asp.net web开发 ASP.NET Core Web API下事件驱动型架构的实(9)

以下是基于Microsoft.Extensions.DependencyInjection框架来实现的事件处理器执行上下文完整代码，这里有个兼容性问题，就是构造函数的第二个参数：serviceProviderFactory。在Microsoft.Extensions.DependencyInjection框架2.0版本之前，IServiceCollection.BuildServiceProvider方法的返回类型是IServiceProvider，但从2.0开始，它的返回类型已经从IServiceProvider接口，变成了ServiceProvider类。这里引出了框架设计的另一个原则，就是依赖较低版本的.NET Core，以便获得更好的兼容性。如果我们的EdaSample是使用.NET Core 1.1开发的，那么当下面这个类被直接用在ASP.NET Core 2.0的项目中时，如果不通过构造函数参数传入ServiceProvider创建委托，而是直接在代码中使用registry.BuildServiceProvider调用，就会出现异常。asp.net web开发

public class EventHandlerExecutionContext : IEventHandlerExecutionContext

{

private readonly IServiceCollection registry;

private readonly Func<IServiceCollection, IServiceProvider> serviceProviderFactory;

private readonly ConcurrentDictionary<Type, List<Type>> registrations = new ConcurrentDictionary<Type, List<Type>>();

public EventHandlerExecutionContext(IServiceCollection registry,

Func<IServiceCollection, IServiceProvider> serviceProviderFactory = null )

{

this .registry = registry;

this .serviceProviderFactory = serviceProviderFactory ?? (sc => registry.BuildServiceProvider());

}

public async Task HandleEventAsync(IEvent @ event , CancellationToken cancellationToken = default (CancellationToken))

{

var eventType = @ event .GetType();

if ( this .registrations.TryGetValue(eventType, out List<Type> handlerTypes) &&

handlerTypes?.Count > 0)

{

var serviceProvider = this .serviceProviderFactory( this .registry);

using ( var childScope = serviceProvider.CreateScope())

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