* Event - An Event is a message produced by an Emitter. Usually denoting a [state](https://en.wikipedia.org/wiki/State_(computer_science)) change.
* Listener - A Listener is any [PHP callable](https://www.php.net/manual/en/language.types.callable.php) that expects to
be passed an Event. Zero or more Listeners may be passed the same Event. A Listener MAY enqueue some other asynchronous
behavior if it so chooses.
* Emitter - An Emitter is any arbitrary code that wishes to dispatch an Event. This is also known as the "calling code".
* Dispatcher - The Dispatcher is given an Event by an Emitter. The Dispatcher is responsible for ensuring that the Event
is passed to all relevant Listeners.
Pattern
-------
We implement the [Observer pattern](https://en.wikipedia.org/wiki/Observer_pattern) using the [Mediator pattern](https://en.wikipedia.org/wiki/Mediator_pattern).
The key is that the emitter should not know what is listening to its events. The dispatcher avoids modules communicate
directly but instead through a mediator. This helps the [Single Responsibility principle](https://en.wikipedia.org/wiki/Single-responsibility_principle)
by allowing communication to be offloaded to a class that just handles communication.
How does it work? The *dispatcher*, the central object of the event dispatcher system, notifies *listeners* of an *event*
dispatched to it. Put another way: your code dispatches an event to the dispatcher, the dispatcher notifies all registered
listeners for the event, and each listener does whatever it wants with the event.
Example 1: Adding elements to the core UI
-------
> An emitter in a core twig template
```html
{% for block in handle_event('ViewAttachment' ~ attachment.getMimetypeMajor() | capitalize , {'attachment': attachment, 'thumbnail_parameters': thumbnail_parameters}) %}
{{ block | raw }}
{% endfor %}
```
> Listener
```php
/**
* Generates the view for attachments of type Image
*
*@param array $vars Input from the caller/emitter
*@param array $res I/O parameter used to accumulate or return values from the listener to the emitter
*
*@return bool true if not handled or if the handling should be accumulated with other listeners,
* false if handled well enough and no other listeners are needed
*/
public function onViewAttachmentImage(array $vars, array &$res): bool
