DOTNET CORE源码分析之IOC容器结果获取内容补充

　　补充一下ServiceProvider的内容

　　可能上一篇文章DOTNET CORE源码分析之IServiceProvider、ServiceProvider、IServiceProviderEngine、ServiceProviderEngine和ServiceProviderEngineScope 中还没有关联上ServiceProvider和ServiceCollection就直接通过GetService获取了值，这样不科学啊。其实是有关联的，请看一下上篇文章同样存在的一个代码段：

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");
  }
}

　　是的在这个构造函数中第一个参数：IEnumerable<ServiceDescriptor> serviceDescriptors，这个就是ServiceCollection的对应参数，这样，保存了用户添加的注入信息就和ServiceProviderEngine关联上了，然后ServiceProviderEngine可以为调用方提供数据，换句话说，ServiceProvider不是直接提供数据响应，而是借用ServiceProviderEngine的子类来提供，并且提供了ServiceProviderMode中提供的几种方式。

　　我们再看看ServiceProviderMode的构造函数，如下：

protected ServiceProviderEngine(
  IEnumerable<ServiceDescriptor> serviceDescriptors,
  IServiceProviderEngineCallback callback)
{
  this._createServiceAccessor = new Func<Type, Func<ServiceProviderEngineScope, object>>(this.CreateServiceAccessor);
  this._callback = callback;
  this.Root = new ServiceProviderEngineScope(this);
  this.RuntimeResolver = new CallSiteRuntimeResolver();
  this.CallSiteFactory = new CallSiteFactory(serviceDescriptors);
  this.CallSiteFactory.Add(typeof (IServiceProvider), (IServiceCallSite) new ServiceProviderCallSite());
  this.CallSiteFactory.Add(typeof (IServiceScopeFactory), (IServiceCallSite) new ServiceScopeFactoryCallSite());
  this.RealizedServices = new ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object>>();
}

　　这个过程做了很多初始化，我们把关注点集中在这一句：this.CallSiteFactory = new CallSiteFactory(serviceDescriptors)，这句把ServiceCollection传递给了CallSiteFactory。CallSiteFactory是创建ServiceCallSite的工厂，具体作用下如下代码中：

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);
}

　　这里的IServiceCallSite callSite = this.CallSiteFactory.CreateCallSite(serviceType, new CallSiteChain()) 函数CreateCallSite实现如下：

internal IServiceCallSite CreateCallSite(
      Type serviceType,
      CallSiteChain callSiteChain)
{
  lock (this._callSiteCache)
  {
	IServiceCallSite serviceCallSite1;
	if (this._callSiteCache.TryGetValue(serviceType, out serviceCallSite1))
	  return serviceCallSite1;
	IServiceCallSite serviceCallSite2;
	try
	{
	  callSiteChain.CheckCircularDependency(serviceType);
	  serviceCallSite2 = this.TryCreateExact(serviceType, callSiteChain) ?? this.TryCreateOpenGeneric(serviceType, callSiteChain) ?? this.TryCreateEnumerable(serviceType, callSiteChain);
	}
	finally
	{
	  callSiteChain.Remove(serviceType);
	}
	this._callSiteCache[serviceType] = serviceCallSite2;
	return serviceCallSite2;
  }
}

　　其中this.TryCreateExact(serviceType, callSiteChain) ?? this.TryCreateOpenGeneric(serviceType, callSiteChain) ?? this.TryCreateEnumerable(serviceType, callSiteChain) 这个返回值是带有结果的ServiceCallSite,然后通过这个ServiceCallSite获取最终结果。

　　接着介绍昨天没完成的内容

　　昨天介绍到这个获取服务的函数，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);
  });
}

　　关注点停留在this.RuntimeResolver.Resolve(callSite, scope)，这个函数在调用过程的时候带有两个参数callSite和scope,callSite就是刚刚介绍的ServiceCallSite，Scope是ServiceProviderEngineScope。RuntimeResolver是CallSiteRuntimeResolver的实例，Resolve方法如下：

public object Resolve(IServiceCallSite callSite, ServiceProviderEngineScope scope)
{
  return this.VisitCallSite(callSite, scope);
}

　　其中VisitCallSite就是：

protected virtual TResult VisitCallSite(IServiceCallSite callSite, TArgument argument)
{
  switch (callSite.Kind)
  {
	case CallSiteKind.Factory:
	  return this.VisitFactory((FactoryCallSite) callSite, argument);
	case CallSiteKind.Constructor:
	  return this.VisitConstructor((ConstructorCallSite) callSite, argument);
	case CallSiteKind.Constant:
	  return this.VisitConstant((ConstantCallSite) callSite, argument);
	case CallSiteKind.IEnumerable:
	  return this.VisitIEnumerable((IEnumerableCallSite) callSite, argument);
	case CallSiteKind.ServiceProvider:
	  return this.VisitServiceProvider((ServiceProviderCallSite) callSite, argument);
	case CallSiteKind.Scope:
	  return this.VisitScoped((ScopedCallSite) callSite, argument);
	case CallSiteKind.Transient:
	  return this.VisitTransient((TransientCallSite) callSite, argument);
	case CallSiteKind.CreateInstance:
	  return this.VisitCreateInstance((CreateInstanceCallSite) callSite, argument);
	case CallSiteKind.ServiceScopeFactory:
	  return this.VisitServiceScopeFactory((ServiceScopeFactoryCallSite) callSite, argument);
	case CallSiteKind.Singleton:
	  return this.VisitSingleton((SingletonCallSite) callSite, argument);
	default:
	  throw new NotSupportedException(string.Format("Call site type {0} is not supported", (object) callSite.GetType()));
  }
}

　　然后：

protected override object VisitConstant(
      ConstantCallSite constantCallSite,
      ServiceProviderEngineScope scope)
{
    return constantCallSite.DefaultValue;
}

　　这个constantCallSite.Defaultvalue就是需要的返回结果了。

　　上述的源码过程解析如下代码：

IServiceCollection serviceCollection = new ServiceCollection();
serviceCollection.AddSingleton(typeof(string), "lizizhang");
ServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();
string name = (string)serviceProvider.GetService(typeof(string));

　　如果错误之处，请及时指正！