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[ASP.net教程]ASP.NET Core中的依赖注入(5):ServicePrvider实现揭秘【补充漏掉的细节】


到目前为止,我们定义的ServiceProvider已经实现了基本的服务提供和回收功能,但是依然漏掉了一些必需的细节特性。这些特性包括如何针对IServiceProvider接口提供一个ServiceProvider对象,何创建ServiceScope,以及如何提供一个服务实例的集合。

一、提供一个ServiceProvider对象

我们知道当将服务类型指定为IServiceProvider接口并调用ServiceProvider的GetService方法是,ServiceProvider对象本身将会作为服务实例返回,这个特性可以利用一个自定义的Service来实现。如下面的代码片段所示,我们定义的这个ServiceProviderService既是一个Service,又是一个ServiceCallSite。它默认采用生命周期管理模式为Scoped,在Invoke和Build方法中,它直接将当前ServiceProvider作为提供的服务实例。在初始化ServiceTable的时候,我们额外添加一个针对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并添加相应ServiceEntry
 27:     this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());
 28:   }
 29: }



二、创建ServiceScope

创建ServiceScope的目的在于创建作为当前ServiceProvider儿子的另一个ServiceProvider,新创建的ServiceProvider不仅与原来的ServiceProvider具有相同的根,同时共享所有的服务注册信息。利用这个新的ServiceProvider来代替现有的ServiceProvider,其主要的目的还是使我们能够及时地回收提供的服务实例。ServiceScope是通过它的工厂ServiceScopeFactory来创建的,所以先创建了如下一个ServiceScopeFactory类和对应的ServiceScope,它们的定义与我们在前面一节介绍的完全一致。

  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: }


为了让ServiceProvider的GetService方法在服务类型指定为IServiceScopeFactory接口的时候能够自动返回上面我们定义的ServiceScopeFactory对象,我们依然和上面一样创建了一个自定义的Service,并将其命名为ServiceScopeFactoryService。与ServiceProviderService一样,ServiceScopeFactoryService同时也是一个ServiceCallSite,在Build和Invoke方法中它会返回一个ServiceScopeFactory对象。为了让这个它能够生效,我们依然在ServiceTable初始化的时自动添加一个相应的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并添加相应ServiceEntry
 27:     this.ServieEntries[typeof(IServiceProvider)] = new ServiceEntry(new ServiceProviderService());
 28:     this.ServieEntries[typeof(IServiceScopeFactory)] = new ServiceEntry(new ServiceScopeFactoryService());
 29:   }
 30: }



三、提供一组服务的集合

到目前为止,我们自定义的ServiceProvider尚不具备原生ServiceProvider的一项特性,那就是当调用GetService方法时将服务类型指定为IEnumerable<T>或者直接调用扩展方法GetServices时,得到的是一个服务实例的集合。这个特性可以通过一个自定义的ServiceCallSite来完成,我们将其命名为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: }


如上面的代码片段所示,EnumerableCallSite具有两个两个只读属性(ElementType和ServiceCallSites),前者表示返回的服务集合的元素类型,后者则返回一组用于提供集合元素的ServiceCallSite。在Invoke和Build方法中,我们只需要根据元素类型创建一个数组,并利用这组ServiceCallSite创建所有的元素即可。这个EnumerableCallSite最终按照如下的方式应用到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:   //其他成员
 35: }


 

ASP.NET Core中的依赖注入(1):控制反转(IoC)
ASP.NET Core中的依赖注入(2):依赖注入(DI)
ASP.NET Core中的依赖注入(3):服务注册与提取
ASP.NET Core中的依赖注入(4):构造函数的选择与生命周期管理
ASP.NET Core中的依赖注入(5):ServicePrvider实现揭秘【总体设计】
ASP.NET Core中的依赖注入(5):ServicePrvider实现揭秘【解读ServiceCallSite】
ASP.NET Core中的依赖注入(5):ServicePrvider实现揭秘【补充漏掉的细节】