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!\n");
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!\n");
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!\n");
//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!\n");
};
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!\n");
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!\n");
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。
原文地址:https://www.cnblogs.com/kenwoo/p/9404671.html