Current usage of events in Tuleap presents some drawbacks:

* Developers must carefully read the Event::XXX docblock to know what exist as parameters (when docblock exists and when docblock is still accurate),
* Plugins can sometimes erase data in the array parameters (data given by the core, or data given by previous listeners),
* Need to add variables' docblock to know that $params['group'] is of type Project,
* Lack of navigability unless you add @see Event::XXX docblocks everywhere,
* ...

The proposal is to send to listeners a real object as event:

* First class collection instead of associative array
* Explicit method names instead of inaccurate docblock
* Logic for managing "output" data is left to the event object, not the listener
* Type hinting and meaningful @return instead of wild guess
* Better navigability