Locator interceptors

[spion-h4]

In order to add features such as logging and tracing, it would be nice if h4bff supported interceptors for the locator.

Motivation:

Lets say you would like to implement an XRay tracer that will give you a listing of all the service method calls for a request. You've made a tracing proxy that wraps a service method calls with the right xray xray logging calls. How to apply it to all service instances?

Sketch API / motivation

app.addServiceInterceptor((ServiceClass, oldGetter) => { 
  if (ServiceClass.traced || TracedClasses.has(ServiceClass)) { 
    return xrayProxy(oldGetter(ServiceClass)); 
  } 
  else { return oldGetter(ServiceClass); } 
})

This would wrap all services annotated with a "traced" property (or those explicitly added to the "TracedClasses" map) with a tracing proxy. Its sort of like a middleware system, but not to be confused with middleware - MW pertains to individual client requests (one per service context) while interceptors are for each service instantiation.

[spion-h4]

The interceptor is general enough to actually implement overrides using it, instead of having it as a primitive:

app.addServiceInterceptor((ServiceClass, oldGetter) => { 
  if (overrides.has(ServiceClass)) { 
    let NewClass = overrides.get(ServiceClass); 
    return oldGetter(NewClass); 
  }
  else { return oldGetter(ServiceClass); } 
})

[spion-h4]

see https://github.com/inversify/InversifyJS/blob/master/wiki/middleware.md