[Web API] Web API 2 深入系列(3) 激活HttpController

目錄

1. HttpController

2. 創建HttpController

   • IAssembliesResolver
   • IHttpControllerTypeResolver
   • HttpControllerTypeCache
   • IHttpControllerSelector

3. ServicesContainer

從上節我們知道,在消息管道中,最終在HttpControllerDispatcher的SendAsync方法中會創建IHttpController.

HttpController

我們先看看IHttpController

public interface IHttpController
{
    Task<HttpResponseMessage> ExecuteAsync(HttpControllerContext controllerContext, CancellationToken cancellationToken);
}

IHttpController設計很像HttpMessageHandler,只不過參數由HttpRequestMessage變成了HttpControllerContext

public  public class HttpControllerContext
{
    HttpControllerContext(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor)
}

可以看出HttpControllerContext實際就是包裝了一下HttpRequestMessage和HttpControllerDescriptor

而HttpControllerDescriptor則是真正創建IHttpController的對象

public class HttpControllerDescriptor
{
    public virtual IHttpController CreateController(HttpRequestMessage request)
    {
    //通過內置的輕量級容器創建
      return this.Configuration.Services.GetHttpControllerActivator().Create(request, this, this.ControllerType);
    }
}

本節重點就是介紹HttpControllerDescriptor的CreateController方法

創建HttpController

在談創建HttpController前,我們需要了解HttpControllerDescriptor是如何被創建的
整個創建過程需要經歷以下步驟

IAssembliesResolver

首先通過IAssembliesResolver找出符合的程序集

默認的實現是找出當前應用程序域中的程序集

public class DefaultAssembliesResolver : IAssembliesResolver
{
    public virtual ICollection<Assembly> GetAssemblies()
    {
        return (ICollection<Assembly>) ((IEnumerable<Assembly>) AppDomain.CurrentDomain.GetAssemblies()).ToList<Assembly>();
    }
}

而在WebHost下,在HttpConfiguration的創建過程中替換為WebHostAssembliesResolver

internal sealed class WebHostAssembliesResolver : IAssembliesResolver
{
    ICollection<Assembly> IAssembliesResolver.GetAssemblies()
    {
        //所有引用的程序集
        return (ICollection<Assembly>) BuildManager.GetReferencedAssemblies().OfType<Assembly>().ToList<Assembly>();
    }
}

IHttpControllerTypeResolver

在列出可用的Assemblies后,會通過IHttpControllerTypeResolver找出所有的IHttpController
默認實現為DefaultHttpControllerTypeResolver

public class DefaultHttpControllerTypeResolver : IHttpControllerTypeResolver
{
    public virtual ICollection<Type> GetControllerTypes(IAssembliesResolver assembliesResolver)
    {
        foreach (Assembly assembly in (IEnumerable<Assembly>) assembliesResolver.GetAssemblies())
        {
            typeArray = assembly.GetTypes();
            return typeArray.Where<Type>(x => this.IsControllerTypePredicate(x));
        }
    }

    internal static bool IsControllerType(Type t)
    {
        //IsClass IsVisible !IsAbstract IHttpController
      if (t != (Type) null && t.IsClass && (t.IsVisible && !t.IsAbstract) && typeof (IHttpController).IsAssignableFrom(t))
        return DefaultHttpControllerTypeResolver.HasValidControllerName(t);
      return false;
    }
}

IHttpControllerSelector

當找到所有HttpController后,IHttpControllerSelector用來選擇能夠生成HttpControllerDescriptor(當同一個Controller名在不同的命名空間下,這時是WebAPI會拋棄這2個Controller)

默認實現為DefaultHttpControllerSelector

public class DefaultHttpControllerSelector : IHttpControllerSelector
{
    //根據請求 選擇對應的HttpControllerDescriptor
    public virtual HttpControllerDescriptor SelectController(HttpRequestMessage request)
    {
        string controllerName = this.GetControllerName(request);
        HttpControllerDescriptor controllerDescriptor;
        if (GetControllerDescriptor(controllerName, out controllerDescriptor))
            return controllerDescriptor;
        return null;
    }

    //從路由變量controller中獲取ControllerName
    public virtual string GetControllerName(HttpRequestMessage request)
    {
      string str;
      request.GetRouteData().Values.TryGetValue<string>("controller", out str);
      return str;
    }

    public HttpControllerDescriptor GetControllerDescriptor(string controllerName,out HttpControllerDescriptor controllerDescriptor)
    {
        //去除同名Controller
        var keys = ControllerTypeCache.Cache.Where(x=>x.Value.Count == 1).Select(x=>x.Key);
        foreach(var key in keys){
            //...
            return new HttpControllerDescriptor(...);
        }
    }
}

HttpControllerTypeCache

這里再補充一點:
在DefaultHttpControllerSelector的代碼中,我們使用ControllerTypeCache.Cache

實際上由於頻繁的反射創建Controller,WebAPI通過緩存Controller避免了不必要的損失.

同時在DefaultHttpControllerSelector代碼中也存在相應的緩存措施(文中的代碼為了便於理解).

internal sealed class HttpControllerTypeCache
{
    public HttpControllerTypeCache(HttpConfiguration configuration)
    {
        Cache = GetCache(configuration);//該處同樣為偽代碼
    }
    internal Dictionary<string, ILookup<string, Type>> Cache {get;}
}

通過上面的流程,我們清楚了HttpControllerDescriptor是如何創建的,同時創建完HttpControllerDescriptor后,調用CreateController即可創建 public virtual IHttpController CreateController(HttpRequestMessage request)

ServicesContainer

在Web API消息管道中,定義了很多接口,對應的也有很多實現類.

WebAPI自定義了一套DI容器ServicesContainer.而這個容器直接掛在HttpConfiguration上

public class HttpConfiguration : IDisposable
{
    public ServicesContainer Services { get; internal set; }
    public HttpConfiguration(HttpRouteCollection routes)
    {
        this.Services = (ServicesContainer) new DefaultServices(this);
    }
}

在默認的實現DefaultServices 則定義了WebAPI 默認對應的實現

public class DefaultServices : ServicesContainer
{
    private readonly Dictionary<Type, object> _defaultServicesSingle = new Dictionary<Type, object>();

    public DefaultServices(HttpConfiguration configuration)
    {
        //...
        this.SetSingle<IAssembliesResolver>((IAssembliesResolver) new DefaultAssembliesResolver());
        this.SetSingle<IHttpControllerActivator>((IHttpControllerActivator) new DefaultHttpControllerActivator());
        this.SetSingle<IHttpControllerSelector>((IHttpControllerSelector) new DefaultHttpControllerSelector(configuration));
        this.SetSingle<IHttpControllerTypeResolver>((IHttpControllerTypeResolver) new DefaultHttpControllerTypeResolver());
    }
    //設置接口實現
    private void SetSingle<T>(T instance) where T : class
    {
      this._defaultServicesSingle[typeof (T)] = (object) instance;
    }
    //獲取接口實現
    public override object GetService(Type serviceType)
    {
        return this._defaultServicesSingle[serviceType];
    }
}

備注:

• 文章中的代碼並非完整WebAPI代碼,一般是經過自己精簡后的.

• 本篇內容使用MarkDown語法編輯

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