[Asp.net 5] DependencyInjection项目代码分析2-Autofac

Microsoft.Framework.DependencyInjection.Autofac源码分析

该工程只有一个代码静态类AutofacRegistration，但是该类有3个扩展方法，以及3个内部类。扩展方法如下：

public static class AutofacRegistration
    {
          public static void Populate(this ContainerBuilder builder,IEnumerable<ServiceDescriptor> descriptors);

           private static void Register(ContainerBuilder builder,IEnumerable<ServiceDescriptor> descriptors);

           private static IRegistrationBuilder<object, T, U> ConfigureLifecycle<T, U>(this IRegistrationBuilder<object, T, U> registrationBuilder,ServiceLifetime lifecycleKind)
    }

在Autofac中，ContainerBuilder是用于类型注册的，3个扩展方法都是基于此展开的。

Populate方法如下：先对IServiceProvider、IServiceScopeFactory接口进行注册，之后调用Register方法，对于外部的具体类型进行注册。该方法的入参是IEnumerable<ServiceDescriptor> descriptors类型，所以参数可以直接使用IServiceCollection（也就是List<ServiceDescriptor>）。

public static void Populate(this ContainerBuilder builder,
                            IEnumerable<ServiceDescriptor> descriptors)
        {
            builder.RegisterType<AutofacServiceProvider>().As<IServiceProvider>();
            builder.RegisterType<AutofacServiceScopeFactory>().As<IServiceScopeFactory>();

            Register(builder, descriptors);
        }

对于Register方法相对复杂些，需要将ServiceDescriptor对象转化为实际的注册类型，代码如下所示：

private static void Register(
                ContainerBuilder builder,
                IEnumerable<ServiceDescriptor> descriptors)
        {
            foreach (var descriptor in descriptors)
            {
                if (descriptor.ImplementationType != null)
                {
                    // Test if the an open generic type is being registered
                    var serviceTypeInfo = descriptor.ServiceType.GetTypeInfo();
                    if (serviceTypeInfo.IsGenericTypeDefinition)
                    {
                        builder
                            .RegisterGeneric(descriptor.ImplementationType)
                            .As(descriptor.ServiceType)
                            .ConfigureLifecycle(descriptor.Lifetime);
                    }
                    else
                    {
                        builder
                            .RegisterType(descriptor.ImplementationType)
                            .As(descriptor.ServiceType)
                            .ConfigureLifecycle(descriptor.Lifetime);
                    }
                }
                else if (descriptor.ImplementationFactory != null)
                {
                    var registration = RegistrationBuilder.ForDelegate(descriptor.ServiceType, (context, parameters) =>
                    {
                        var serviceProvider = context.Resolve<IServiceProvider>();
                        return descriptor.ImplementationFactory(serviceProvider);
                    })
                    .ConfigureLifecycle(descriptor.Lifetime)
                    .CreateRegistration();

                    builder.RegisterComponent(registration);
                }
                else
                {
                    builder
                        .RegisterInstance(descriptor.ImplementationInstance)
                        .As(descriptor.ServiceType)
                        .ConfigureLifecycle(descriptor.Lifetime);
                }
            }
        }

代码看起来挺多，实际也很简单对于列表进行循环遍历，对于每一项：如果注册类型不为空，使用ContainerBuilder的类型注册方法进行注册（包括泛型和非泛型的）；如果类型没提供，判断是否为类型提供工厂delegate,如果提供，使用ContainerBuilder注册代理的方式注册；最后都没有直接将实例类型进行注册。在ContainerBuilder中涉及到了ConfigureLifecycle方法，世界该方法仅仅是将DependencyInjection中定义的ServiceLifetime枚举转化为Autofac能够识别的类型。

private static IRegistrationBuilder<object, T, U> ConfigureLifecycle<T, U>(
                this IRegistrationBuilder<object, T, U> registrationBuilder,
                ServiceLifetime lifecycleKind)
        {
            switch (lifecycleKind)
            {
                case ServiceLifetime.Singleton:
                    registrationBuilder.SingleInstance();
                    break;
                case ServiceLifetime.Scoped:
                    registrationBuilder.InstancePerLifetimeScope();
                    break;
                case ServiceLifetime.Transient:
                    registrationBuilder.InstancePerDependency();
                    break;
            }

            return registrationBuilder;
        }

*IRegistrationBuilder是Autofac中对于注入类型范围进行控制的接口

回过头来我们看下AutofacServiceProvider和AutofacServiceScopeFactory类（Populate方法进行的注入实现类）。

AutofacServiceScopeFactory类，实现IServiceScopeFactory接口，并且CreateScope方法创建一个新AutofacServiceScope实例

private class AutofacServiceScopeFactory : IServiceScopeFactory
        {
            private readonly ILifetimeScope _lifetimeScope;

            public AutofacServiceScopeFactory(ILifetimeScope lifetimeScope)
            {
                _lifetimeScope = lifetimeScope;
            }

            public IServiceScope CreateScope()
            {
                return new AutofacServiceScope(_lifetimeScope.BeginLifetimeScope());
            }
        }

AutofacServiceScope类。使用内部ILifetimeScope创建新的IServiceProvider对象。当当前作用域失效时，调用Dispose方法，以释放内部的IServiceProvider对象，保证不同的ILifetimeScope（Scope）的IServiceProvider生成的对象不同。

private class AutofacServiceScope : IServiceScope
        {
            private readonly ILifetimeScope _lifetimeScope;
            private readonly IServiceProvider _serviceProvider;

            public AutofacServiceScope(ILifetimeScope lifetimeScope)
            {
                _lifetimeScope = lifetimeScope;
                _serviceProvider = _lifetimeScope.Resolve<IServiceProvider>();
            }

            public IServiceProvider ServiceProvider
            {
                get { return _serviceProvider; }
            }

            public void Dispose()
            {
                _lifetimeScope.Dispose();
            }
        }

AutofacServiceProvider类用于创建具体的对象。代码如下。IComponentContext参数会在Aotufac注入实例化时自动初始化。

 private class AutofacServiceProvider : IServiceProvider
        {
            private readonly IComponentContext _componentContext;

            public AutofacServiceProvider(IComponentContext componentContext)
            {
                _componentContext = componentContext;
            }

            public object GetService(Type serviceType)
            {
                return _componentContext.ResolveOptional(serviceType);
            }
        }