asp.net core 依賴註入實現全過程粗略剖析（1）

轉載請註明出處： https://home.cnblogs.com/u/zhiyong-ITNote/

常用擴展方法 註入依賴服務：

new ServiceCollection().AddSingleton<IApplicationBuilder, ApplicationBuilder>();

// AddSingleton多個重載方法 源碼

public static IServiceCollection AddSingleton<TService, TImplementation>(this IServiceCollection services)
where TService : class
where TImplementation : class, TService
{
if (services == null)
{
throw new ArgumentNullException(nameof(services));
}

return services.AddSingleton(typeof(TService), typeof(TImplementation));
}

// Singleton模式 最終的調用
public static IServiceCollection AddSingleton(
this IServiceCollection services,
Type serviceType,
Type implementationType)
{
if (services == null)
{
throw new ArgumentNullException(nameof(services));
}

if (serviceType == null)
{
throw new ArgumentNullException(nameof(serviceType));
}

if (implementationType == null)
{
throw new ArgumentNullException(nameof(implementationType));
}

return Add(services, serviceType, implementationType, ServiceLifetime.Singleton);
}

// 所有的Addxxx 最終都是調用Add方法，將ServiceDescriptor添加到IServiceCollection中：
private static IServiceCollection Add(
IServiceCollection collection,
Type serviceType,
Type implementationType,
ServiceLifetime lifetime)
{
var descriptor = new ServiceDescriptor(serviceType, implementationType, lifetime);
collection.Add(descriptor);
return collection;
}

// IServiceCollection源碼：
public interface IServiceCollection : IList<ServiceDescriptor>
{}

如上，我們一般在ConfigureService中使用Addxxx將服務註入框架的過程。大概做個總結，其實就是屌用IServiceCollection的Addxxx 擴展方法，隨後調用Add方法，初始化一個ServiceDescriptor，參數是我們註入的介面和類，還有就是生命周期。隨後添加到IServiceCollection中，根據該介面的定義就是一個ServiceDescriptor的集合。

我們看看ServiceProvider的源碼：

 public sealed class ServiceProvider : IServiceProvider, IDisposable, IServiceProviderEngineCallback
    {
        private readonly IServiceProviderEngine _engine;

        private readonly CallSiteValidator _callSiteValidator;

        internal ServiceProvider(IEnumerable<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
        {
            IServiceProviderEngineCallback callback = null;
            if (options.ValidateScopes)
            {
                callback = this;
                _callSiteValidator = new CallSiteValidator();
            }
            switch (options.Mode)
            {
                case ServiceProviderMode.Dynamic:
                    _engine = new DynamicServiceProviderEngine(serviceDescriptors, callback);
                    break;
                case ServiceProviderMode.Runtime:
                    _engine = new RuntimeServiceProviderEngine(serviceDescriptors, callback);
                    break;
#if IL_EMIT
                case ServiceProviderMode.ILEmit:
                    _engine = new ILEmitServiceProviderEngine(serviceDescriptors, callback);
                    break;
#endif
                case ServiceProviderMode.Expressions:
                    _engine = new ExpressionsServiceProviderEngine(serviceDescriptors, callback);
                    break;
                default:
                    throw new NotSupportedException(nameof(options.Mode));
            }
        }

        /// <summary>
        /// Gets the service object of the specified type.
        /// </summary>
        /// <param name="serviceType"></param>
        /// <returns></returns>
        public object GetService(Type serviceType) => _engine.GetService(serviceType);

        /// <inheritdoc />
        public void Dispose() => _engine.Dispose();

        void IServiceProviderEngineCallback.OnCreate(IServiceCallSite callSite)
        {
            _callSiteValidator.ValidateCallSite(callSite);
        }

        void IServiceProviderEngineCallback.OnResolve(Type serviceType, IServiceScope scope)
        {
            _callSiteValidator.ValidateResolution(serviceType, scope, _engine.RootScope);
        }
    }

該類就是初始化並獲取如下屬性：

/// <inheritdoc />
public ServiceLifetime Lifetime { get; }

/// <inheritdoc />
public Type ServiceType { get; }

/// <inheritdoc />
public Type ImplementationType { get; }

/// <inheritdoc />
public object ImplementationInstance { get; }

/// <inheritdoc />
public Func<IServiceProvider, object> ImplementationFactory { get; }

到此，我們的服務註入到asp.net core框架中就完事了，那麼服務的實例化呢？

首先我們理下思路，IServiceProvider介面對應的實現是ServiceProvider,這個類就是實例化了IServiceProvider介面，而IServiceProvider介面只有一個方法：

public object GetService(Type serviceType);

該方法就是獲取註入的服務。但是ServiceProvider類不單單是獲取註入的服務，服務的實例化還是在該類中實現的，我們看下：

public sealed class ServiceProvider : IServiceProvider, IDisposable, IServiceProviderEngineCallback
{
private readonly IServiceProviderEngine _engine;

private readonly CallSiteValidator _callSiteValidator;

internal ServiceProvider(IEnumerable<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
{
IServiceProviderEngineCallback callback = null;
if (options.ValidateScopes)
{
callback = this;
_callSiteValidator = new CallSiteValidator();
}
switch (options.Mode)
{
case ServiceProviderMode.Dynamic:
_engine = new DynamicServiceProviderEngine(serviceDescriptors, callback);
break;
case ServiceProviderMode.Runtime:
_engine = new RuntimeServiceProviderEngine(serviceDescriptors, callback);
break;
#if IL_EMIT
case ServiceProviderMode.ILEmit:
_engine = new ILEmitServiceProviderEngine(serviceDescriptors, callback);
break;
#endif
case ServiceProviderMode.Expressions:
_engine = new ExpressionsServiceProviderEngine(serviceDescriptors, callback);
break;
default:
throw new NotSupportedException(nameof(options.Mode));
}
}

/// <summary>
/// Gets the service object of the specified type.
/// </summary>
/// <param name="serviceType"></param>
/// <returns></returns>
public object GetService(Type serviceType) => _engine.GetService(serviceType);

/// <inheritdoc />
public void Dispose() => _engine.Dispose();

void IServiceProviderEngineCallback.OnCreate(IServiceCallSite callSite)
{
_callSiteValidator.ValidateCallSite(callSite);
}

void IServiceProviderEngineCallback.OnResolve(Type serviceType, IServiceScope scope)
{
_callSiteValidator.ValidateResolution(serviceType, scope, _engine.RootScope);
}
}

可以看到該類的構造函數中就是實例化服務的過程了。很直白的可以看出我們常見的幾種方法來實例化類：反射，Emit，表達式樹等...

目前也算是交代清楚了相關的類。那麼框架具體是如何來實例化的呢？整個的流程是怎麼樣的。篇2再敘

轉載請註明出處： https://home.cnblogs.com/u/zhiyong-ITNote/

源碼地址：https://github.com/aspnet/DependencyInjection