ASP.NET Core 1.0中的管道-中間件模式
ASP.NET Core 1.0借鑒了Katana項目的管道設計(Pipeline)。日志記錄、用戶認證、MVC等模塊都以中間件(Middleware)的方式注冊在管道中。顯而易見這樣的設計非常松耦合并且非常靈活，你可以自己定義任意功能的Middleware注冊在管道中。這一設計非常適用于“請求-響應”這樣的場景——消息從管道頭流入最后反向流出。
在本文中暫且為這種模式起名叫做“管道-中間件（Pipeline-Middleware)”模式吧。
本文將描述”管道-中間件模式”的“契約式”設計和“函數(shù)式”設計兩種方案。
一、什么是管道-中間件模式？
在此模式中抽象了一個類似管道的概念，所有的組件均以中間件的方式注冊在此管道中，當請求進入管道后：中間件依次對請求作出處理，然后從最后一個中間件開始處理響應內容，最終反向流出管道。
二、契約式設計
契約式設計是從面向對象的角度來思考問題，根據管道-中間件的理解，中間件(Middleware)有兩個職責：
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public?interface?IMiddleware{????Request ProcessRequest(Request request);????Response ProcessResponse(Response response);}管道(Pipeline)抽象應該能夠注冊中間件(Middleware)：
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public?interface?IApplicationBuilder{?
????void?Use(IMiddleware middleware);?
????void?UseArrange(List<IMiddleware> middlewares);?
????Context Run(Context context);}實現(xiàn)IApplicationBuilder：
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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()方法將依次枚舉Middleware并對消息的請求和響應進行處理，最后返回最終處理過的消息。
接下來需要實現(xiàn)一個Middleware：
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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;?????}?}為了將Middleware注冊進管道，我們還可以寫一個擴展方法增加代碼的可讀性：
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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);}寫個測試看看吧：
寫第二個Middleware：
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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;????}}編寫測試：
三、函數(shù)式設計方案
此方案也是Owin和ASP.NET Core采用的方案，如果站在面向對象的角度，第一個方案是非常清晰的，管道最終通過枚舉所有Middleware來依次處理請求。
站在函數(shù)式的角度來看，Middleware可以用Func<Context, Context>來表示，再來看看這張圖：
一個Middleware的邏輯可以用Func<Func<Context, Context>, Func<Context, Context>>來表示，整個Middleware的邏輯可以用下面的代碼描述：
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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;}這一過程是理解函數(shù)式方案的關鍵，所有Middleware可以聚合為一個Func<Context,Context>,為了易于閱讀，我們可以定義一個委托：
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public?delegate?Context RequestDelegate(Context context);給定初始RequestDelegate，聚合所有Middleware：
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public?IApplication Build(){????RequestDelegate request = context => context;?
????_middlewares.Reverse();?
????foreach?(var?middleware in?_middlewares)????{????????request = middleware(request);????}?
????return?new?Application(request);}自定義一個函數(shù)式Middleware：
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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;????}}標簽：
