Reflection overview

What is reflection?

Reflection allows a program to inspect itself and/or modify itself. Although reflection is a concept mostly related to programming languages, however is still useful in frameworks like Tappin.

For example, HTTP module uses reflection to detect all controllers/routes that have specific metadata.

Metadata

Every service or a module can have some metadata. It is set using set function on ModuleDsl or ServiceDsl.

Using reflection

Reflection is accessed using reflectModule it is imported from reflect package. You inject reflectService with typings available in ReflectService interface.

ReflectService has three functions:

Example: Getting all services that have Symbol("discoverable") set to true

const discoverableKey = Symbol("discoverable");

const discoverableService = createService((dsl) =>
  dsl.set(discoverableKey, true)
);

const discoverer = createService((dsl) =>
  dsl
    .inject(reflectService)
    .provide((reflect: ReflectService) => {
      const discoverable = reflect.getServices((service) =>
        service.metadata.get(discoverableKey) === true
      );

      return {
        discoverable,
      };
    })
);