[asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（4）

這個系列已經寫了6篇，鏈接地址如下：

[Asp.net 5] DependencyInjection項目代碼分析

[Asp.net 5] DependencyInjection項目代碼分析2-Autofac

[Asp.net 5] DependencyInjection項目代碼分析3-Ninject

[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（1）

[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（2）

[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（3）

如果想對本篇有個更好的了解，建議需要先看

“[Asp.net 5] DependencyInjection項目代碼分析”

“[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（1）”

“[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（2）”。

"[Asp.net 5] DependencyInjection項目代碼分析4-微軟的實現（3）"

繼續ServicePRovider類

在之前的講解中我們提到過Service類調用CreateCallSite方法時會遞歸調用，但是我們沒具體說明如何遞歸調的。實際上Service類，通過反射創建實例的時候，會實例化的參數對象，而實例話參數對象通過ServiceProvider類創建，而ServiceProvider類創建參數的實例，又需要通過Service類（如果是通過Type注冊的）創建。下面我們把Service的CreateInstanceCallSite方法以及ServiceProvider相關的方法列出來。

public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain)        {            ConstructorInfo[] constructors = _descriptor.ImplementationType.GetTypeInfo()                .DeclaredConstructors                .Where(IsInjectable)                .ToArray();            // TODO: actual service-fulfillment constructor selection            if (constructors.Length == 1)            {                ParameterInfo[] parameters = constructors[0].GetParameters();                IServiceCallSite[] parameterCallSites = new IServiceCallSite[parameters.Length];                for (var index = 0; index != parameters.Length; ++index)                {                    parameterCallSites[index] = provider.GetServiceCallSite(parameters[index].ParameterType, callSiteChain);                                    if (parameterCallSites[index] == null && parameters[index].HasDefaultValue)                    {                        parameterCallSites[index] = new ConstantCallSite(parameters[index].DefaultValue);                    }                    if (parameterCallSites[index] == null)                    {                        throw new InvalidOperationException(Resources.FormatCannotResolveService(                                parameters[index].ParameterType,                                 _descriptor.ImplementationType));                    }                }                return new ConstructorCallSite(constructors[0], parameterCallSites);            }            return new CreateInstanceCallSite(_descriptor);        }

internal IServiceCallSite GetServiceCallSite(Type serviceType, ISet<Type> callSiteChain)        {            try            {                if (callSiteChain.Contains(serviceType))                {                    throw new InvalidOperationException(Resources.FormatCircularDependencyException(serviceType));                }                callSiteChain.Add(serviceType);                ServiceEntry entry;                if (_table.TryGetEntry(serviceType, out entry))                {                    return GetResolveCallSite(entry.Last, callSiteChain);                }                object emptyIEnumerableOrNull = GetEmptyIEnumerableOrNull(serviceType);                if (emptyIEnumerableOrNull != null)                {                    return new EmptyIEnumerableCallSite(serviceType, emptyIEnumerableOrNull);                }                return null;            }            finally            {                callSiteChain.Remove(serviceType);            }        }        internal IServiceCallSite GetResolveCallSite(IService service, ISet<Type> callSiteChain)        {            IServiceCallSite serviceCallSite = service.CreateCallSite(this, callSiteChain);            if (service.Lifetime == ServiceLifetime.Transient)            {                return new TransientCallSite(serviceCallSite);            }            else if (service.Lifetime == ServiceLifetime.Scoped)            {                return new ScopedCallSite(service, serviceCallSite);            }            else            {                return new SingletonCallSite(service, serviceCallSite);            }        }

對于Service的CreateCallSite方法，之前我們已經介紹過，現在我們重點講下ServiceProvider的GetServiceCallSite方法。從上面代碼中我們發現參數中含有“ISet<Type> callSiteChain”，這個參數是防止發生A的構造函數有B類型參數，B的構織函數中有A類型參數，當A，B都是通過類型注入的，那么系統會陷入死循環。而callSiteChain得作用就是防止這樣的死循環發生，當創建A時，會在callSiteChain中查詢歷史中是否有A的創建過程，如果有則說明發生死循環了，直接拋出異常，結束；如果沒有將A加入到callSiteChain中，繼續創建其參數。GetResolveCallSite方法比較簡單，對于ServiceProvider已經能夠獲取的IServiceCallSite實例，進行包裝，已保證生成的實例能夠適應不同的Scoped(該處應該使用設計模式中的代理模式，不過我設計模式不過關，請幫忙確認)。

對于TransientCallSite、ScopedCallSite、SingletonCallSite以及EmptyIEnumerableCallSite代碼，如下所示：

 private class EmptyIEnumerableCallSite : IServiceCallSite        {            private readonly object _serviceInstance;            private readonly Type _serviceType;            public EmptyIEnumerableCallSite(Type serviceType, object serviceInstance)            {                _serviceType = serviceType;                _serviceInstance = serviceInstance;            }            public object Invoke(ServiceProvider provider)            {                return _serviceInstance;            }            public Expression Build(Expression provider)            {                return Expression.Constant(_serviceInstance, _serviceType);            }        }        private class TransientCallSite : IServiceCallSite        {            private readonly IServiceCallSite _service;            public TransientCallSite(IServiceCallSite service)            {                _service = service;            }            public object Invoke(ServiceProvider provider)            {                return provider.CaptureDisposable(_service.Invoke(provider));            }            public Expression Build(Expression provider)            {                return Expression.Call(                    provider,                    CaptureDisposableMethodInfo,                    _service.Build(provider));            }        }        private class ScopedCallSite : IServiceCallSite        {            private readonly IService _key;            private readonly IServiceCallSite _serviceCallSite;            public ScopedCallSite(IService key, IServiceCallSite serviceCallSite)            {                _key = key;                _serviceCallSite = serviceCallSite;            }            public virtual object Invoke(ServiceProvider provider)            {                object resolved;                lock (provider._sync)                {                    if (!provider._resolvedServices.TryGetValue(_key, out resolved))                    {                        resolved = provider.CaptureDisposable(_serviceCallSite.Invoke(provider));