我正在尝试使用装饰器来包装Web API控制器(IHttpController实现),但是当我这样做时,Web API会抛出异常,因为不知何故它会期待实际的实现.
将装饰器应用于控制器是我成功应用于MVC控制器的一个技巧,我显然希望在Web API中也这样做.
我创建了一个自定义的IHttpControllerActivator,它允许解析装饰的IHttpController实现.这是一个剥离的实现：
public class CrossCuttingConcernHttpControllerActivator : IHttpControllerActivator {
private readonly Container container;
public CrossCuttingConcernHttpControllerActivator(Container container) {
this.container = container;
}
public IHttpController Create(HttpRequestMessage request,HttpControllerDescriptor controllerDescriptor,Type controllerType)
{
var controller = (IHttpController)this.container.GetInstance(controllerType);
// Wrap the instance in one or multiple decorators. Note that in reality,the
// decorator is applied by the container,but that doesn't really matter here.
return new MyHttpControllerDecorator(controller);
}
}
我的装饰师看起来像这样：
public class MyHttpControllerDecorator : IHttpController {
private readonly IHttpController decoratee;
public MyHttpControllerDecorator(IHttpController decoratee) {
this.decoratee = decoratee;
}
public Task<HttpResponseMessage> ExecuteAsync(
HttpControllerContext controllerContext,CancellationToken cancellationToken)
{
// this decorator does not add any logic. Just the minimal amount of code to
// reproduce the issue.
return this.decoratee.ExecuteAsync(controllerContext,cancellationToken);
}
}
但是,当我运行我的应用程序并请求ValuesController时,Web API会抛出以下InvalidCastException：
Unable to cast object of type ‘WebApiTest.MyHttpControllerDecorator’
to type ‘WebApiTest.Controllers.ValuesController’.
堆栈跟踪：
at lambda_method(Closure,Object,Object[] ) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass13.<GetExecutor>b__c(Object instance,Object[] methodParameters) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.Execute(Object instance,Object[] arguments) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.<>c__DisplayClass5.<ExecuteAsync>b__4() at System.Threading.Tasks.TaskHelpers.RunSynchronously[TResult](Func`1 func,CancellationToken cancellationToken)
这就好像Web API为我们提供了IHttpController抽象,但跳过了它仍然依赖于实现本身.这当然会严重违反依赖性倒置原则,并使抽象完全无用.所以我可能会做错事.
我做错了什么？我怎样才能愉快地装饰我的API控制器？
解决方法
我会说,如何在ASP.NET Web API中实现此行为的自然设计方式是使用
Custom Message Handlers / Delegation Handlers
例如,我确实有这个DelegationHandler
public class AuthenticationDelegationHandler : DelegatingHandler
{
protected override System.Threading.Tasks.Task<HttpResponseMessage>
SendAsync(HttpRequestMessage request,CancellationToken cancellationToken)
{
// I. do some stuff to create Custom Principal
// e.g.
var principal = CreatePrincipal();
...
// II. return execution to the framework
return base.SendAsync(request,cancellationToken).ContinueWith(t =>
{
HttpResponseMessage resp = t.Result;
// III. do some stuff once finished
// e.g.:
// SetHeaders(resp,principal);
return resp;
});
}
这就是如何将其注入结构：
public static class WebApiConfig
{
public static void Register(HttpConfiguration config)
{
config.MessageHandlers.Add(new AuthenticationDelegationHandler());
