.NET Core开发日志——RequestDelegate
2019-08-06

本文主要是对.NET Core开发日志——Middleware的补遗,但是会从看起来平平无奇的RequestDelegate开始叙述,所以以其作为标题,也是合情合理。

RequestDelegate是一种委托类型,其全貌为public delegate Task RequestDelegate(HttpContext context),MSDN上对它的解释,"A function that can process an HTTP request."——处理HTTP请求的函数。唯一参数,是最熟悉不过的HttpContext,返回值则是表示请求处理完成的异步操作类型。

可以将其理解为ASP.NET Core中对一切HTTP请求处理的抽象(委托类型本身可视为函数模板,其实现具有统一的参数列表及返回值类型),没有它整个框架就失去了对HTTP请求的处理能力。

并且它也是构成Middleware的基石。或者更准确地说参数与返回值都是其的Func<RequestDelegate, RequestDelegate>委托类型正是维持Middleware运转的核心齿轮。

组装齿轮的地方位于ApplicationBuilder类之内,其中包含着所有齿轮的集合。

private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();

以及添加齿轮的方法:

public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware){ _components.Add(middleware); return this;}

在Startup类的Configure方法里调用以上ApplicationBuilder的Use方法,就可以完成一个最简单的Middleware。

public void Configure(IApplicationBuilder app){ app.Use(_ => { return context => { return context.Response.WriteAsync("Hello, World!"); }; });}

齿轮要想变成Middleware,在完成添加后,还需要经过组装。

public RequestDelegate Build(){ RequestDelegate app = context => { context.Response.StatusCode = 404; return Task.CompletedTask; }; foreach (var component in _components.Reverse()) { app = component(app); } return app;}

Build方法里先定义了最底层的零件——app,context => { context.Response.StatusCode = 404; return Task.CompletedTask; },这段代码意味着,如果没有添加任何Middleware的话,ASP.NET Core站点启动后,会直接出现404的错误。

接下的一段,遍历倒序排列的齿轮,开始正式组装。

在上述例子里,只使用了一个齿轮:

_ =>{ return context => { return context.Response.WriteAsync("Hello, World!"); }; }

那么第一次也是最后一次循环后,执行component(app)操作,app被重新赋值为:

context => context.Response.WriteAsync("Hello, World!");

组装的结果便是app的值。

这个组装过程在WebHost进行BuildApplication时开始操作。从此方法的返回值类型可以看出,虽然明义上是创建Application,其实生成的是RequestDelegate。

private RequestDelegate BuildApplication(){ try { ... var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>(); var builder = builderFactory.CreateBuilder(Server.Features); ... Action<IApplicationBuilder> configure = _startup.Configure; ... configure(builder); return builder.Build(); } ...}

而这个RequestDelegate最终会在HostingApplication类的ProcessRequestAsync方法里被调用。

public virtual async Task StartAsync(CancellationToken cancellationToken = default){ ... var application = BuildApplication(); ... var hostingApp = new HostingApplication(application, _logger, diagnosticSource, httpContextFactory); ...} public HostingApplication( RequestDelegate application, ILogger logger, DiagnosticListener diagnosticSource, IHttpContextFactory httpContextFactory){ _application = application; _diagnostics = new HostingApplicationDiagnostics(logger, diagnosticSource); _httpContextFactory = httpContextFactory;}public Task ProcessRequestAsync(Context context){ return _application(context.HttpContext);}

上例中的执行结果即是显示Hello, World!字符。

404的错误不再出现,意味着这种Middleware只会完成自己对HTTP请求的处理,并不会将请求传至下一层的Middleware。

要想达成不断传递请求的目的,需要使用另一种Use扩展方法。

public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware){ return app.Use(next => { return context => { Func<Task> simpleNext = () => next(context); return middleware(context, simpleNext); }; });}

在实际代码中可以这么写:

public void Configure(IApplicationBuilder app){ app.Use(async (context, next) => { await context.Response.WriteAsync("I am a Middleware!"); await next.Invoke(); }); app.Use(_ => { return context => { return context.Response.WriteAsync("Hello, World!"); }; });}

现在多了个Middleware,继续上面的组装过程。app的值最终被赋值为:

async context =>{ Func<Task> simpleNext = () => context.Response.WriteAsync("Hello, World!"); await context.Response.WriteAsync("I am a Middleware!"); await simpleNext.Invoke();};

显示结果为:

I am a Middleware!Hello, World!

下面的流程图中可以清楚地说明这个过程。

如果把await next.Invoke()注释掉的话,

public void Configure(IApplicationBuilder app){ app.Use(async (context, next) => { await context.Response.WriteAsync("I am a Middleware!"); //await next.Invoke(); }); app.Use(_ => { return context => { return context.Response.WriteAsync("Hello, World!"); }; });}

上例中第一个Middleware处理完后,不会继续交给第二个Middleware处理。注意以下simpleNext的方法只被定义而没有被调用。

async context =>{ Func<Task> simpleNext = () => context.Response.WriteAsync("Hello, World!"); await context.Response.WriteAsync("I am a Middleware!");};

这种情况被称为短路(short-circuiting)。

做短路处理的Middleware一般会放在所有Middleware的最后,以作为整个pipeline的终点。

并且更常见的方式是用Run扩展方法。

public static void Run(this IApplicationBuilder app, RequestDelegate handler){ ... app.Use(_ => handler);}

所以可以把上面例子的代码改成下面的形式:

public void Configure(IApplicationBuilder app){ app.Use(async (context, next) => { await context.Response.WriteAsync("I am a Middleware!"); await next.Invoke(); }); app.Run(async context => { await context.Response.WriteAsync("Hello, World!"); });}

除了短路之外,Middleware处理时还可以有分支的情况。

public void Configure(IApplicationBuilder app){ app.Map("/branch1", ab => { ab.Run(async context => { await context.Response.WriteAsync("Map branch 1"); }); }); app.Map("/branch2", ab => { ab.Run(async context => { await context.Response.WriteAsync("Map branch 2"); }); }); app.Use(async (context, next) => { await context.Response.WriteAsync("I am a Middleware!"); await next.Invoke(); }); app.Run(async context => { await context.Response.WriteAsync("Hello, World!"); });}

URL地址后面跟着branch1时:

URL地址后面跟着branch2时:

其它情况下:

Map扩展方法的代码实现:

public static IApplicationBuilder Map(this IApplicationBuilder app, PathString pathMatch, Action<IApplicationBuilder> configuration){ ... // create branch var branchBuilder = app.New(); configuration(branchBuilder); var branch = branchBuilder.Build(); var options = new MapOptions { Branch = branch, PathMatch = pathMatch, }; return app.Use(next => new MapMiddleware(next, options).Invoke);}

创建分支的办法就是重新实例化一个ApplicationBuilder。

public IApplicationBuilder New(){ return new ApplicationBuilder(this);}

对分支的处理则是封装在MapMiddleware类之中。

public async Task Invoke(HttpContext context){ ... PathString matchedPath; PathString remainingPath; if (context.Request.Path.StartsWithSegments(_options.PathMatch, out matchedPath, out remainingPath)) { // Update the path var path = context.Request.Path; var pathBase = context.Request.PathBase; context.Request.PathBase = pathBase.Add(matchedPath); context.Request.Path = remainingPath; try { await _options.Branch(context); } finally { context.Request.PathBase = pathBase; context.Request.Path = path; } } else { await _next(context); }}

说到MapMiddleware,不得不提及各种以Use开头的扩展方法,比如UseStaticFiles,UseMvc,UsePathBase等等。

这些方法内部都会调用UseMiddleware方法以使用各类定制的Middleware类。如下面UsePathBase的代码:

public static IApplicationBuilder UsePathBase(this IApplicationBuilder app, PathString pathBase){ ... // Strip trailing slashes pathBase = pathBase.Value?.TrimEnd("/"); if (!pathBase.HasValue) { return app; } return app.UseMiddleware<UsePathBaseMiddleware>(pathBase);}

而从UseMiddleware方法中可以获知,Middleware类需满足两者条件之一才能被有效使用。其一是实现IMiddleware,其二,必须有Invoke或者InvokeAsync方法,且方法至少要有一个HttpContext类型参数(它还只能是放第一个),同时返回值需要是Task类型。

internal const string InvokeMethodName = "Invoke";internal const string InvokeAsyncMethodName = "InvokeAsync";public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args){ if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo())) { ... return UseMiddlewareInterface(app, middleware); } var applicationServices = app.ApplicationServices; return app.Use(next => { var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public); var invokeMethods = methods.Where(m => string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal) || string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal) ).ToArray(); ... var ctorArgs = new object[args.Length + 1]; ctorArgs[0] = next; Array.Copy(args, 0, ctorArgs, 1, args.Length); var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs); if (parameters.Length == 1) { return (RequestDelegate)methodinfo.CreateDelegate(typeof(RequestDelegate), instance); } var factory = Compile<object>(methodinfo, parameters); return context => { var serviceProvider = context.RequestServices ?? applicationServices; ... return factory(instance, context, serviceProvider); }; });}

对ASP.NET Core中Middleware的介绍到此终于可以告一段落,希望这两篇文章能够为读者提供些许助力。