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

目录

1. HttpController

2. 创建HttpController

   • IAssembliesResolver
   • IHttpControllerTypeResolver
   • HttpControllerTypeCache
   • IHttpControllerSelector

3. ServicesContainer

从上节我们知道,在消息管道中,最终在HttpControllerDispatcher的SendAsync方法中会创建IHttpController.

HttpController

我们先看看IHttpController

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

IHttpController设计很像HttpMessageHandler,只不过参数由HttpRequestMessage变成了HttpControllerContext

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

可以看出HttpControllerContext实际就是包装了一下HttpRequestMessage和HttpControllerDescriptor

而HttpControllerDescriptor则是真正创建IHttpController的对象

public class HttpControllerDescriptor
{
    public virtual IHttpController CreateController(HttpRequestMessage request)
    {
    //通过内置的轻量级容器创建
      return this.Configuration.Services.GetHttpControllerActivator().Create(request, this, this.ControllerType);
    }
}

本节重点就是介绍HttpControllerDescriptor的CreateController方法

创建HttpController

在谈创建HttpController前,我们需要了解HttpControllerDescriptor是如何被创建的
整个创建过程需要经历以下步骤

IAssembliesResolver

首先通过IAssembliesResolver找出符合的程序集

默认的实现是找出当前应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的创建过程中替换为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,会通过IHttpControllerTypeResolver找出所有的IHttpController
默认实现为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

当找到所有HttpController后,IHttpControllerSelector用来选择能够生成HttpControllerDescriptor(当同一个Controller名在不同的命名空间下,这时是WebAPI会抛弃这2个Controller)

默认实现为DefaultHttpControllerSelector

public class DefaultHttpControllerSelector : IHttpControllerSelector
{
    //根据请求 选择对应的HttpControllerDescriptor
    public virtual HttpControllerDescriptor SelectController(HttpRequestMessage request)
    {
        string controllerName = this.GetControllerName(request);
        HttpControllerDescriptor controllerDescriptor;
        if (GetControllerDescriptor(controllerName, out controllerDescriptor))
            return controllerDescriptor;
        return null;
    }

    //从路由变量controller中获取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

这里再补充一点:
在DefaultHttpControllerSelector的代码中,我们使用ControllerTypeCache.Cache

实际上由于频繁的反射创建Controller,WebAPI通过缓存Controller避免了不必要的损失.

同时在DefaultHttpControllerSelector代码中也存在相应的缓存措施(文中的代码为了便于理解).

internal sealed class HttpControllerTypeCache
{
    public HttpControllerTypeCache(HttpConfiguration configuration)
    {
        Cache = GetCache(configuration);//该处同样为伪代码
    }
    internal Dictionary<string, ILookup<string, Type>> Cache {get;}
}

通过上面的流程,我们清楚了HttpControllerDescriptor是如何创建的,同时创建完HttpControllerDescriptor后,调用CreateController即可创建 public virtual IHttpController CreateController(HttpRequestMessage request)

ServicesContainer

在Web API消息管道中,定义了很多接口,对应的也有很多实现类.

WebAPI自定义了一套DI容器ServicesContainer.而这个容器直接挂在HttpConfiguration上

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

在默认的实现DefaultServices 则定义了WebAPI 默认对应的实现

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());
    }
    //设置接口实现
    private void SetSingle<T>(T instance) where T : class
    {
      this._defaultServicesSingle[typeof (T)] = (object) instance;
    }
    //获取接口实现
    public override object GetService(Type serviceType)
    {
        return this._defaultServicesSingle[serviceType];
    }
}

备注:

• 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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