【ASP.NET Core】运行原理(2):启动WebHost

本系列将分析ASP.NET Core运行原理

本节将分析WebHost.StartAsync();代码，确定是如何一步一步到我们注册的中间件，并介绍几种Configure的方式。

源代码参考.NET Core 2.0.0

Server.StartAsync

在上节我们知道WebHost.StartAsync内部是调用Server.StartAsync的。

public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken)
{
    async Task OnBind(ListenOptions endpoint)
    {
        var connectionHandler = new ConnectionHandler<TContext>(endpoint, ServiceContext, application);
        var transport = _transportFactory.Create(endpoint, connectionHandler);
        _transports.Add(transport);

        await transport.BindAsync().ConfigureAwait(false);
    }

    await AddressBinder.BindAsync(_serverAddresses, Options.ListenOptions, Trace, OnBind).ConfigureAwait(false);
}

参数application即为之前的new HostingApplication。在这里说下大概的流程：

KestrelServer.StartAsync -> new ConnectionHandler<TContext>().OnConnection -> new FrameConnection().StartRequestProcessing() -> 
new Frame<TContext>().ProcessRequestsAsync() -> _application.CreateContext(this) && _application.ProcessRequestAsync(context)

如果你需要更细节的流程，可参考如下：

LibuvTransportFactory -> LibuvTransport.BindAsync() -> ListenerPrimary.StartAsync() -> 
listener.ListenSocket.Listen(LibuvConstants.ListenBacklog, ConnectionCallback, listener) -> listener.OnConnection(stream, status) -> ConnectionCallback() ->
new LibuvConnection(this, socket).Start() -> ConnectionHandler.OnConnection() -> connection.StartRequestProcessing() -> 
ProcessRequestsAsync -> CreateFrame -> await _frame.ProcessRequestsAsync()

_application 为上面的HostingApplication;
每个WebHost.StartAsync 将创建唯一的一个HostingApplication实例并在每次请求时使用。
由Frame类调用HostingApplication的方法。

下面展示Frame以及HostingApplication:

Frame

public class Frame<TContext> : Frame
{
    public override async Task ProcessRequestsAsync()
    {
        while (!_requestProcessingStopping)
        {
            Reset();

            EnsureHostHeaderExists();

            var messageBody = MessageBody.For(_httpVersion, FrameRequestHeaders, this);
            InitializeStreams(messageBody);

            var context = _application.CreateContext(this);
            try
            {
                await _application.ProcessRequestAsync(context);
            }
            finally
            {
                _application.DisposeContext(context, _applicationException);
            }
        }
    }
}

HostingApplication

public class HostingApplication : IHttpApplication<HostingApplication.Context>
{
    private readonly RequestDelegate _application;
    private readonly IHttpContextFactory _httpContextFactory;

    public HostingApplication(
        RequestDelegate application,
        IHttpContextFactory httpContextFactory)
    {
        _application = application;
        _httpContextFactory = httpContextFactory;
    }

    // Set up the request
    public Context CreateContext(IFeatureCollection contextFeatures)
    {
        var context = new Context();
        var httpContext = _httpContextFactory.Create(contextFeatures);
        context.HttpContext = httpContext;
        return context;
    }

    // Execute the request
    public Task ProcessRequestAsync(Context context)
    {
        return _application(context.HttpContext);
    }

    // Clean up the request
    public void DisposeContext(Context context, Exception exception)
    {
        var httpContext = context.HttpContext;
        _httpContextFactory.Dispose(httpContext);
    }

    public struct Context
    {
        public HttpContext HttpContext { get; set; }
    }
}

由此我们发现HttpContext是由HttpContextFactory创建的，其中_httpContextFactory则是上节在WebHostBuilder的BuildCommon注入的
同时在HostingApplication的ProcessRequestAsync方法中，我们看到我们的_application(Startup注册的中间件)被调用了。
IHttpContextFactory。

HttpContextFactory

public HttpContext Create(IFeatureCollection featureCollection)
{
    var httpContext = new DefaultHttpContext(featureCollection);
    if (_httpContextAccessor != null)
        _httpContextAccessor.HttpContext = httpContext;
    return httpContext;
}

而创建的HttpContext则是DefaultHttpContext类型：

public class DefaultHttpContext : HttpContext
{
    public virtual void Initialize(IFeatureCollection features)
    {
        _features = new FeatureReferences<FeatureInterfaces>(features);
        _request = InitializeHttpRequest();
        _response = InitializeHttpResponse();
    }

    public override HttpRequest Request => _request;

    public override HttpResponse Response => _response;
}

Configure

IApplicationBuilder

我们知道在Startup的Configure方法中，通过IApplicationBuilder可以注册中间件。

public interface IApplicationBuilder
{
    IServiceProvider ApplicationServices { get; set; }
    RequestDelegate Build();
    IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware);
}

默认实现类为：

public class ApplicationBuilder : IApplicationBuilder
{
    private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();
    public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
    {
        _components.Add(middleware);
        return this;
    }

    public RequestDelegate Build()
    {
        RequestDelegate app = context =>
        {
            context.Response.StatusCode = 404;
            return Task.CompletedTask;
        };

        foreach (var component in _components.Reverse())
            app = component(app);

        return app;
    }
}

其中Use方法为注册中间件。中间件的本质就是一个Func<RequestDelegate, RequestDelegate>对象。
该对象的传入参数为下一个中间件，返回对象为本中间件。

而Build方法为生成一个RequestDelegate，在HostingApplication构造函数中的参数即为该对象。
在Build方法中，我们看到最后一个中间件为404中间件。其他的中间件都是通过Use方法注册到内部维护的_components对象上。

Use

我们通过一个Use示例，来看下中间件的流程：

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.Use(next => async context =>
    {
        Console.WriteLine("A begin");
        await next(context);
        Console.WriteLine("A end");
    });

    app.Use(next => async context =>
    {
        Console.WriteLine("B begin");
        await next(context);
        Console.WriteLine("B end");
    });
}

访问结果：
A begin
B begin
B end
A end

流程图：

Run

当我们不使用next 下一个中间件的时候，我们可以使用Run方法来实现
Run方法接受一个RequestDelegate对象，本身是IApplicationBuilder的扩展方法。

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

Run示例

app.Run(context=>context.Response.WriteAsync("Run Core"));

该示例相当于：

app.Use(next => context => context.Response.WriteAsync("Run Core"));

UseMiddleware

而通常我们添加中间件的方式是通过UseMiddleware来更加方便的操作。

先看下IMiddleware：

public interface IMiddleware
{
    Task InvokeAsync(HttpContext context, RequestDelegate next);
}

参数next即为下一个中间件。

有2种实现UseMiddleware的方式：

实现IMiddleware接口。
基于接口约定的方法。

IMiddleware接口

public class DemoMiddle : IMiddleware
{
    public Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        return context.Response.WriteAsync("hello middleware");
    }
}

在使用IMiddleware接口的时候，还需要注册该类到DI系统中。

约定

public class DemoMiddle
{
    private RequestDelegate _next;
    public DemoMiddle(RequestDelegate next)
    {
        _next = next;
    }
    public Task InvokeAsync(HttpContext context)
    {
        return context.Response.WriteAsync("hello middleware");
    }
}

这种方式，不用再注册到DI中，如果需要对该类构造函数传入参数，直接在app.UseMiddleware<DemoMiddle>("hi1");传入参数即可。

UseWhen

app.Use(next => async context => { await context.Response.WriteAsync("Begin"); await next(context); });

app.UseWhen(context => context.Request.Path.Value == "/hello", branch => branch.Use(
    next => async context => { await context.Response.WriteAsync("hello"); await next(context); }));

app.Run(context => context.Response.WriteAsync("End"));

当我们访问/hello时，结果为：BeginhelloEnd
分析源码得知在构建管道的时候，克隆一个另外的IApplicationBuilder。

public static IApplicationBuilder UseWhen(this IApplicationBuilder app, Predicate predicate, Action<IApplicationBuilder> configuration)
{
    var branchBuilder = app.New();
    configuration(branchBuilder);

    return app.Use(main =>
    {
        // This is called only when the main application builder
        // is built, not per request.
        branchBuilder.Run(main);// 添加(调用)原来的中间件
        var branch = branchBuilder.Build();

        return context => predicate(context) ? branch(context): main(context);
    });
}

MapWhen

app.Use(next => async context => { await context.Response.WriteAsync("Begin"); await next(context); });

app.MapWhen(context => context.Request.Path.Value == "/hello", app2 => app2.Run(context => context.Response.WriteAsync("hello")));

app.Run(context => context.Response.WriteAsync("End"));

当我们访问/hello时，结果为：Beginhello。
分析源码得知在构建管道的时候，新分支并没有再调用原来的中间件。

public static IApplicationBuilder MapWhen(this IApplicationBuilder app, Predicate predicate, Action<IApplicationBuilder> configuration)
{
    var branchBuilder = app.New();
    configuration(branchBuilder);
    var branch = branchBuilder.Build();
    return app.Use(next => context => predicate(context) ? branch(context): next(context));
}

Map

app.Map("/hello", app2 => app2.Run(context => context.Response.WriteAsync("hello")));

当我们访问/hello时，结果为：Beginhello。与MapWhen效果一样。
如果我们只是判断URLPath的话，通常我们会使用Map方法。

以上是常用的注册中间件的方式。

本文链接：http://neverc.cnblogs.com/p/8029419.html

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原文地址：https://www.cnblogs.com/neverc/p/8029419.html