.NET Core開發日誌——Action

在敘述Controller一文中，有一處未做解釋，即CreateControllerFactory方法中ControllerActionDescriptor參數是如何產生的。這是因為其與Action的關聯性更大，所以放在本文中繼續描述。

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();
    };

    ...
}

不難發現作為源頭的ActionContext中傳入了actionDescriptor，而這個參數的值是在ActionSelector中被篩選出來的。

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;
}

然後可供篩選的ActionDescriptors集合又是來自ActionDescriptorCollectionProvider類。

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;
    }
}

它的內部又再調用了ControllerActionDescriptorProvider類的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));
}

調用鏈繼續深入到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;
}

到了這裡終於可以看到Action的影子，雖然現在還只是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類的Build方法，可以得到預期的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包含了足以構建Controller與Action的屬性。

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的構建已經介紹過了，現在該談談關於Action的。

先找到創建ControllerActionInvokerCacheEntry對象的ControllerActionInvokerCache類的GetCachedResult方法。可以看到兩個關鍵參數objectMethodExecutor與actionMethodExecutor的創建方式。

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類的Next方法中跟蹤到State.ActionInside環節：

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

        goto case State.ActionEnd;
    }

終於可以找到創建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());
    }
    ...
}

核心的代碼是這一句actionMethodExecutor.Execute(objectMethodExecutor, controller, orderedArguments)。

actionMethodExecutor與objectMethodExecutor即是之前生成ControllerActionInvokerCacheEntry對象時傳入的兩個參數，controller是在State.ActionBegin環節通過_instance = _cacheEntry.ControllerFactory(controllerContext);生成的。orderedArguments是Action方法所需的參數。

至於更詳細的創建過程，可以到ActionMethodExecutor類與ObjectMethodExecutor類中探尋，主要是涉及反射相關的知識，這裡就不做進一步解釋了。