文章出處

到目前為止，我們定義的ServiceProvider已經實現了基本的服務提供和回收功能，但是依然漏掉了一些必需的細節特性。這些特性包括如何針對IServiceProvider接口提供一個ServiceProvider對象，何創建ServiceScope，以及如何提供一個服務實例的集合。

一、提供一個ServiceProvider對象

我們知道當將服務類型指定為IServiceProvider接口并調用ServiceProvider的GetService方法是，ServiceProvider對象本身將會作為服務實例返回，這個特性可以利用一個自定義的Service來實現。如下面的代碼片段所示，我們定義的這個ServiceProviderService既是一個Service，又是一個ServiceCallSite。它默認采用生命周期管理模式為Scoped，在Invoke和Build方法中，它直接將當前ServiceProvider作為提供的服務實例。在初始化ServiceTable的時候，我們額外添加一個針對ServiceProviderService的ServideEntry。

   1: internal class ServiceProviderService : IService, IServiceCallSite   2: {   3:     public ServiceLifetime Lifetime => ServiceLifetime.Scoped;   4:     public IService Next { get; set; }   5:     6:     public Expression Build(Expression provider)   7:     {   8:         return provider;   9:     }  10:    11:     public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain)  12:     {  13:         return this;  14:     }  15:    16:     public object Invoke(ServiceProvider provider)  17:     {  18:         return provider;  19:     }  20: }  21:    22: internal class ServiceTable  23: {  24:     public ServiceTable(IServiceCollection services)  25:     {  26:         //解析ServiceCollection并添加相應ServiceEntry  27:         this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());  28:     }  29: }

二、創建ServiceScope

創建ServiceScope的目的在于創建作為當前ServiceProvider兒子的另一個ServiceProvider，新創建的ServiceProvider不僅與原來的ServiceProvider具有相同的根，同時共享所有的服務注冊信息。利用這個新的ServiceProvider來代替現有的ServiceProvider，其主要的目的還是使我們能夠及時地回收提供的服務實例。ServiceScope是通過它的工廠ServiceScopeFactory來創建的，所以先創建了如下一個ServiceScopeFactory類和對應的ServiceScope，它們的定義與我們在前面一節介紹的完全一致。

   1: internal class ServiceScope : IServiceScope   2: {   3:     public IServiceProvider ServiceProvider { get; private set; }   4:     5:     public ServiceScope(ServiceProvider serviceProvider)   6:     {   7:         this.ServiceProvider = serviceProvider;   8:     }   9:    10:     public void Dispose()  11:     {  12:         (this.ServiceProvider as IDisposable)?.Dispose();  13:     }  14: }  15:    16: internal class ServiceScopeFactory : IServiceScopeFactory  17: {  18:     public ServiceProvider ServiceProvider { get; private set; }  19:    20:     public ServiceScopeFactory(ServiceProvider serviceProvider)  21:     {  22:         this.ServiceProvider = serviceProvider;  23:     }  24:    25:     public IServiceScope CreateScope()  26:     {  27:         return new ServiceScope(this.ServiceProvider);  28:     }  29: }  30:    31: internal class ServiceProvider : IServiceProvider, IDisposable  32: {  33:       34:     public ServiceProvider(ServiceProvider parent)  35:     {  36:         this.Root = parent.Root;  37:         this.ServiceTable = parent.ServiceTable;  38:     }  39: }

為了讓ServiceProvider的GetService方法在服務類型指定為IServiceScopeFactory接口的時候能夠自動返回上面我們定義的ServiceScopeFactory對象，我們依然和上面一樣創建了一個自定義的Service，并將其命名為ServiceScopeFactoryService。與ServiceProviderService一樣，ServiceScopeFactoryService同時也是一個ServiceCallSite，在Build和Invoke方法中它會返回一個ServiceScopeFactory對象。為了讓這個它能夠生效，我們依然在ServiceTable初始化的時自動添加一個相應的ServiceEntry。

   1: internal class ServiceScopeFactoryService : IService, IServiceCallSite   2: {   3:     public ServiceLifetime Lifetime=> ServiceLifetime.Scoped;   4:     public IService Next { get; set; }   5:     6:     public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain)   7:     {   8:         return this;   9:     }  10:    11:     public Expression Build(Expression provider)  12:     {  13:         return Expression.New(typeof(ServiceScopeFactory).GetConstructors().Single(), provider);  14:     }  15:    16:     public object Invoke(ServiceProvider provider)  17:     {  18:         return new ServiceScopeFactory(provider);  19:     }  20: }  21:    22: internal class ServiceTable  23: {  24:     public ServiceTable(IServiceCollection services)  25:     {  26:         //解析ServiceCollection并添加相應ServiceEntry  27:         this.ServieEntries[typeof(IServiceProvider)] =  new ServiceEntry(new ServiceProviderService());  28:         this.ServieEntries[typeof(IServiceScopeFactory)] = new ServiceEntry(new ServiceScopeFactoryService());  29:     }  30: }

三、提供一組服務的集合

到目前為止，我們自定義的ServiceProvider尚不具備原生ServiceProvider的一項特性，那就是當調用GetService方法時將服務類型指定為IEnumerable<T>或者直接調用擴展方法GetServices時，得到的是一個服務實例的集合。這個特性可以通過一個自定義的ServiceCallSite來完成，我們將其命名為EnumerableCallSite。

   1: internal class EnumerableCallSite : IServiceCallSite   2: {   3:     public Type ElementType { get; private set; }   4:     public IServiceCallSite[] ServiceCallSites { get; private set; }   5:     6:     public EnumerableCallSite(Type elementType, IServiceCallSite[] serviceCallSites)   7:     {   8:         this.ElementType = elementType;   9:         this.ServiceCallSites = serviceCallSites;  10:     }  11:    12:     public Expression Build(Expression provider)  13:     {  14:         return Expression.NewArrayInit(this.ElementType, this.ServiceCallSites.Select(  15:             it => Expression.Convert(it.Build(provider), this.ElementType)));  16:     }  17:    18:     public object Invoke(ServiceProvider provider)  19:     {  20:         var array = Array.CreateInstance(this.ElementType, this.ServiceCallSites.Length);  21:         for (var index = 0; index < this.ServiceCallSites.Length; index++)  22:         {  23:             array.SetValue(this.ServiceCallSites[index].Invoke(provider), index);  24:         }  25:         return array;  26:     }  27: }

如上面的代碼片段所示，EnumerableCallSite具有兩個兩個只讀屬性（ElementType和ServiceCallSites），前者表示返回的服務集合的元素類型，后者則返回一組用于提供集合元素的ServiceCallSite。在Invoke和Build方法中，我們只需要根據元素類型創建一個數組，并利用這組ServiceCallSite創建所有的元素即可。這個EnumerableCallSite最終按照如下的方式應用到ServiceProvider的GetServiceCallSite方法中。

   1: internal class ServiceProvider : IServiceProvider, IDisposable   2: {    3:     public IServiceCallSite GetServiceCallSite(Type serviceType, ISet<Type> callSiteChain)   4:     {   5:         try   6:         {   7:             if (callSiteChain.Contains(serviceType))   8:             {   9:                 throw new InvalidOperationException(string.Format("A circular dependency was detected for the service of type '{0}'",serviceType.FullName);  10:             }  11:             callSiteChain.Add(serviceType);  12:             ServiceEntry serviceEntry;  13:             if (this.ServiceTable.ServieEntries.TryGetValue(serviceType, out serviceEntry))  14:             {  15:                 return serviceEntry.Last.CreateCallSite(this, callSiteChain);  16:             }  17:    18:             if (serviceType.IsGenericType && serviceType.GetGenericTypeDefinition()== typeof(IEnumerable<>))  19:             {  20:                 Type elementType = serviceType.GetGenericArguments()[0];  21:                 IServiceCallSite[] serviceCallSites = this.ServiceTable.ServieEntries.TryGetValue(elementType, out serviceEntry)  22:                     ? serviceEntry.All.Select(it => it.CreateCallSite(this, callSiteChain)).ToArray()  23:                     : new IServiceCallSite[0];  24:                 return new EnumerableCallSite(elementType, serviceCallSites);  25:             }  26:    27:             return null;  28:         }  29:         finally  30:         {  31:             callSiteChain.Remove(serviceType);  32:         }  33:     }  34:     //其他成員  35: }