Event library that implements variations of the mediator and observer patterns
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README.rst

Event

Event library that implements variations of the mediator and observer patterns.

https://travis-ci.com/kuria/event.svg?branch=master

Features

  • emitting events with any number of arguments
  • managing listeners for specific or all events
  • ordering listeners by priority
  • stopping event propagation
  • multiple ways to embed the event system

Requirements

  • PHP 7.1+

Components

Event emitter

The EventEmitter class maintains a list of listeners and dispatches events to them.

It is intended to be used as a mediator.

<?php

use Kuria\Event\EventEmitter;

$emitter = new EventEmitter();

EventEmitter implements ObservableInterface.

Observable

The abstract Observable class implements the ObservableInterface using an inner event emitter.

It is intended to be extended by child classes that will emit their own events:

<?php

use Kuria\Event\Observable;

class MyComponent extends Observable
{
    function doSomething()
    {
        $this->emit('something');
    }
}

Alternatively, you can use the ObservableTrait to achieve the same result:

<?php

use Kuria\Event\EventEmitterPropInterface;
use Kuria\Event\ObservableInterface;
use Kuria\Event\ObservableTrait;

class MyComponent implements ObservableInterface, EventEmitterPropInterface
{
    use ObservableTrait;

    // ...
}

Usage

The following applies to both event emitter and observable as they both implement the ObservableInterface.

Listening to events

Using a callback

To register a callback to be called when a specific event occurs, register it using the on() method. Any event arguments will be passed directly to it.

<?php

$observable->on('some.event', function ($arg1, $arg2) {
    // do something
});
  • the callback can stop event propagation by returning FALSE
  • listener priority can be specified using the 3rd argument of on()

To unregister a callback, call the off() method with the same callback (in case of closures this means the same object):

<?php

$observable->off('some.event', $callback); // returns TRUE on success

Using an event listener

To register an event listener, use the addListener() method:

<?php

use Kuria\Event\EventListener;

$observable->addListener(
    new EventListener(
        'some.event',
        function ($arg1, $arg2) {}
    )
);
  • listener priority can be specified by using the 3rd argument of the EventListener constructor
  • the callback can stop event propagation by returning FALSE

To unregister a listener, call the removeListener() method with the same event listener object:

<?php

$observable->removeListener($eventListener); // returns TRUE on success

Using an event subscriber

Event subscribers subscribe to a list of events. Each event is usually mapped to one method of the subscriber.

The listeners can be created using the convenient listen() method (as shown in the example below) or by manually creating EventListener instances.

  • any callback or method can stop event propagation by returning FALSE
  • listener priority can be specified using 3rd argument of listen() or the EventListener constructor
<?php

use Kuria\Event\EventSubscriber;

class MySubscriber extends EventSubscriber
{
    protected function getListeners(): array
    {
        return [
            $this->listen('foo.bar', 'onFooBar'),
            $this->listen('lorem.ipsum', 'onLoremIpsum', 10),
            $this->listen('dolor.sit', 'onDolorSitA'),
            $this->listen('dolor.sit', 'onDolorSitB', 5),
        ];
    }

    function onFooBar() { /* do something */ }
    function onLoremIpsum() { /* do something */ }
    function onDolorSitA() { /* do something */ }
    function onDolorSitB() { /* do something */ }
}

$subscriber = new MySubscriber();

Registering the event subscriber:

<?php

$subscriber->subscribeTo($observable);

Unregistering the event subsriber:

<?php

$subscriber->unsubscribeFrom($observable);

Stopping event propagation

Any listener can stop further propagation of the current event by returning FALSE.

This prevents any other listeners from being invoked.

Listener priority

Listener priority determines the order in which the listeners are invoked:

  • listeners with greater priority are invoked sooner
  • listeners with lesser priority are invoked later
  • if the priorities are equal, the order of invocation is undefined
  • priority can be negative
  • default priority is 0

Listening to all events

To listen to all events, use ObservableInterface::ANY_EVENT in place of the event name:

<?php

use Kuria\Event\EventListener;
use Kuria\Event\ObservableInterface;

$observable->on(
    ObservableInterface::ANY_EVENT,
    function ($event, $arg1, $arg2) {}
);

$observable->addListener(
    new EventListener(
        ObservableInterface::ANY_EVENT,
        function ($event, $arg1, $arg2) {}
    )
);
  • global listeners are invoked before listeners of specific events
  • global listeners get an extra event name argument before the emitted event arguments
  • global listeners can also stop event propagation by returning FALSE and may have specified listener priority

Emitting events

Events are emitted using the emit() method.

<?php

$observable->emit('foo');

Any extra arguments will be passed to the listeners.

<?php

$observable->emit('foo', 'hello', 123);

Note

Variable references cannot be emitted directly as an argument. If you need to use references, wrap them in an object or an array.

Documenting events

While the event library itself doesn't require this, it is a good idea to explicitly define possible event names and their arguments somewhere.

The example below defines a FieldEvents class for this purpose. Constants of this class are then used in place of event names and their annotations serve as documentation. This also allows for code-completion.

<?php

use Kuria\Event\Observable;

/**
 * @see Field
 */
abstract class FieldEvents
{
    /**
     * Emitted when field value is about to be changed.
     *
     * @param Field $field
     * @param mixed $oldValue
     * @param mixed $newValue
     */
    const CHANGE = 'change';

    /**
     * Emitted when field value is about to be cleared.
     *
     * @param Field $field
     */
    const CLEAR = 'clear';
}

/**
 * @see FieldEvents
 */
class Field extends Observable
{
    private $name;
    private $value;

    function __construct(string $name, $value = null)
    {
        $this->name = $name;
        $this->value = $value;
    }

    function getName(): string
    {
        return $this->name;
    }

    function getValue()
    {
        return $this->value;
    }

    function setValue($value): void
    {
        $this->emit(FieldEvents::CHANGE, $this, $this->value, $value);

        $this->value = $value;
    }

    function clear()
    {
        $this->emit(FieldEvents::CLEAR, $this);

        $this->value = null;
    }
}

Note

Using @param annotations on class constants is non-standard, but IDE's dont mind it and some documentation-generators (such as Doxygen) even display them nicely.

Usage example

<?php

$field = new Field('username');

$field->on(FieldEvents::CHANGE, function (Field $field, $oldValue, $newValue) {
    echo "Field '{$field->getName()}' has been changed from '{$oldValue}' to '{$newValue}'\n";
});

$field->on(FieldEvents::CLEAR, function (Field $field) {
    echo "Field '{$field->getName()}' has been cleared\n";
});

$field->setValue('john.smith');
$field->setValue('foo.bar123');
$field->clear();

Output:

Field 'username' has been changed from '' to 'john.smith'
Field 'username' has been changed from 'john.smith' to 'foo.bar123'
Field 'username' has been cleared