Creating plugins

Guzzle is extremely extensible because of the behavioral modifications that can be added to requests, clients, and commands using an event system. Before and after the majority of actions are taken in the library, an event is emitted with the name of the event and context surrounding the event. Observers can subscribe to a subject and modify the subject based on the events received. Guzzle's event system utilizes the Symfony2 EventDispatcher and is the backbone of its plugin architecture.


Plugins must implement the Symfony\Component\EventDispatcher\EventSubscriberInterface interface. The EventSubscriberInterface requires that your class implements a static method, getSubscribedEvents(), that returns an associative array mapping events to methods on the object. See the Symfony2 documentation for more information.

Plugins can be attached to any subject, or object in Guzzle that implements that Guzzle\Common\HasDispatcherInterface.

Subscribing to a subject

You can subscribe an instantiated observer to an event by calling addSubscriber on a subject.

$testPlugin = new TestPlugin();

You can also subscribe to only specific events using a closure:

$client->getEventDispatcher()->addListener('request.create', function(Event $event) {
    echo $event->getName();
    echo $event['request'];

Guzzle\Common\Event objects are passed to notified functions. The Event object has a getName() method which return the name of the emitted event and may contain contextual information that can be accessed like an array.

Knowing what events to listen to

Any class that implements the Guzzle\Common\HasDispatcherInterface must implement a static method, getAllEvents(), that returns an array of the events that are emitted from the object. You can browse the source to see each event, or you can call the static method directly in your code to get a list of available events.

Examples of the event system

Simple Echo plugin

This simple plugin prints a string containing the request that is about to be sent by listening to the request.before_send event:

use Symfony\Component\EventDispatcher\EventSubscriberInterface;

class EchoPlugin implements EventSubscriberInterface
    public static function getSubscribedEvents()
        return array('request.before_send' => 'onBeforeSend');

    public function onBeforeSend(Guzzle\Common\Event $event)
        echo 'About to send a request: ' . $event['request'] . "\n";

$client = new Guzzle\Service\Client('');

// Create the plugin and add it as an event subscriber
$plugin = new EchoPlugin();

// Send a request and notice that the request is printed to the screen

Running the above code will print a string containing the HTTP request that is about to be sent.

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