Yet another subscription API

As requested here I wrote a new IDL for yet another variant of the subscription API. Please see the following Web IDL:

[Exposed=Window]
interface ConsumedThing {
  /*... other methods */
  ObservationHandle createObservation(DOMString name, optional InteractionOptions options = null);
  SubscriptionHandle createSubscription(DOMString name, optional InteractionOptions options = null);
};


[Exposed=Window]
interface AffordanceHandle : EventTarget {
     attribute EventHandler onerror;
};

[Exposed=Window]
interface SubscriptionHandle : AffordanceHandle {
     undefined subscribeEvent();
     undefined unsubscribeEvent();
     attribute EventHandler onunsubscribeevent;
     attribute EventHandler onevent;
     attribute EventHandler onsubscribeevent;
};

[Exposed=Window]
interface ObservationHandle : AffordanceHandle {
     undefined observeProperty();
     undefined unobserveProperty();
     attribute EventHandler onpropertychange;
     attribute EventHandler onobserveproperty;
     attribute EventHandler onunobserveproperty;
};

The design idea is to re-use as much as we can what is already out there. Therefore, here we are using EventTarget and EventHandler.

Pros

• Compliant to [Web Platform Design Principles]https://w3ctag.github.io/design-principles/#dont-invent-event-like
• no callback - promise duality
• In the future, we could adapt this pattern also for actions. For example, if we find a way to define long-standing actions in the TD, we could define an ActionHandle
• It still evokes TD basic operation verbs (i.e. observeProperty, onevent)

Cons

• More verbose
• Harder error handling for subscribe, unsubscribe, observe, and unobserve (i.e. you can't try {}catch{} but you have to use the EventHandler

What do you think? 🤔

Start by reading the linked pull request and its referenced discussion, then compare this proposed Web IDL with the subscription API already described in the project. The issue identifies no implementation files or tests; done would require an agreed API design and the corresponding project changes.