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# API
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The API that événement exposes is defined by the
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`Evenement\EventEmitterInterface`. The interface is useful if you want to
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define an interface that extends the emitter and implicitly defines certain
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events to be emitted, or if you want to type hint an `EventEmitter` to be
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passed to a method without coupling to the specific implementation.
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## on($event, callable $listener)
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Allows you to subscribe to an event.
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Example:
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```php
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$emitter->on('user.created', function (User $user) use ($logger) {
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$logger->log(sprintf("User '%s' was created.", $user->getLogin()));
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});
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```
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Since the listener can be any callable, you could also use an instance method
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instead of the anonymous function:
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```php
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$loggerSubscriber = new LoggerSubscriber($logger);
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$emitter->on('user.created', array($loggerSubscriber, 'onUserCreated'));
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```
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This has the benefit that listener does not even need to know that the emitter
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exists.
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You can also accept more than one parameter for the listener:
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```php
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$emitter->on('numbers_added', function ($result, $a, $b) {});
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```
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## once($event, callable $listener)
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Convenience method that adds a listener which is guaranteed to only be called
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once.
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Example:
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```php
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$conn->once('connected', function () use ($conn, $data) {
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$conn->send($data);
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});
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```
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## emit($event, array $arguments = [])
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Emit an event, which will call all listeners.
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Example:
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```php
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$conn->emit('data', [$data]);
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```
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The second argument to emit is an array of listener arguments. This is how you
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specify more args:
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```php
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$result = $a + $b;
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$emitter->emit('numbers_added', [$result, $a, $b]);
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```
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## listeners($event)
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Allows you to inspect the listeners attached to an event. Particularly useful
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to check if there are any listeners at all.
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Example:
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```php
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$e = new \RuntimeException('Everything is broken!');
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if (0 === count($emitter->listeners('error'))) {
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throw $e;
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}
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```
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## removeListener($event, callable $listener)
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Remove a specific listener for a specific event.
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## removeAllListeners($event = null)
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Remove all listeners for a specific event or all listeners all together. This
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is useful for long-running processes, where you want to remove listeners in
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order to allow them to get garbage collected.
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