[Web API] Web API 2 深入系列(4) Action的选择

目录

1. ApiController

2. HttpActionDescriptor

3. IHttpActionSelector

ApiController

在上节中,讲到如何选择并激活对应的IHttpController,而一般我们在开发中使用的是ApiController

public abstract class ApiController : IHttpController, IDisposable
{
    public virtual Task<HttpResponseMessage> ExecuteAsync(HttpControllerContext controllerContext, CancellationToken cancellationToken)
    {
        this.Initialize(controllerContext);
        HttpActionDescriptor actionDescriptor = services.GetActionSelector().SelectAction(controllerContext);
        HttpActionContext actionContext = new HttpActionContext(controllerContext,actionDescriptor);
        return services.GetActionInvoker().InvokeActionAsync(actionContext, cancellationToken);
    }
}

在ApiController中,我们看到通过内置的DI容器选择出对应的HttpActionDescriptor.本节重点内容就是介绍SelectAction方法.

HttpActionDescriptor

在介绍IHttpActionSelector前,我们需要了解HttpActionDescriptor

public abstract class HttpActionDescriptor
{
    //相关联的HttpControllerDescriptor
    public HttpControllerDescriptor ControllerDescriptor { get; } { set; }
    //Action名称(通过使用ActionName特性,Action名称可以和方法名称不同)
    public abstract string ActionName { get; }
    //Action返回值类型(void为null)
    public abstract Type ReturnType { get; }
    //支持的HttpMethod(默认一个Action方法支持一种HttpMethod,且默认为Post Method,可以使用AcceptVerbs特性支持多个)
    public virtual Collection<HttpMethod> SupportedHttpMethods { get; }
    //Action方法所有参数
    public abstract Collection<HttpParameterDescriptor> Parameters { get; };
}

而HttpActionDescriptor默认实现为ReflectedHttpActionDescriptor,这里稍微展示下初始化HttpActionDescriptor过程

public class ReflectedHttpActionDescriptor : HttpActionDescriptor
{
    //初始化HttpActionDescriptor的属性
    public ReflectedHttpActionDescriptor(HttpControllerDescriptor controllerDescriptor, MethodInfo methodInfo)
      : base(controllerDescriptor)
    {
      this.InitializeProperties(methodInfo);
      this._parameters = new Lazy<Collection<HttpParameterDescriptor>>(() => this.InitializeParameterDescriptors());
    }    private Collection<HttpParameterDescriptor> InitializeParameterDescriptors()
    {
        return this.ParameterInfos.Select(item => new ReflectedHttpParameterDescriptor((HttpActionDescriptor) this, item)).ToList();
    }
}

IHttpActionSelector

IHttpActionSelector是选择Action最核心的类

public interface IHttpActionSelector
{
    //选择符合标准的Action
    HttpActionDescriptor SelectAction(HttpControllerContext controllerContext);    //获取HttpController所有Action
    ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor);
}

IHttpActionSelector默认的实现为ApiControllerActionSelector

public class ApiControllerActionSelector : IHttpActionSelector
{
    ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor);
    {
        //找到所有符合要求的方法
        var methods = controllerDescriptor.ControllerType
        .GetMethods(BindingFlags.Instance | BindingFlags.Public), methodInfo =>
        !methodInfo.IsSpecialName && !methodInfo.GetBaseDefinition().DeclaringType.IsAssignableFrom(TypeHelper.ApiControllerType)
        && methodInfo.GetCustomAttribute<NonActionAttribute>() == null);
        return methods.Select(method => new HttpActionDescriptor(controllerDescriptor,method))...;
    }
}

注意:GetActionMapping会在请求每个HttpController第一次的时候 缓存当前所有HttpActionDescriptor

当我们找到HttpController下所有HttpActionDescriptor,还需要最后一步通过SelectAction筛选出最终的Action

由于这块源码稍微复杂,这里把关键的几步及对应的方法名说明下

public virtual HttpActionDescriptor SelectAction(HttpControllerContext controllerContext)
{
    1. Action名称过滤    如果路由中配置Action变量,则会先过滤ActionName.调用方法GetInitialCandidateList    2. Http方法过滤    对于Get Put Post默认都通过,调用方法FindActionsForVerb    3. 必须的参数能绑定上    在Action方法上的必须参数,在路由参数和Request参数中能获取到,调用方法FindActionMatchRequiredRouteAndQueryParameters    4. 参数个数符合最多的匹配    当多个Action方法都满足以上3个条件时,取最多参数符合的那个,调用方法FindActionMatchMostRouteAndQueryParameters    5. 取唯一匹配
    最终匹配结果为1个Action方法则成功,多个或零失败,在SelectAction方法本身调用
}

这里稍微举个例子来解释SelectAction

public class DemoController : ApiController
{
    public string Get(int x)    public string Get(int x, int y)    public string Get(string x, string y)    public string Get(string x, string y, string z)
}

分别请求如下地址,并列出对应的匹配方法

• demo?x=1 OK

  public string Get(int x)

• demo?x=1&y=2 Erro

  public string Get(int x, int y)

  public string Get(string x, string y)

• demo?x=1&y=2&z=3 OK

  public string Get(string x, string y, string z)

(OK表示请求成功,Erro表示请求失败)

备注:

– Action的选择明显比Controller的激活要复杂

- 文章中的代码并非完整WebAPI代码,一般是经过自己精简后的.- 本篇内容使用MarkDown语法编辑

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