[Web API] Web API 2 深入系列(4) Action的選擇

目錄

1. ApiController

2. HttpActionDescriptor

3. IHttpActionSelector

ApiController

在上節中,講到如何選擇並激活對應的IHttpController,而一般我們在開發中使用的是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,我們看到通過內置的DI容器選擇出對應的HttpActionDescriptor.本節重點內容就是介紹SelectAction方法.

HttpActionDescriptor

在介紹IHttpActionSelector前,我們需要瞭解HttpActionDescriptor

public abstract class HttpActionDescriptor
{
    //相關聯的HttpControllerDescriptor
    public HttpControllerDescriptor ControllerDescriptor { get; } { set; }
    //Action名稱(通過使用ActionName特性,Action名稱可以和方法名稱不同)
    public abstract string ActionName { get; }
    //Action返回值類型(void為null)
    public abstract Type ReturnType { get; }
    //支持的HttpMethod(預設一個Action方法支持一種HttpMethod,且預設為Post Method,可以使用AcceptVerbs特性支持多個)
    public virtual Collection<HttpMethod> SupportedHttpMethods { get; }
    //Action方法所有參數
    public abstract Collection<HttpParameterDescriptor> Parameters { get; };
}

而HttpActionDescriptor預設實現為ReflectedHttpActionDescriptor,這裡稍微展示下初始化HttpActionDescriptor過程

public class ReflectedHttpActionDescriptor : HttpActionDescriptor
{
    //初始化HttpActionDescriptor的屬性
    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是選擇Action最核心的類

public interface IHttpActionSelector
{
    //選擇符合標準的Action
    HttpActionDescriptor SelectAction(HttpControllerContext controllerContext);

    //獲取HttpController所有Action
    ILookup<string, HttpActionDescriptor> GetActionMapping(HttpControllerDescriptor controllerDescriptor);
}

IHttpActionSelector預設的實現為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))...;
    }
}

註意:GetActionMapping會在請求每個HttpController第一次的時候 緩存當前所有HttpActionDescriptor

當我們找到HttpController下所有HttpActionDescriptor,還需要最後一步通過SelectAction篩選出最終的Action

由於這塊源碼稍微複雜,這裡把關鍵的幾步及對應的方法名說明下

public virtual HttpActionDescriptor SelectAction(HttpControllerContext controllerContext)
{
    1. Action名稱過濾

    如果路由中配置Action變數,則會先過濾ActionName.調用方法GetInitialCandidateList

    2. Http方法過濾

    對於Get Put Post預設都通過,調用方法FindActionsForVerb

    3. 必須的參數能綁定上

    在Action方法上的必須參數,在路由參數和Request參數中能獲取到,調用方法FindActionMatchRequiredRouteAndQueryParameters

    4. 參數個數符合最多的匹配

    當多個Action方法都滿足以上3個條件時,取最多參數符合的那個,調用方法FindActionMatchMostRouteAndQueryParameters

    5. 取唯一匹配
    最終匹配結果為1個Action方法則成功,多個或零失敗,在SelectAction方法本身調用
}

這裡稍微舉個例子來解釋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)
}

分別請求如下地址,併列出對應的匹配方法

• 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表示請求成功,Erro表示請求失敗)

備註:
- Action的選擇明顯比Controller的激活要複雜

- 文章中的代碼並非完整WebAPI代碼,一般是經過自己精簡後的.

- 本篇內容使用MarkDown語法編輯

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