ASP.NET Core 1.0中的管道-中间件模式

目录

ASP.NET Core 1.0借鉴了Katana项目的管道设计(Pipeline)。日志记录、用户认证、MVC等模块都以中间件(Middleware)的方式注册在管道中。显而易见这样的设计非常松耦合并且非常灵活，你可以自己定义任意功能的Middleware注册在管道中。这一设计非常适用于“请求-响应”这样的场景——消息从管道头流入最后反向流出。

在本文中暂且为这种模式起名叫做“管道-中间件（Pipeline-Middleware)”模式吧。

本文将描述”管道-中间件模式”的“契约式”设计和“函数式”设计两种方案。

一、什么是管道-中间件模式？

在此模式中抽象了一个类似管道的概念，所有的组件均以中间件的方式注册在此管道中，当请求进入管道后：中间件依次对请求作出处理，然后从最后一个中间件开始处理响应内容，最终反向流出管道。

二、契约式设计

契约式设计是从面向对象的角度来思考问题，根据管道-中间件的理解，中间件(Middleware)有两个职责：

public interface IMiddleware
{
    Request ProcessRequest(Request request);
    Response ProcessResponse(Response response);
}

管道(Pipeline)抽象应该能够注册中间件(Middleware)：

public interface IApplicationBuilder
{
 
    void Use(IMiddleware middleware);
 
    void UseArrange(List<IMiddleware> middlewares);
 
    Context Run(Context context);
}

实现IApplicationBuilder：

public class ApplicationBuilder : IApplicationBuilder
{
    public IWindsorContainer Container { get; private set; }
    private readonly List<IMiddleware> _middlewares;
 
    public ApplicationBuilder(IWindsorContainer container)
    {
        Contract.Requires(container!=null,"container!=null");
 
        _middlewares=new List<IMiddleware>();
        Container = container;
    }
 
    public void Use(IMiddleware middleware)
    {
        Contract.Requires(middleware != null, "middleware!=null");
 
        _middlewares.Add(middleware);
    }
 
    public void UseArrange(List<IMiddleware> middlewares)
    {
        Contract.Requires(middlewares != null, "middlewares!=null");
 
        _middlewares.AddRange(middlewares);
    }
 
    public Context Run(Context context)
    {
        Contract.Requires(context!=null,"context!=null");
 
        var request=context.Request;
        var response=context.Response;
 
        foreach (var middleware in _middlewares)
        {
            request = middleware.ProcessRequest(request);
        }
 
        _middlewares.Reverse();
 
        foreach (var middleware in _middlewares)
        {
            response = middleware.ProcessResponse(response);
        }
 
        return new Context(request,response);
    }
}

Run()方法将依次枚举Middleware并对消息的请求和响应进行处理，最后返回最终处理过的消息。

接下来需要实现一个Middleware：

public class DefaultMiddleware:IMiddleware
 {
     public Request ProcessRequest(Request request)
     {
         request.Process("default request", "processed by defaultMiddleware");
         return request;
     }
 
     public Response ProcessResponse(Response response)
     {
         response.Process("default response", "processed by defaultMiddleware");
         return response;
     }
 }

为了将Middleware注册进管道，我们还可以写一个扩展方法增加代码的可读性：

public static void UseDefaultMiddleware(this IApplicationBuilder applicationBuilder)
{
    applicationBuilder.Use<DefaultMiddleware>();
}
 
public static void Use<TMiddleware>(this IApplicationBuilder applicationBuilder)
    where TMiddleware:IMiddleware
{
    var middleware = applicationBuilder.Container.Resolve<TMiddleware>();
 
    applicationBuilder.Use(middleware);
}

写个测试看看吧：

写第二个Middleware：

public class GreetingMiddleware:IMiddleware
{
    public Request ProcessRequest(Request request)
    {
        request.Process("hello, request","processed by greetingMiddleware");
 
        return request;
    }
 
    public Response ProcessResponse(Response response)
    {
        response.Process("hello, request", "processed by greetingMiddleware");
 
        return response;
    }
}

编写测试：

三、函数式设计方案

此方案也是Owin和ASP.NET Core采用的方案，如果站在面向对象的角度，第一个方案是非常清晰的，管道最终通过枚举所有Middleware来依次处理请求。

站在函数式的角度来看，Middleware可以用Func<Context, Context>来表示，再来看看这张图：

一个Middleware的逻辑可以用Func<Func<Context, Context>, Func<Context, Context>>来表示，整个Middleware的逻辑可以用下面的代码描述：

public Func<Func<Context, Context>, Func<Context, Context>> Process()
{
    Func<Func<Context, Context>, Func<Context, Context>> middleware = next =>
    {
        Func<Context, Context> process = context =>
        {
            /*process request*/
           
            next(context);
 
            /*process response*/
 
            return context;
        };
 
        return process;
    };
 
    return middleware;
}

这一过程是理解函数式方案的关键，所有Middleware可以聚合为一个Func<Context,Context>,为了易于阅读，我们可以定义一个委托：

1	`public` `delegate` `Context RequestDelegate(Context context);`

给定初始RequestDelegate，聚合所有Middleware：

public IApplication Build()
{
    RequestDelegate request = context => context;
 
    _middlewares.Reverse();
 
    foreach (var middleware in _middlewares)
    {
        request = middleware(request);
    }
 
    return new Application(request);
}

自定义一个函数式Middleware：

public class DefaultMiddleware:IMiddleware
{
    public Func<RequestDelegate, RequestDelegate> Request()
    {
        Func<RequestDelegate, RequestDelegate> request = next =>
        {
            return context =>
            {
                context.Request.Process("default request", "processed by defaultMiddleware");
 
                next(context);
 
                context.Response.Process("default response", "processed by defaultMiddleware");
 
                return context;
            };
 
        };
 
        return request;
    }
}