Applying multiple OpenRasta OperationInterceptors

OpenRasta has the concept on an OperationInterceptor, a special class that runs before and after your handler method is called. It’s useful when you have a cross-cutting action that should apply across all your resources. A couple of typical examples are:

1. Validation, when you need to validate the incoming resource object on a Put or Post
2. Authorization, when you need to check that the authenticate user has rights to perform this action on the resource.

So you create a subtype of OperationInterceptor, and you hook it up in your IConfigurationSource like this:

ResourceSpace.Uses
    .CustomDependency<IOperationInterceptor, MyOperationInterceptor>
        (DependencyLifetime.Transient);

Unfortunately, OpenRasta only supports one OperationInterceptor. That makes the OpenRasta API much simpler: if OpenRasta supported multiple interceptors, it would need a mechanism for ordering each of the OperationInterceptor instances that you pass it.

Thanksfully it’s easy to get around because we can use create a CompositeOperationInterceptor that does our ordering for us. It’s dead simple and you’ll find the code below. This’ll work if you’ve got a ValidationInterceptor and an AuthorizationInterceptor (you’ll need to write these yourself, of course).

public class CompositeOperationInterceptor : OperationInterceptor
{
    readonly IEnumerable<OperationInterceptor> _interceptors;
    public CompositeOperationInterceptor(IDependencyResolver resolver)
    {
        _interceptors = new List<OperationInterceptor> {
            resolver.Resolve(typeof(ValidationInterceptor))
                                     as OperationInterceptor,
            resolver.Resolve(typeof(AuthorizationInterceptor))
                                    as OperationInterceptor
        };
    }
    public override bool BeforeExecute(IOperation operation)
    {
        return _interceptors.All(i => i.BeforeExecute(operation));
    }
     public override bool AfterExecute(IOperation operation,
                      IEnumerable<OutputMember> outputMembers)
    {
        return _interceptors.All(i => i.AfterExecute(operation, outputMembers));
    }

}

And in your IConfigurationSource you need to hook up your interceptors:

ResourceSpace.Uses.
    CustomDependency<ValidationInterceptor, ValidationInterceptor>
                                  (DependencyLifetime.Transient);
ResourceSpace.Uses.
    CustomDependency<AuthorizationInterceptor, AuthorizationInterceptor>
                                  (DependencyLifetime.Transient);
ResourceSpace.Uses
     .CustomDependency<IOperationInterceptor, CompositeOperationInterceptor>
                                  (DependencyLifetime.Transient);

Easy

Just on a final note: the exclusion of this feature in OpenRasta is a great example of API design. It keeps the design simple, minimises the API surface area and yet doesn’t limit functionality. Nice.