在理清路由的工作流程后，接下来需要考虑的，是MVC框架如何生成Controller以及它的生成时机。
根据以前ASP.NET MVC的经验，Controller应该是由一个ControllerFactory构建的。查看ASP.NET Core MVC的源码，果然是有一个DefaultControllerFactory类，并且不出意外的，它拥有一个CreateController方法。
public virtual object CreateController(ControllerContext context)
{
...
var controller = _controllerActivator.Create(context);
foreach (var propertyActivator in _propertyActivators)
{
propertyActivator.Activate(context, controller);
}
return controller;
}但细推其使用的场合，只出现在ControllerFactoryProvider的构造方法内部，且仅是用于判断所传入的controllerFactory类型是否是DefaultControllerFactory。
public ControllerFactoryProvider(
IControllerActivatorProvider activatorProvider,
IControllerFactory controllerFactory,
IEnumerable<IControllerPropertyActivator> propertyActivators)
{
...
_activatorProvider = activatorProvider;
// Compat: Delegate to the IControllerFactory if it's not the default implementation.
if (controllerFactory.GetType() != typeof(DefaultControllerFactory))
{
_factoryCreateController = controllerFactory.CreateController;
_factoryReleaseController = controllerFactory.ReleaseController;
}
_propertyActivators = propertyActivators.ToArray();
}再看ControllerFactoryProvider内部的CreateControllerFactory方法。这更像是一个真正创建Controller的工厂方法。
public Func<ControllerContext, object> CreateControllerFactory(ControllerActionDescriptor descriptor)
{
...
if (_factoryCreateController != null)
{
return _factoryCreateController;
}
var controllerActivator = _activatorProvider.CreateActivator(descriptor);
var propertyActivators = GetPropertiesToActivate(descriptor);
object CreateController(ControllerContext controllerContext)
{
var controller = controllerActivator(controllerContext);
for (var i = 0; i < propertyActivators.Length; i++)
{
var propertyActivator = propertyActivators[i];
propertyActivator(controllerContext, controller);
}
return controller;
}
return CreateController;
}创建方式分为两种，一种是使用自定义的工厂方法，另一种是通过ControllerActivatorProvider的CreateActivator方法。
public Func<ControllerContext, object> CreateActivator(ControllerActionDescriptor descriptor)
{
...
var controllerType = descriptor.ControllerTypeInfo?.AsType();
...
if (_controllerActivatorCreate != null)
{
return _controllerActivatorCreate;
}
var typeActivator = ActivatorUtilities.CreateFactory(controllerType, Type.EmptyTypes);
return controllerContext => typeActivator(controllerContext.HttpContext.RequestServices, arguments: null);
}明白了如何创建Controller，下面开始调查创建Controller的时机。
ControllerFactoryProvider类的CreateControllerFactory方法是被ControllerActionInvokerCache类的GetCachedResult方法调用。
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 filterFactoryResult = FilterFactory.GetAllFilters(_filterProviders, controllerContext);
filters = filterFactoryResult.Filters;
var parameterDefaultValues = ParameterDefaultValues
.GetParameterDefaultValues(actionDescriptor.MethodInfo);
var objectMethodExecutor = ObjectMethodExecutor.Create(
actionDescriptor.MethodInfo,
actionDescriptor.ControllerTypeInfo,
parameterDefaultValues);
var controllerFactory = _controllerFactoryProvider.CreateControllerFactory(actionDescriptor);
var controllerReleaser = _controllerFactoryProvider.CreateControllerReleaser(actionDescriptor);
var propertyBinderFactory = ControllerBinderDelegateProvider.CreateBinderDelegate(
_parameterBinder,
_modelBinderFactory,
_modelMetadataProvider,
actionDescriptor);
var actionMethodExecutor = ActionMethodExecutor.GetExecutor(objectMethodExecutor);
cacheEntry = new ControllerActionInvokerCacheEntry(
filterFactoryResult.CacheableFilters,
controllerFactory,
controllerReleaser,
propertyBinderFactory,
objectMethodExecutor,
actionMethodExecutor);
cacheEntry = cache.Entries.GetOrAdd(actionDescriptor, cacheEntry);
}
else
{
// Filter instances from statically defined filter descriptors + from filter providers
filters = FilterFactory.CreateUncachedFilters(_filterProviders, controllerContext, cacheEntry.CachedFilters);
}
return (cacheEntry, filters);
}其值作为ControllerActionInvokerCacheEntry对象的一部分被方法返回。
GetCachedResult方法的上层调用者是ControllerActionInvokerProvider类的OnProvidersExecuting方法。
public void OnProvidersExecuting(ActionInvokerProviderContext context)
{
...
if (context.ActionContext.ActionDescriptor is ControllerActionDescriptor)
{
var controllerContext = new ControllerContext(context.ActionContext);
// PERF: These are rarely going to be changed, so let's go copy-on-write.
controllerContext.ValueProviderFactories = new CopyOnWriteList<IValueProviderFactory>(_valueProviderFactories);
controllerContext.ModelState.MaxAllowedErrors = _maxModelValidationErrors;
var cacheResult = _controllerActionInvokerCache.GetCachedResult(controllerContext);
var invoker = new ControllerActionInvoker(
_logger,
_diagnosticSource,
controllerContext,
cacheResult.cacheEntry,
cacheResult.filters);
context.Result = invoker;
}
}ControllerActionInvokerCacheEntry对象又被作为ControllerActionInvoker对象的一部分为ActionInvokerProviderContext的Result属性赋值。
再往上跟踪，到了ActionInvokerFactory类的CreateInvoker方法。
public IActionInvoker CreateInvoker(ActionContext actionContext)
{
var context = new ActionInvokerProviderContext(actionContext);
foreach (var provider in _actionInvokerProviders)
{
provider.OnProvidersExecuting(context);
}
for (var i = _actionInvokerProviders.Length - 1; i >= 0; i--)
{
_actionInvokerProviders[i].OnProvidersExecuted(context);
}
return context.Result;
}而它的调用者便是MvcRouteHandler或者MvcAttributeRouteHandler。
public Task RouteAsync(RouteContext context)
{
...
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();
};
...
}到了这里创建Controller的工厂方法还没有被实际调用，此时Controller还是不存在的。所以还需要完成执行ControllerActionInvoker的InvokeAsync方法，或者更准确地说是其基类ResourceInvoker的InvokeAsync方法。
public virtual async Task InvokeAsync()
{
try
{
...
using (_logger.ActionScope(_actionContext.ActionDescriptor))
{
...
try
{
await InvokeFilterPipelineAsync();
}
...
}
}
...
}从InvokeFilterPipelineAsync方法开始，一系列的处理流程将依据不同状态逐步展开。
private async Task InvokeFilterPipelineAsync()
{
var next = State.InvokeBegin;
var scope = Scope.Invoker;
var state = (object)null;
var isCompleted = false;
while (!isCompleted)
{
await Next(ref next, ref scope, ref state, ref isCompleted);
}
}而到了State.ActionBegin这一步(ControllerActionInvoker类的Next方法)，终于能找到Controller工厂方法被执行的场合。
private Task Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
{
switch (next)
{
case State.ActionBegin:
{
var controllerContext = _controllerContext;
_cursor.Reset();
_instance = _cacheEntry.ControllerFactory(controllerContext);
_arguments = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
var task = BindArgumentsAsync();
if (task.Status != TaskStatus.RanToCompletion)
{
next = State.ActionNext;
return task;
}
goto case State.ActionNext;
}
...
}
}
} 最后以一张流程图总结上面的探寻过程。
