DOTNET CORE源码分析之IServiceProvider、ServiceProvider、IServiceProviderEngine、ServiceProviderEngine和ServiceProviderEngineScope

　　首先谈一下IServiceProvider

　　IServiceProvider只提供给了一个根据类型获取对象的功能，试想一下IOC总得有一个找到对象，具体如下

public interface IServiceProvider
{
　　object GetService(Type serviceType);
}

　　再谈一下ServiceProvider

　　ServiceProvider实例化了IServiceProvider，也就是实现了GetService，用于获取容器中实际对象，但是它是借助于IServiceProviderEngine（这个接口稍后再介绍），具体如下：

public object GetService(Type serviceType)
{
  return this._engine.GetService(serviceType);
}

　　另外ServiceCollection中有一个方法BuildServiceProvider（存在于类ServiceCollectionContainerBuilderExtensions中），返回值对象类型就是ServiceProvider

/// <summary>
/// Extension methods for building a <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" /> from an <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />.
/// </summary>
public static class ServiceCollectionContainerBuilderExtensions
{
/// <summary>
/// Creates a <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" /> containing services from the provided <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />.
/// </summary>
/// <param name="services">The <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" /> containing service descriptors.</param>
/// <returns>The <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" />.</returns>
public static ServiceProvider BuildServiceProvider(
  this IServiceCollection services)
{
  return services.BuildServiceProvider(ServiceProviderOptions.Default);
}

/// <summary>
/// Creates a <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" /> containing services from the provided <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />
/// optionaly enabling scope validation.
/// </summary>
/// <param name="services">The <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" /> containing service descriptors.</param>
/// <param name="validateScopes">
/// <c>true</c> to perform check verifying that scoped services never gets resolved from root provider; otherwise <c>false</c>.
/// </param>
/// <returns>The <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" />.</returns>
public static ServiceProvider BuildServiceProvider(
  this IServiceCollection services,
  bool validateScopes)
{
  return services.BuildServiceProvider(new ServiceProviderOptions()
  {
	ValidateScopes = validateScopes
  });
}

/// <summary>
/// Creates a <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" /> containing services from the provided <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />
/// optionaly enabling scope validation.
/// </summary>
/// <param name="services">The <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" /> containing service descriptors.</param>
/// <param name="options">
/// Configures various service provider behaviors.
/// </param>
/// <returns>The <see cref="T:Microsoft.Extensions.DependencyInjection.ServiceProvider" />.</returns>
public static ServiceProvider BuildServiceProvider(
  this IServiceCollection services,
  ServiceProviderOptions options)
{
  if (services == null)
	throw new ArgumentNullException(nameof (services));
  if (options == null)
	throw new ArgumentNullException(nameof (options));
  return new ServiceProvider((IEnumerable<ServiceDescriptor>) services, options);
}
}

　　必须要结合一下IServiceProviderEngine、ServiceProviderEngine和ServiceProviderEngineScope对象才能具体知道ServiceProvider是怎么获取IOC容器中的对象了

　　在使用BuildServiceProvider的时候，最终会调用new ServiceProvider((IEnumerable<ServiceDescriptor>)services,options),然后就是调用如下函数：

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

　　就是说最终还是根据ServiceProviderMode来判断需要实例化哪种ServiceProviderEngine，现在简单举例DynamicServiceProviderEngine。DynamicServiceProviderEngine最终还是继承ServiceProviderEngine，

所以上面说的GetService其实就是调用了ServiceProviderEngine的GetService

public object GetService(Type serviceType)
{
  return this.GetService(serviceType, this.Root);
}

　　其中this.Root就是实例化的ServiceProviderScope。上面的this.GetService(serviceType,this.Root)会调用如下函数：

internal object GetService(
  Type serviceType,
  ServiceProviderEngineScope serviceProviderEngineScope)
{
  if (this._disposed)
	ThrowHelper.ThrowObjectDisposedException();
  Func<ServiceProviderEngineScope, object> orAdd = this.RealizedServices.GetOrAdd(serviceType, this._createServiceAccessor);
  this._callback?.OnResolve(serviceType, (IServiceScope) serviceProviderEngineScope);
  ServiceProviderEngineScope providerEngineScope = serviceProviderEngineScope;
  return orAdd(providerEngineScope);
}

　　最终就是调用Func<ServiceProviderEngineScope, object> orAdd = this.RealizedServices.GetOrAdd(serviceType, this._createServiceAccessor);其中RealizedServices是ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>> RealizedServices { get; }，this._createServiceAccessor是Func<ServiceProviderEngineScope, object>,通过这个委托根据ServiceProviderEngineScope获取最终的对象,也就是说，如果服务存在的话就从字典中获取，不存在就添加到队列上，并直接返回。另外在实例化ServiceProviderEngine的时候实例化了this._createServiceAccessor = new Func<Type, Func<ServiceProviderEngineScope, object>>(this.CreateServiceAccessor);其中CreateServiceAccessor是一个函数，定义如下：

private Func<ServiceProviderEngineScope, object> CreateServiceAccessor(
  Type serviceType)
{
  IServiceCallSite callSite = this.CallSiteFactory.CreateCallSite(serviceType, new CallSiteChain());
  if (callSite == null)
	return (Func<ServiceProviderEngineScope, object>) (_ => (object) null);
  this._callback?.OnCreate(callSite);
  return this.RealizeService(callSite);
}

　　就DynamicServiceProviderEngine而言，RealizeService的定义如下：

protected override Func<ServiceProviderEngineScope, object> RealizeService(
  IServiceCallSite callSite)
{
  int callCount = 0;
  return (Func<ServiceProviderEngineScope, object>) (scope =>
  {
	if (Interlocked.Increment(ref callCount) == 2)
	  Task.Run<Func<ServiceProviderEngineScope, object>>((Func<Func<ServiceProviderEngineScope, object>>) (() => base.RealizeService(callSite)));
	return this.RuntimeResolver.Resolve(callSite, scope);
  });
}

　　RuntimeResolver是ServiceProviderEngine的一个属性。到此为止，IOC容器的数据就可以获取到了。下篇博客会继续说明this.RuntimeResover.Resolve下的事情。