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。
