ASP.NET Core学习总结(3)

　　我们重点来看看这个InvokeInnerFilterAsync。

 protected override async Task InvokeInnerFilterAsync()
        {
            var next = State.ActionBegin;
            var scope = Scope.Invoker;
            var state = (object)null;
            var isCompleted = false;

            while (!isCompleted)
            {
                await Next(ref next, ref scope, ref state, ref isCompleted);
            }
        }

　　似曾相识，它与ResourceInvoker的InvokeFilterPipelineAsync几乎是一模一样的。并且也有一个Next方法，里面也是一个大的switch语句，然后是很多case。

 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;
                    }

                case State.ActionNext:
                    {
                        var current = _cursor.GetNextFilter<IActionFilter, IAsyncActionFilter>();
                        if (current.FilterAsync != null)
                        {
                            if (_actionExecutingContext == null)
                            {
                                _actionExecutingContext = new ActionExecutingContext(_controllerContext, _filters, _arguments, _instance);
                            }

                            state = current.FilterAsync;
                            goto case State.ActionAsyncBegin;
                        }
                        else if (current.Filter != null)
                        {
                            if (_actionExecutingContext == null)
                            {
                                _actionExecutingContext = new ActionExecutingContext(_controllerContext, _filters, _arguments, _instance);
                            }

                            state = current.Filter;
                            goto case State.ActionSyncBegin;
                        }
                        else
                        {
                            goto case State.ActionInside;
                        }
                    }

                case State.ActionAsyncBegin:
                    {
                        Debug.Assert(state != null);
                        Debug.Assert(_actionExecutingContext != null);

                        var filter = (IAsyncActionFilter)state;
                        var actionExecutingContext = _actionExecutingContext;

                        _diagnosticSource.BeforeOnActionExecution(actionExecutingContext, filter);

                        var task = filter.OnActionExecutionAsync(actionExecutingContext, InvokeNextActionFilterAwaitedAsync);
                        if (task.Status != TaskStatus.RanToCompletion)
                        {
                            next = State.ActionAsyncEnd;
                            return task;
                        }

                        goto case State.ActionAsyncEnd;
                    }

                case State.ActionAsyncEnd:
                    {
                        Debug.Assert(state != null);
                        Debug.Assert(_actionExecutingContext != null);

                        var filter = (IAsyncActionFilter)state;

                        if (_actionExecutedContext == null)
                        {
                            // If we get here then the filter didn't call 'next' indicating a short circuit.
                            _logger.ActionFilterShortCircuited(filter);

                            _actionExecutedContext = new ActionExecutedContext(
                                _controllerContext,
                                _filters,
                                _instance)
                            {
                                Canceled = true,
                                Result = _actionExecutingContext.Result,
                            };
                        }

                        _diagnosticSource.AfterOnActionExecution(_actionExecutedContext, filter);

                        goto case State.ActionEnd;
                    }

                case State.ActionSyncBegin:
                    {
                        Debug.Assert(state != null);
                        Debug.Assert(_actionExecutingContext != null);

                        var filter = (IActionFilter)state;
                        var actionExecutingContext = _actionExecutingContext;

                        _diagnosticSource.BeforeOnActionExecuting(actionExecutingContext, filter);

                        filter.OnActionExecuting(actionExecutingContext);

                        _diagnosticSource.AfterOnActionExecuting(actionExecutingContext, filter);

                        if (actionExecutingContext.Result != null)
                        {
                            // Short-circuited by setting a result.
                            _logger.ActionFilterShortCircuited(filter);

                            _actionExecutedContext = new ActionExecutedContext(
                                _actionExecutingContext,
                                _filters,
                                _instance)
                            {
                                Canceled = true,
                                Result = _actionExecutingContext.Result,
                            };

                            goto case State.ActionEnd;
                        }

                        var task = InvokeNextActionFilterAsync();
                        if (task.Status != TaskStatus.RanToCompletion)
                        {
                            next = State.ActionSyncEnd;
                            return task;
                        }

                        goto case State.ActionSyncEnd;
                    }

                case State.ActionSyncEnd:
                    {
                        Debug.Assert(state != null);
                        Debug.Assert(_actionExecutingContext != null);
                        Debug.Assert(_actionExecutedContext != null);

                        var filter = (IActionFilter)state;
                        var actionExecutedContext = _actionExecutedContext;

                        _diagnosticSource.BeforeOnActionExecuted(actionExecutedContext, filter);

                        filter.OnActionExecuted(actionExecutedContext);

                        _diagnosticSource.AfterOnActionExecuted(actionExecutedContext, filter);

                        goto case State.ActionEnd;
                    }

　　如果我们查看之前的部分，就会发现。在ResourceInvoker中的动作过滤器部分并没有真正的执行，而是调用了抽象方法InvokeInnerFilterAsync。

    case State.ActionBegin:
                        {
                            var task = InvokeInnerFilterAsync();
                            if (task.Status != TaskStatus.RanToCompletion)
                            {
                                next = State.ActionEnd;
                                return task;
                            }
    
                            goto case State.ActionEnd;
                        }
    
                    case State.ActionEnd:
                        {
                            if (scope == Scope.Exception)
                            {
                                // If we're inside an exception filter, let's allow those filters to 'unwind' before
                                // the result.
                                isCompleted = true;
                                return Task.CompletedTask;
                            }
    
                            Debug.Assert(scope == Scope.Invoker || scope == Scope.Resource);
                            goto case State.ResultBegin;
                        }

　　也就是说，所有的ActionFilter终归到底是在ControlerActionInvoker中执行的。接着我们来看Next方法的后面部分。

 case State.ActionInside:
                    {
                          //关键在这里
                        var task = InvokeActionMethodAsync();
                        if (task.Status != TaskStatus.RanToCompletion)
                        {
                            next = State.ActionEnd;
                            return task;
                        }

                        goto case State.ActionEnd;
                    }

                case State.ActionEnd:
                    {
                        if (scope == Scope.Action)
                        {
                            if (_actionExecutedContext == null)
                            {
                                _actionExecutedContext = new ActionExecutedContext(_controllerContext, _filters, _instance)
                                {
                                    Result = _result,
                                };
                            }

                            isCompleted = true;
                            return Task.CompletedTask;
                        }

                        var actionExecutedContext = _actionExecutedContext;
                        Rethrow(actionExecutedContext);

                        if (actionExecutedContext != null)
                        {
                            _result = actionExecutedContext.Result;
                        }

                        isCompleted = true;
                        return Task.CompletedTask;
                    }

                default:
                    throw new InvalidOperationException();

　　关键应该是那个InvokeActionMethodAsync方法。

 private async Task InvokeActionMethodAsync()
        {
            var controllerContext = _controllerContext;
            var executor = _cacheEntry.ActionMethodExecutor;
            var controller = _instance;
            var arguments = _arguments;
            var orderedArguments = PrepareArguments(arguments, executor);

            var diagnosticSource = _diagnosticSource;
            var logger = _logger;

            IActionResult result = null;
            try
            {
                diagnosticSource.BeforeActionMethod(
                    controllerContext,
                    arguments,
                    controller);
                logger.ActionMethodExecuting(controllerContext, orderedArguments);

                  //关键从这开始
                var returnType = executor.MethodReturnType;
                if (returnType == typeof(void))
                {
                    // Sync method returning void
                    executor.Execute(controller, orderedArguments);
                    result = new EmptyResult();
                }
                else if (returnType == typeof(Task))
                {
                    // Async method returning Task
                    // Avoid extra allocations by calling Execute rather than ExecuteAsync and casting to Task.
                    await (Task)executor.Execute(controller, orderedArguments);
                    result = new EmptyResult();
                }
                else if (returnType == typeof(Task<IActionResult>))
                {
                    // Async method returning Task<IActionResult>
                    // Avoid extra allocations by calling Execute rather than ExecuteAsync and casting to Task<IActionResult>.
                    result = await (Task<IActionResult>)executor.Execute(controller, orderedArguments);
                    if (result == null)
                    {
                        throw new InvalidOperationException(
                            Resources.FormatActionResult_ActionReturnValueCannotBeNull(typeof(IActionResult)));
                    }
                }
                else if (IsResultIActionResult(executor))
                {
                    if (executor.IsMethodAsync)
                    {
                        // Async method returning awaitable-of-IActionResult (e.g., Task<ViewResult>)
                        // We have to use ExecuteAsync because we don't know the awaitable's type at compile time.
                        result = (IActionResult)await executor.ExecuteAsync(controller, orderedArguments);
                    }
                    else
                    {
                        // Sync method returning IActionResult (e.g., ViewResult)
                        result = (IActionResult)executor.Execute(controller, orderedArguments);
                    }

                    if (result == null)
                    {
                        throw new InvalidOperationException(
                            Resources.FormatActionResult_ActionReturnValueCannotBeNull(executor.AsyncResultType ?? returnType));
                    }
                }
                else if (!executor.IsMethodAsync)
                {
                    // Sync method returning arbitrary object
                    var resultAsObject = executor.Execute(controller, orderedArguments);
                    result = resultAsObject as IActionResult ?? new ObjectResult(resultAsObject)
                    {
                        DeclaredType = returnType,
                    };
                }
                else if (executor.AsyncResultType == typeof(void))
                {
                    // Async method returning awaitable-of-void
                    await executor.ExecuteAsync(controller, orderedArguments);
                    result = new EmptyResult();
                }
                else
                {
                    // Async method returning awaitable-of-nonvoid
                    var resultAsObject = await executor.ExecuteAsync(controller, orderedArguments);
                    result = resultAsObject as IActionResult ?? new ObjectResult(resultAsObject)
                    {
                        DeclaredType = executor.AsyncResultType,
                    };
                }

                _result = result;
                logger.ActionMethodExecuted(controllerContext, result);
            }
            finally
            {
                diagnosticSource.AfterActionMethod(
                    controllerContext,
                    arguments,
                    controllerContext,
                    result);
            }
        }

　　上面的方法在于不断判断returnType的类型，根据不同的类型执行不同的操作。不难发现，这些returnType正是我们所写的Action的返回类型。换句话说，executor.Execute执行的正是我们的Action方法。那么，executor又是什么呢？它是一个ObjectMethodExecutor类型的变量。从它构造函数可以看出，它实质上是对一个方法的包装。

private ObjectMethodExecutor(MethodInfo methodInfo, TypeInfo targetTypeInfo, object[] parameterDefaultValues)
        {
            if (methodInfo == null)
            {
                throw new ArgumentNullException(nameof(methodInfo));
            }

            MethodInfo = methodInfo;
            MethodParameters = methodInfo.GetParameters();
            TargetTypeInfo = targetTypeInfo;
            MethodReturnType = methodInfo.ReturnType;

            var isAwaitable = CoercedAwaitableInfo.IsTypeAwaitable(MethodReturnType, out var coercedAwaitableInfo);

            IsMethodAsync = isAwaitable;
            AsyncResultType = isAwaitable ? coercedAwaitableInfo.AwaitableInfo.ResultType : null;

            // Upstream code may prefer to use the sync-executor even for async methods, because if it knows
            // that the result is a specific Task<T> where T is known, then it can directly cast to that type
            // and await it without the extra heap allocations involved in the _executorAsync code path.
              //看这里
            _executor = GetExecutor(methodInfo, targetTypeInfo);

            if (IsMethodAsync)
            {
                _executorAsync = GetExecutorAsync(methodInfo, targetTypeInfo, coercedAwaitableInfo);
            }

            _parameterDefaultValues = parameterDefaultValues;
        }

public object Execute(object target, object[] parameters)
        {
            return _executor(target, parameters);
        }

　　Execute方法也只是委托_executor去执行了。而_executor又是在构造函数中赋值的，下面是GetExecutor方法。

private static MethodExecutor GetExecutor(MethodInfo methodInfo, TypeInfo targetTypeInfo)
        {
            // Parameters to executor
            var targetParameter = Expression.Parameter(typeof(object), "target");
            var parametersParameter = Expression.Parameter(typeof(object[]), "parameters");

            // Build parameter list
            var parameters = new List<Expression>();
            var paramInfos = methodInfo.GetParameters();
            for (int i = 0; i < paramInfos.Length; i++)
            {
                var paramInfo = paramInfos[i];
                var valueObj = Expression.ArrayIndex(parametersParameter, Expression.Constant(i));
                var valueCast = Expression.Convert(valueObj, paramInfo.ParameterType);

                // valueCast is "(Ti) parameters[i]"
                parameters.Add(valueCast);
            }

            // Call method
            var instanceCast = Expression.Convert(targetParameter, targetTypeInfo.AsType());
            var methodCall = Expression.Call(instanceCast, methodInfo, parameters);

            // methodCall is "((Ttarget) target) method((T0) parameters[0], (T1) parameters[1], ...)"
            // Create function
            if (methodCall.Type == typeof(void))
            {
                var lambda = Expression.Lambda<VoidMethodExecutor>(methodCall, targetParameter, parametersParameter);
                var voidExecutor = lambda.Compile();
                return WrapVoidMethod(voidExecutor);
            }
            else
            {
                // must coerce methodCall to match ActionExecutor signature
                var castMethodCall = Expression.Convert(methodCall, typeof(object));
                var lambda = Expression.Lambda<MethodExecutor>(castMethodCall, targetParameter, parametersParameter);
                return lambda.Compile();
            }
        }

　　可以看到，上面利用表达式类编译成了委托，并且区分有无返回值的情况。至此，我们已经过了整个MVC框架的流程。现在，让我们再重新梳理一遍。

​ 　　Internet=>Application=>Middleware=>IRourer=>MvcRouterHandler=>ControllerActionInvoker=》ActionFilter

​ 　　网络和应用程序之间，通过HTTP协议交互信息。而在应用程序内部，又有由一系列中间件编译成的委托链。然后是整个MVC的入口点，即路由中间件。其中使用了IRouter的RouteAsync方法匹配路由。而在IRouter的默认实现MvcRouterHandler中又调用了IActionInvoker的InvokeAsync方法。IActionInvoker的默认实现ControllerActionInvoker又继承了ResourceInvoker。在ResourceInvoker中，执行了整个过滤器管道的流程。而动作过滤器和真正的动作方法则是在ControllerActionInvoker中执行的。