[Proposal] Forbid usage of the MediaKeys type and other EME TS types

[peaBerberian]

Based on #1397.

In the idea of including fake encrypted contents in our integration tests by mocking MSE+EME+HTML 5 media API (not as complex as it sounds) to increase by a lot the types of issues our CI is capable to catch, I noticed an opportunity to even improve on the current code.

Like in #1397, the idea is there to provide typing subsets for the various EME API and to rely on them in the RxPlayer code (and enforcing this through our linter).

This allows to:

• much simplify EME API mocking by not having to implement the full extent of the EME API (though what is missing in the custom types are very minor here: the EventTarget's third options optional parameter which we never use, dispatchEvent, and the onevent methods that we never rely on).

• Allow the definition of environment-specific APIs - such as webkit-vendored API

• Be more aware of which EME API we currently use

The gain is here much more evident than in #1397 as we already had some kind of subtyping with the ICustom... types (e.g. ICustomMediaKeys). By renaming those I... (e.g. IMediaKeys) and ensuring they are actually compatible with the base type (e.g. MediaKeys), we end up in my opinion with simpler code.

[Florent-Bouisset]

I'm wondering if we can type onSessionRelatedEvent with MediaKeyMessageEvent | Event

[peaBerberian]

line 84? What would that change?

[Florent-Bouisset]

I mean events like "keymessage", "message", "keyadded", "ready", "keyerror", "error" seems to be typed as a generic Event but it can be more precize with a type MediaKeyMessageEvent.

I don't really understand why typescript correctly show an error for

 this.trigger("message", evt);

but does not trigger an error for

this.trigger(evt.type as keyof MediaKeySessionEventMap, evt);

[peaBerberian]

I don't know how to fix this, if you have an idea, you can commit it

[Florent-Bouisset]

I'm not sure about how is it supposed to work on the "old webkit", from what I read, we can have an event
triggered from error with a type that is different, it seems weird ?

mediaElement.addEventListener("error", (event) => event.type /* keystatuseschange*/)

[peaBerberian]

? How is it possible? Where did you see that possibility?

[Florent-Bouisset]

I thought that event "type" was more precised in typescript but it's always a string
In fact what we can do is:

    const onSessionRelatedEvent = (evt: MediaKeyMessageEvent | Event) => {
      this.trigger(evt.type, evt);
    };

but I'm not sure if it's better than casting

[peaBerberian]

Yes I'm kind of lost on that one.

In any case, the compat EME files are the "ugliest" TypeScript files in the RxPlayer to me but it makes sense that it is (due to the beauty of EME implementations out there).