A callback is used to receive notifications of signal or status changes for a service.

When a subscriber is registered with a context, it will be given notice of a created service. At this point, the subscriber can subscribe using a callback to receive future signals and status changes for the particular service.

The callback interface can also be used to record outcomes for templated utility methods in the library, restricting the callback just to just one particular signaling code block.

The single operation in the callback interface accepts two parameters: