.ResourceNotFound, request.RequestUri),
SRResources.NoControllerSelected));
}

IHttpController httpController = httpControllerDescriptor.CreateController(request);
if (httpController == null)
{
return TaskHelpers.FromResult(request.CreateErrorResponse(
HttpStatusCode.NotFound,
Error.Format(SRResources.ResourceNotFound, request.RequestUri),
SRResources.NoControllerCreated));
}

// Set the controller configuration on the request properties
HttpConfiguration requestConfig = request.GetConfiguration();
if (requestConfig == null)
{
request.Properties.Add(HttpPropertyKeys.HttpConfigurationKey, httpControllerDescriptor.Configuration);
}
else
{
if (requestConfig != httpControllerDescriptor.Configuration)
{
request.Properties[HttpPropertyKeys.HttpConfigurationKey] = httpControllerDescriptor.Configuration;
}
}

// Create context
HttpControllerContext controllerContext = new HttpControllerContext(httpControllerDescriptor.Configuration, routeData, request);
controllerContext.Controller = httpController;
controllerContext.ControllerDescriptor = httpControllerDescriptor;

return httpController.ExecuteAsync(controllerContext, cancellationToken);
}

在这个方法里面主要是获取Controller，然后再执行它。

首先调用 IHttpRouteData routeData = request.GetRouteData();来获取路由信息，其主要实现代码如下：

public static IHttpRouteData GetRouteData(this HttpRequestMessage request)
{
if (request == null)
{
throw Error.ArgumentNull("request");
}

return request.GetProperty<IHttpRouteData>(HttpPropertyKeys.HttpRouteDataKey);
}

private static T GetProperty<T>(this HttpRequestMessage request, string key)
{
T value;
request.Properties.TryGetValue(key, out value);
return value;
}
public static bool TryGetValue<T>(this IDictionary<string, object> collection, string key, out T value)
{
if (collection == null)
{
throw Error.ArgumentNull("collection");
}

object valueObj;
if (collection.TryGetValue(key, out valueObj))
{
if (valueObj is T)
{
value = (T)valueObj;
return true;
}
}

value = default(T);
return false;
}

然后调用  HttpControllerDescriptor httpControllerDescriptor = ControllerSelector.SelectController(request);来获取HttpControllerDescriptor实例，在DefaultServices类中有如下代码： SetSingle<IHttpControllerSelector>(new DefaultHttpControllerSelector(configuration));

所以这里的ControllerSelector属性其实是一个DefaultHttpControllerSelector实例。那么我们来看看DefaultHttpControllerSelector的SelectController方法吧：

   string controllerName = GetControllerName(request);

    HttpControllerDescriptor controllerDescriptor;

    if (_controllerInfoCache.Value.TryGetValue(controllerName, out controllerDescriptor))

        {

            return controllerDescriptor;

        }

这里首先是获取Controller的名称，GetControllerName方法的实现如下：

 public virtual string GetControllerName(HttpRequestMessage request)

        {

            IHttpRouteData routeData = request.GetRouteData();

            string controllerName = null;

            routeData.Values.TryGetValue(ControllerKey, out controllerName);

            return controllerName;

        }

  private const string ControllerKey = "controller";

我们知道这里的routeData是HostedHttpRouteData实例，那么它的Values是什么了，HostedHttpRouteData的Values如下

   public IDictionary<string, object> Values

        {

            get { return OriginalRouteData.Values; }

        }

这里的OriginalRouteData是真正的路由信息。

现在我们来看看_controllerInfoCache这个东东是什么时候这是的了？在DefaultHttpControllerSelector的构造函数中有如下代码：

  public DefaultHttpControllerSelector(HttpConfiguration configuration)

        {

            if (configuration == null)

            {

                throw Error.ArgumentNull("configuration");

            }

            _controllerInfoCache = new Lazy<ConcurrentDictionary<string, HttpControllerDescriptor>>(InitializeControllerInfoCache);

            _configuration = configuration;

            _controllerTypeCache = new HttpControllerTypeCache(_configuration);

        }

这里我们还是先看看_controllerTypeCache的设置吧，直接实例化一个HttpControllerTypeCache，而HttpControllerTypeCache的构造函数如下：

  public HttpControllerTypeCache(HttpConfiguration configuration)

        {

            if (configuration == null)

            {

                throw Error.ArgumentNull("configuration");

            }

            _configuration = configuration;

            _cache = new Lazy<Dictionary<string, ILookup<string, Type>>>(InitializeCache);

        }

我们具体看看HttpControllerTypeCache的InitializeCache方法是如何实现的：

 private Dictionary<string, ILookup<string, Type>> InitializeCache()

        {

            IAssembliesResolver assembliesResolver = _configuration.Services.GetAssembliesResolver();

            IHttpControllerTypeResolver controllersResolver = _configuration.Services.GetHttpControllerTypeResolver();

            ICollection<Type> controllerTypes = controllersResolver.GetControllerTypes(assembliesResolver);

            var groupedByName = controllerTypes.GroupBy(

                t => t.Name.Substring(0, t.Name.Length - DefaultHttpControllerSelector.ControllerSuffix.Length),

                StringComparer.OrdinalIgnoreCase);

            return groupedByName.ToDictionary(

                g => g.Key,

                g => g.ToLookup(t => t.Namespace ?? String.Empty, StringComparer.OrdinalIgnoreCase),

                StringComparer.OrdinalIgnoreCase);

        }

从DefaultServices的构造函数我们知道这里的assembliesResolver
是一个DefaultAssembliesResolver实例，controllersResolver
是一个DefaultHttpControllerTypeResolver实例，但是在GlobalConfiguration有这么一句config.Services.Replace(typeof(IHttpControllerTypeResolver), new WebHostHttpControllerTypeResolver());。其中DefaultAssembliesResolver的实现如下

 public class DefaultAssembliesResolver : IAssembliesResolver

    {

        public virtual ICollection<Assembly> GetAssemblies()

        {

            return AppDomain.CurrentDomain.GetAssemblies().ToList();

        }

    }

}

它主要是获取所有的程序集，而DefaultHttpControllerTypeResolver的GetControllerTypes大致实现如下：

 public virtual ICollection<Type> GetControllerTypes(IAssembliesResolver assembliesResolver)

        {

            List<Type> result = new List<Type>();

            ICollection<Assembly> assemblies = assembliesResolver.GetAssemblies();

            foreach (Assembly assembly in assemblies)

            {

                Type[] exportedTypes = null;

                try

                {

                    exportedTypes = assembly.GetExportedTypes();

                }

                catch (ReflectionTypeLoadException ex)

                {

                    exportedTypes = ex.Types;

                }

                if (exportedTypes != null)

                {

                    result.AddRange(exportedTypes.Where(x => IsControllerTypePredicate(x)));

                }

            }

            return result;

        }

这里的IsControllerTypePredicate主要是检查我们的type是否是一个ControllerType，它必须是一个可见的非抽象类，并且它实现了IHttpController接口，且类名必须以Controller结尾。

所以HttpControllerTypeCache的InitializeCache方法返回了一个以Controller名称为key的字典。

现在轮到我们的WebHostHttpControllerTypeResolver了，

internal sealed class WebHostHttpControllerTypeResolver : DefaultHttpControllerTypeResolver

    {

        public override ICollection<Type> GetControllerTypes(IAssembliesResolver assembliesResolver)

        {

            HttpControllerTypeCacheSerializer serializer = new HttpControllerTypeCacheSerializer();

            // First, try reading from the cache on disk

            List<Type> matchingTypes = ReadTypesFromCache(TypeCacheName, IsControllerTypePredicate, serializer);

            if (matchingTypes != null)

            {

                return matchingTypes;

            }

            // If reading from the cache failed, enumerate over every assembly looking for a matching type

            matchingTypes = base.GetControllerTypes(assembliesResolver).ToList();

            // Finally, save the cache back to disk

            SaveTypesToCache(TypeCacheName, matchingTypes, serializer);

            return matchingTypes;

        }

    }

现在我们有回到DefaultHttpControllerSelector的构造方法中来，这里的_controllerTypeCache已经明白是怎么一回事了，我们该来看看DefaultHttpControllerSelector的InitializeControllerInfoCache是怎么实现的。

 private ConcurrentDictionary<string, HttpControllerDescriptor> InitializeControllerInfoCache()

        {

            var result = new ConcurrentDictionary<string, HttpControllerDescriptor>(StringComparer.OrdinalIgnoreCase);

            var duplicateControllers = new HashSet<string>();

            Dictionary<string, ILookup<string, Type>> controllerTypeGroups = _controllerTypeCache.Cache;

            foreach (KeyValuePair<string, ILookup<string, Type>> controllerTypeGroup in controllerTypeGroups)

            {

                string controllerName = controllerTypeGroup.Key;

                foreach (IGrouping<string, Type> controllerTypesGroupedByNs in controllerTypeGroup.Value)

                {

                    foreach (Type controllerType in controllerTypesGroupedByNs)

                    {

                        if (result.Keys.Contains(controllerName))

                        {

                            duplicateControllers.Add(controllerName);

                            break;

                        }

                        else

                        {

                            result.TryAdd(controllerName, new HttpControllerDescriptor(_configuration, controllerName, controllerType));

                        }

                    }

                }

            }

            foreach (string duplicateController in duplicateControllers)

            {

                HttpControllerDescriptor descriptor;

                result.TryRemove(duplicateController, out descriptor);

            }

            return result;
        }

这个方法就很简单了，我也不多说了，主要是创建HttpControllerDescriptor实例，在HttpControllerDescriptor构造函数中调有一个Initialize方法来做一些初始化的工作，而它也主要调用InvokeAttributesOnControllerType方法，具体实现：

 private static void InvokeAttributesOnControllerType(HttpControllerDescriptor controllerDescriptor, Type type)

        {

            InvokeAttributesOnControllerType(controllerDescriptor, type.BaseType);

            object[] attrs = type.GetCustomAttributes(inherit: false);

            foreach (object attr in attrs)

            {

                var controllerConfig = attr as IControllerConfiguration;

                if (controllerConfig != null)

                {

                    var originalConfig = controllerDescriptor.Configuration;

                    var controllerSettings = new HttpControllerSettings(originalConfig);

                    controllerConfig.Initialize(controllerSettings, controllerDescriptor);

                    controllerDescriptor.Configuration = HttpConfiguration.ApplyControllerSettings(controllerSettings, originalConfig);

                }

            }

        }

由于默认情况下我们没有IControllerConfiguration特性所以这里这个也就忽略吧。到这里DefaultHttpControllerSelector中SelectController的_controllerInfoCache.Value.TryGetValue(controllerName, out controllerDescriptor)也就很好明白了。

现在我们回到HttpControllerDispatcher中SendAsyncInternal方法来，这里有

 IHttpController httpController = httpControllerDescriptor.CreateController(request);，我们知道httpControllerDescriptor是一个HttpControllerDescriptor实例，它的CreateController方法如下：

  public class HttpControllerDescriptor

    {

     public virtual IHttpController CreateController(HttpRequestMessage request)

        {

            IHttpControllerActivator activator = Configuration.Services.GetHttpControllerActivator();

            IHttpController instance = activator.Create(request, this, ControllerType);

            return instance;

        }

    }

从DefaultServices中我们可以知道这里的activator 是DefaultHttpControllerActivator实例，这里的DefaultHttpControllerActivator.Create方法实现非常复杂，里面涉及到表达式树。

现在我们回到HttpControllerDispatcher的SendAsyncInternal方法中来，现在IHttpController实例我们已经创建好了，然后创建HttpControllerContext实例，

 HttpControllerContext controllerContext = new HttpControllerContext(httpControllerDescriptor.Configuration, routeData, request);

            controllerContext.Controller = httpController;

            controllerContext.ControllerDescriptor = httpControllerDescriptor;

最后调用IHttpController的ExecuteAsync方法，到这里我们才真正进入到controller的方法调用中来