Context

Since  menu  type plugins are siblings in the DOMTree, it is not possible for menu  type plugins to communicate with each other. Next to the menu  type plugin, it is not possible to communicate from an editor  type plugin to another editor  type plugin or a menu  type plugin.

Also, since Addons are not extending from a HTMLElement, Addons cannot subscribe to events on themselves.

HTML Events are only going up in the DOMTree, not down and/or to siblings.

Decision

If we want plugins for OpenSCD to be asynchronous accessible to each other, OpenSCD Core should make use of a central event-bus.
Instead of dispatching an event on the plugin itself, the plugin can dispatch an event on the central event-bus. This will take care of the current problems we're facing:

Addons 

```ts
export  class EventBus implements EventTarget {

    private  _eventListeners: Map<string, EventListenerOrEventListenerObject[]>;

    constructor() {
        this._eventListeners = new  Map<string, EventListenerOrEventListenerObject[]>();
    }

    addEventListener(type: string, callback: EventListenerOrEventListenerObject): void {
        if (!this._eventListeners.has(type)) {
            this._eventListeners.set(type, []);
        }

        this._eventListeners.get(type)!.push(callback);
    }
    
    dispatchEvent(event: Event): boolean {
        if (this._eventListeners.has(event.type)) {
            this._eventListeners
                .get(event.type)
                !.forEach((cb) =>
                    typeof  cb === 'function' ? cb(event) : cb.handleEvent(event)
                );

            return  true;
        }

        return  false;
    }

    removeEventListener(type: string, callback: EventListenerOrEventListenerObject): void {
        if (this._eventListeners.has(type)) {
            this._eventListeners.set(type, this._eventListeners.get(type)!.filter((cb) =>  cb !== callback));
        }
    }

}
```

Alternatives

Find a (seperate solution) for all problems mentioned.

Pro's:

Con's

Consequences

Implementing a central event-bus means that current OSCD plugins need some refactoring to dispatch events to the event-bus instead of dispatching events on itself.