在叙述Controller一文中，有一处未做解释，即CreateControllerFactory方法中ControllerActionDescriptor参数是如何产生的。这是因为其与Action的关联性更大，所以放在本文中继续描述。
回到MvcRouteHandler或者MvcAttributeRouteHandler的方法中：
public Task RouteAsync(RouteContext context){
... var candidates = _actionSelector.SelectCandidates(context); if (candidates == null || candidates.Count == 0)
{
_logger.NoActionsMatched(context.RouteData.Values); return Task.CompletedTask;
} var actionDescriptor = _actionSelector.SelectBestCandidate(context, candidates); if (actionDescriptor == null)
{
_logger.NoActionsMatched(context.RouteData.Values); return Task.CompletedTask;
}
context.Handler = (c) =>
{ var routeData = c.GetRouteData(); var actionContext = new ActionContext(context.HttpContext, routeData, actionDescriptor); if (_actionContextAccessor != null)
{
_actionContextAccessor.ActionContext = actionContext;
} var invoker = _actionInvokerFactory.CreateInvoker(actionContext); if (invoker == null)
{ throw new InvalidOperationException(
Resources.FormatActionInvokerFactory_CouldNotCreateInvoker(
actionDescriptor.DisplayName));
} return invoker.InvokeAsync();
};
...
}不难发现作为源头的ActionContext中传入了actionDescriptor，而这个参数的值是在ActionSelector中被筛选出来的。
public IReadOnlyList<ActionDescriptor> SelectCandidates(RouteContext context){
... var cache = Current; // The Cache works based on a string[] of the route values in a pre-calculated order. This code extracts
// those values in the correct order.
var keys = cache.RouteKeys; var values = new string[keys.Length]; for (var i = 0; i < keys.Length; i++)
{
context.RouteData.Values.TryGetValue(keys[i], out object value); if (value != null)
{
values[i] = value as string ?? Convert.ToString(value);
}
}
if (cache.OrdinalEntries.TryGetValue(values, out var matchingRouteValues) ||
cache.OrdinalIgnoreCaseEntries.TryGetValue(values, out matchingRouteValues))
{
Debug.Assert(matchingRouteValues != null); return matchingRouteValues;
}
_logger.NoActionsMatched(context.RouteData.Values); return EmptyActions;
}然后可供筛选的ActionDescriptors集合又是来自ActionDescriptorCollectionProvider类。
private Cache Current
{ get
{ var actions = _actionDescriptorCollectionProvider.ActionDescriptors; var cache = Volatile.Read(ref _cache); if (cache != null && cache.Version == actions.Version)
{ return cache;
}
cache = new Cache(actions);
Volatile.Write(ref _cache, cache); return cache;
}
}它的内部又再调用了ControllerActionDescriptorProvider类的OnProvidersExecuting方法。
public ActionDescriptorCollection ActionDescriptors
{ get
{ if (_collection == null)
{
UpdateCollection();
} return _collection;
}
}private void UpdateCollection(){ var context = new ActionDescriptorProviderContext(); for (var i = 0; i < _actionDescriptorProviders.Length; i++)
{
_actionDescriptorProviders[i].OnProvidersExecuting(context);
} for (var i = _actionDescriptorProviders.Length - 1; i >= 0; i--)
{
_actionDescriptorProviders[i].OnProvidersExecuted(context);
}
_collection = new ActionDescriptorCollection( new ReadOnlyCollection<ActionDescriptor>(context.Results),
Interlocked.Increment(ref _version));
}调用链继续深入到DefaultApplicationModelProvider之中。
public void OnProvidersExecuting(ActionDescriptorProviderContext context){ if (context == null)
{ throw new ArgumentNullException(nameof(context));
} foreach (var descriptor in GetDescriptors()) {
context.Results.Add(descriptor);
}
}protected internal IEnumerable<ControllerActionDescriptor> GetDescriptors(){ var applicationModel = BuildModel();
ApplicationModelConventions.ApplyConventions(applicationModel, _conventions); return ControllerActionDescriptorBuilder.Build(applicationModel);
}protected internal ApplicationModel BuildModel(){ var controllerTypes = GetControllerTypes(); var context = new ApplicationModelProviderContext(controllerTypes); for (var i = 0; i < _applicationModelProviders.Length; i++)
{
_applicationModelProviders[i].OnProvidersExecuting(context);
} for (var i = _applicationModelProviders.Length - 1; i >= 0; i--)
{
_applicationModelProviders[i].OnProvidersExecuted(context);
} return context.Result;
}private IEnumerable<TypeInfo> GetControllerTypes(){ var feature = new ControllerFeature();
_partManager.PopulateFeature(feature); return feature.Controllers;
}到了这里终于可以看到Action的影子，虽然现在还只是ActionModel。
public virtual void OnProvidersExecuting(ApplicationModelProviderContext context){
... foreach (var controllerType in context.ControllerTypes)
{ var controllerModel = CreateControllerModel(controllerType); if (controllerModel == null)
{ continue;
}
context.Result.Controllers.Add(controllerModel);
controllerModel.Application = context.Result;
... foreach (var methodInfo in controllerType.AsType().GetMethods())
{ var actionModel = CreateActionModel(controllerType, methodInfo); if (actionModel == null)
{ continue;
}
actionModel.Controller = controllerModel;
controllerModel.Actions.Add(actionModel); foreach (var parameterInfo in actionModel.ActionMethod.GetParameters())
{ var parameterModel = CreateParameterModel(parameterInfo); if (parameterModel != null)
{
parameterModel.Action = actionModel;
actionModel.Parameters.Add(parameterModel);
}
}
}
}
}利用ControllerActionDescriptorBuilder类的Build方法，可以得到预期的ControllerActionDescriptor。
public static IList<ControllerActionDescriptor> Build(ApplicationModel application){ var actions = new List<ControllerActionDescriptor>(); var methodInfoMap = new MethodToActionMap(); var routeTemplateErrors = new List<string>(); var attributeRoutingConfigurationErrors = new Dictionary<MethodInfo, string>(); foreach (var controller in application.Controllers)
{ // Only add properties which are explicitly marked to bind.
// The attribute check is required for ModelBinder attribute.
var controllerPropertyDescriptors = controller.ControllerProperties
.Where(p => p.BindingInfo != null)
.Select(CreateParameterDescriptor)
.ToList(); foreach (var action in controller.Actions)
{ // Controllers with multiple [Route] attributes (or user defined implementation of
// IRouteTemplateProvider) will generate one action descriptor per IRouteTemplateProvider
// instance.
// Actions with multiple [Http*] attributes or other (IRouteTemplateProvider implementations
// have already been identified as different actions during action discovery.
var actionDescriptors = CreateActionDescriptors(application, controller, action); foreach (var actionDescriptor in actionDescriptors)
{
actionDescriptor.ControllerName = controller.ControllerName;
actionDescriptor.ControllerTypeInfo = controller.ControllerType;
AddApiExplorerInfo(actionDescriptor, application, controller, action);
AddRouteValues(actionDescriptor, controller, action);
AddProperties(actionDescriptor, action, controller, application);
actionDescriptor.BoundProperties = controllerPropertyDescriptors; if (IsAttributeRoutedAction(actionDescriptor))
{ // Replaces tokens like [controller]/[action] in the route template with the actual values
// for this action.
ReplaceAttributeRouteTokens(actionDescriptor, routeTemplateErrors);
}
}
methodInfoMap.AddToMethodInfo(action, actionDescriptors);
actions.AddRange(actionDescriptors);
}
}
... return actions;
}ControllerActionDescriptor包含了足以构建Controller与Action的属性。
public string ControllerName { get; set; }public virtual string ActionName { get; set; }public MethodInfo MethodInfo { get; set; }public TypeInfo ControllerTypeInfo { get; set; }public IList<ParameterDescriptor> Parameters { get; set; }Controller的构建已经介绍过了，现在该谈谈关于Action的。
先找到创建ControllerActionInvokerCacheEntry对象的ControllerActionInvokerCache类的GetCachedResult方法。可以看到两个关键参数objectMethodExecutor与actionMethodExecutor的创建方式。
public (ControllerActionInvokerCacheEntry cacheEntry, IFilterMetadata[] filters) GetCachedResult(ControllerContext controllerContext)
{ var cache = CurrentCache; var actionDescriptor = controllerContext.ActionDescriptor;
IFilterMetadata[] filters; if (!cache.Entries.TryGetValue(actionDescriptor, out var cacheEntry))
{
... var objectMethodExecutor = ObjectMethodExecutor.Create(
actionDescriptor.MethodInfo,
actionDescriptor.ControllerTypeInfo,
parameterDefaultValues);
... var actionMethodExecutor = ActionMethodExecutor.GetExecutor(objectMethodExecutor);
cacheEntry = new ControllerActionInvokerCacheEntry(
filterFactoryResult.CacheableFilters,
controllerFactory,
controllerReleaser,
propertyBinderFactory,
objectMethodExecutor,
actionMethodExecutor);
cacheEntry = cache.Entries.GetOrAdd(actionDescriptor, cacheEntry);
}
... return (cacheEntry, filters);
}再到ControllerActionInvoker类的Next方法中跟踪到State.ActionInside环节：
case State.ActionInside:
{ var task = InvokeActionMethodAsync(); if (task.Status != TaskStatus.RanToCompletion)
{
next = State.ActionEnd; return task;
} goto case State.ActionEnd;
}终于可以找到创建Action的方法。
private async Task InvokeActionMethodAsync(){ var controllerContext = _controllerContext; var objectMethodExecutor = _cacheEntry.ObjectMethodExecutor; var controller = _instance; var arguments = _arguments; var actionMethodExecutor = _cacheEntry.ActionMethodExecutor; var orderedArguments = PrepareArguments(arguments, objectMethodExecutor); var diagnosticSource = _diagnosticSource; var logger = _logger;
IActionResult result = null; try
{
diagnosticSource.BeforeActionMethod(
controllerContext,
arguments,
controller);
logger.ActionMethodExecuting(controllerContext, orderedArguments); var stopwatch = ValueStopwatch.StartNew(); var actionResultValueTask = actionMethodExecutor.Execute(objectMethodExecutor, controller, orderedArguments); if (actionResultValueTask.IsCompletedSuccessfully)
{
result = actionResultValueTask.Result;
} else
{
result = await actionResultValueTask;
}
_result = result;
logger.ActionMethodExecuted(controllerContext, result, stopwatch.GetElapsedTime());
}
...
}核心的代码是这一句actionMethodExecutor.Execute(objectMethodExecutor, controller, orderedArguments)。
actionMethodExecutor与objectMethodExecutor即是之前生成ControllerActionInvokerCacheEntry对象时传入的两个参数，controller是在State.ActionBegin环节通过_instance = _cacheEntry.ControllerFactory(controllerContext);生成的。orderedArguments是Action方法所需的参数。
至于更详细的创建过程，可以到ActionMethodExecutor类与ObjectMethodExecutor类中探寻，主要是涉及反射相关的知识，这里就不做进一步解释了。