Use 3rd party event library for events in 2.1

[SamMousa]

The wiki mentions Events as a candidate for generic PHP packages. (ie removal from the core).
When looking on packagist, there are several big (in terms of # downloads) event-related packages:

• symfony/event-dispatcher (54 million downloads, lots of dependencies)
• zendframework/zend-eventmanager (11 million, no dependencies, good documentation, last commit < 6 months ago)
• evenement/evenement (3.1 million, no dependencies, few docs, last commit > 3 years ago)
• illuminate/events (2.5 million, some dependencies, last commit < 6 months ago)
• league/event (1.9 million, no dependencies, good documentation, last commit > 1 year ago)

I've had good experiences with packages from The PHP League as well as their handling of contributions.
Zend EventManager seems to be well documented, maintained and its has a high number of installs.

Going through the documentation for ZendEventManager I feel it supports everything that Yii needs, and more. One thing it explicitly does not support that Yii does is listing event handlers; at Zend they removed this in the latest version stating that there is no real use case for it.
Yii often checks for event handlers before firing an event:

if (...hasEventHandlers()) {
   trigger the event.
}

I feel we could live without it.

The specific library to use could even be made configurable by the developer. We could define a simple interface that is used by Component for use within Yii. Then anyone can implement a connector and use the library of their choice.

[dynasource]

what about the introduction of some mechanism in which the developer can choose its own library?

[SamMousa]

@dynasource As my last paragraph mentions, that is definitely possible; still we should pick a default.

[samdark]

There's PSR-14 about events management: https://github.com/php-fig/fig-standards/blob/master/proposed/event-manager.md. It's still in the draft but worth looking at.

[samdark]

#11389

[klimov-paul]

@samdark, can you clarify usage of PSR-14?
In which way particular component (like yii\web\User or yii\db\ActiveRecord) should interact with Psr\EventManager\EventManagerInterface? Should every class which uses events (e.g. current yii\base\Component descendants) implement this interface or they should store event manager instance as an internal property?
What about class-level events? Does PSR-14 handles them somehow, or they should be dropped?
And what about issue #14978 then? Its implementation is possible only in case of single-application-level event dispatcher usage. But in case EventManagerInterface should appear in single instance, how developer will be able to attach event for particular class instance without affecting other ones, or he should not?

[samdark]

I'll check it in detail this week.

[samdark]

1. Event manager should be a component by itself.
2. Storing an instance (i.e. having it as dependency) sounds more like it.
3. If you need events at class level you can create another manager so events are isolated. We may provide a trait for it but I'd avoid it because of Catch all application events #14978.
4. I don't see many use cases for object-specific events.