Net6 DI源码分析Part5 在Kestrel内Di Scope生命周期是如何根据请求走的？

Net6 DI源码分析Part5 在Kestrel内Di Scope生命周期是如何根据请求走的？

在asp.net core中的DI生命周期有一个Scoped是根据请求走的，也就是说在处理一次请求时，Scope生命周期所提供的服务是同一个实例。它是用IServiceScope是实现的。但是我们要知道何时构建的IServiceScope以及IServiceScope何时被销毁掉

先说结论IServiceScope是根据当前的RequestServicesFeature内作为_scope成员存在的，只要知道RequestServicesFeature何时创建，何时销毁就了解整http request DI的生命周期

netcore中DI生命周期。

在net core 默认的DI中你直接build 出来的servicepProvider 用它去获取的实例对象Scoped & Singleton 是具有相同生命周期的。

如果需要有Scoped生命周期的实例，你需要通过serviceProvider创建一个IServiceScope实例，然后通过该实例获取到的服务实例会跟着IServiceScope一同销毁从而达到Scoped生命周期。

DI生命周期案例 1

ServiceCollection serviceCollection = new ServiceCollection();
serviceCollection.AddSingleton<MySingletonClass>();
serviceCollection.AddScoped<MyScopedClass>();
serviceCollection.AddTransient<MyTransientClass >();
var servicepProvider = serviceCollection.BuildServiceProvider();
using (IServiceScope scope = servicepProvider.CreateScope(), scope2 = servicepProvider.CreateScope())
{
    var scopeObj1 = scope.ServiceProvider.GetService<MyScopedClass>();
    var scopeObj2 = scope2.ServiceProvider.GetService<MyScopedClass>();
    Console.WriteLine(object.ReferenceEquals(scopeObj1, scopeObj2)); //false
}
//Scoped Constructor
//Scoped Constructor
//False
//Scoped Dispose
//Scoped Dispose
Console.ReadLine();

RequestServicesFeature

为什么说http request DI 的生命周期是根据RequestServicesFeature来的。通过DI生命周期案例了解到需要创建一个scope生命周期，要有一个IServiceScope实例。

那么在http feature中RequestServicesFeature做了对该接口的封装。也用于提供RequestSerivces服务。

1. IServiceScope的创建： HttpContext获取RequstServiceProivder是根据RequestServicesFeature.RequestServices。在RequestServices属性的get方法内创建_scope。并返回给该scope对应的ServiceProvide供后续使用。
2. IServiceScope的销毁：在RequestServicesFeature.Dispose 方法内又调用了_scope属性（IServiceScope）的同名方法从而进行销毁由此提供的所有service。那么只要销毁了当前请求的RequestServicesFeature 实例就销毁了。当前http requst 的scope生命周期的所有服务。

RequestServicesFeature 是何时被销毁的。

1. HttpProtocol.ProcessRequests 方法内调用await FireOnCompleted();
2. FireOnCompleted内部会循环执行 Stack<KeyValuePair<Func<object, Task>, object>>? _onCompleted;堆栈委托
3. requestServiceFeature.Dispose在此刻被调用，其内部Dispose了IServiceScope。自此ServiceScope生命周期结束。

RequestServicesFeature 简化代码

public class RequestServicesFeature : IServiceProvidersFeature, IDisposable, IAsyncDisposable
{
    private IServiceScope? _scope;
    private readonly HttpContext _context;

    public RequestServicesFeature(HttpContext context, IServiceScopeFactory? scopeFactory)
    {
        _context = context;
        _scopeFactory = scopeFactory;
    }

    public IServiceProvider RequestServices
    {
        get
        {
            _context.Response.RegisterForDisposeAsync(this);
            _scope = _scopeFactory.CreateScope();
            _requestServices = _scope.ServiceProvider;
            return _requestServices!;
        }

    }

    /// <inheritdoc />
    public ValueTask DisposeAsync()
    {
        switch (_scope)
        {
            case IAsyncDisposable asyncDisposable:
                var vt = asyncDisposable.DisposeAsync();
                if (!vt.IsCompletedSuccessfully)
                {
                    return Awaited(this, vt);
                }
                vt.GetAwaiter().GetResult();
                break;
            case IDisposable disposable:
                disposable.Dispose();
                break;
        }
    }
}

HttpProtocol

该类是处理http协议的 ProcessRequests作为重要方法之一，用来使用我们构建好的IHttpApplication 处理request

httpontext的创建，以及我们编排好的http中间件管道都是在这里被执行的。同时调用FireOnCompleted, RequestServicesFeature就是在这里被销毁的。

private async Task ProcessRequests<TContext>(IHttpApplication<TContext> application) where TContext : notnull
{
    while (_keepAlive)
    {
        var context = application.CreateContext(this);
        // Run the application code for this request
        await application.ProcessRequestAsync(context);
        if (_onCompleted?.Count > 0)
        {
            await FireOnCompleted();
        }
        application.DisposeContext(context, _applicationException);
    }
}

protected Task FireOnCompleted()
    {
        var onCompleted = _onCompleted;
        if (onCompleted?.Count > 0)
        {
            return ProcessEvents(this, onCompleted);
        }

        return Task.CompletedTask;

        static async Task ProcessEvents(HttpProtocol protocol, Stack<KeyValuePair<Func<object, Task>, object>> events)
        {
            while (events.TryPop(out var entry))
            {
                try
                {
                    await entry.Key.Invoke(entry.Value);
                }
                catch (Exception ex)
                {
                    protocol.Log.ApplicationError(protocol.ConnectionId, protocol.TraceIdentifier, ex);
                }
            }
        }
    }

RequestServicesFeature 是如何注册到HttpProtocol _onCompleted;堆栈委托中的

HttpProtocol委托堆栈中 RequestServicesFeature 是什么时候被注册进去的呢？

1. RequestServicesFeature.RequestServices属性的Get方法调用了方法内有一句这样的代码 _context.Response.RegisterForDisposeAsync(this);
2. HttpResponse的RegisterForDisposeAsync调用了抽象方法abstract void OnCompleted。实现该方法的是在完成的DefaultHttpResponse
3. DefaultHttpResponse的OnCompleted把委托注册到HttpProtocol.OnCompleted方法注册到_onCompleted中，实际代码体现为 HttpResponseFeature.OnCompleted(callback, state);这里HttpResponseFeature.就是就是HttpProtocol，（HttpProtoccol是个IFeatureCollection接口的实现。）