yii2/docs/guide/concept-events.md

Events

Events allow you to inject custom code into existing code at certain execution points. You can attach custom code to an event so that when the event is triggered, the code gets executed automatically. For example, a mailer object may trigger a messageSent event when it successfully sends a message. If you want to keep track of the messages that are successfully sent, you could then simply attach the tracking code to the messageSent event.

Yii introduces a base class called yii\base\Component to support events. If a class needs to trigger events, it should extend from yii\base\Component or a child class.

Triggering Events

Events are triggered by calling the yii\base\Component::trigger() method. The method requires an event name, and optionally an event object that describes the parameters to be passed to the event handlers. For example:

namespace app\components;

use yii\base\Component;
use yii\base\Event;

class Foo extends Component
{
    const EVENT_HELLO = 'hello';

    public function bar()
    {
        $this->trigger(self::EVENT_HELLO);
    }
}

In the above code, when you call bar(), it will trigger an event named hello.

Tip: It is recommended to use class constants to represent event names. In the above example, the constant EVENT_HELLO is used to represent hello. This approach has two benefits. First, it prevents typos and can impact IDE auto-completion support. Second, you can tell what events are supported by a class by simply checking the constant declarations.

Sometimes when triggering an event, you may want to pass along additional information to the event handlers. For example, a mailer may want pass the message information to the handlers of the messageSent event so that the handlers can know the particulars of the sent messages. To do so, you can provide an event object as the second parameter to the yii\base\Component::trigger() method. The event object must be an instance of the yii\base\Event class or a child class. For example:

namespace app\components;

use yii\base\Component;
use yii\base\Event;

class MessageEvent extends Event
{
    public $message;
}

class Mailer extends Component
{
    const EVENT_MESSAGE_SENT = 'messageSent';

    public function send($message)
    {
        // ...sending $message...

        $event = new MessageEvent;
        $event->message = $message;
        $this->trigger(self::EVENT_MESSAGE_SENT, $event);
    }
}

When the yii\base\Component::trigger() method is called, it will call handlers that are attached to the named event.

Event Handlers

An event handler is a PHP callback that gets executed when the event it is attached to is triggered. You can use one of the following callbacks:

• a global PHP function specified in terms of a string, e.g., 'trim()';
• an object method specified in terms of an array of an object and a method name, e.g., [$object, $method];
• a static class method specified in terms of an array of a class name and a method name, e.g., [$class, $method];
• an anonymous function, e.g., function ($event) { ... }.

The signature of an event handler is:

function ($event) {
    // $event is an object of yii\base\Event or its child class
}

Through the $event parameter, an event handler may get the following information about an event:

• yii\base\Event::name
• yii\base\Event::sender: the object whose trigger() method is called.
• yii\base\Event::data: the data that is provided when attaching the event handler (to be explained shortly).

Attaching Event Handlers

You can attach a handler to an event by calling the yii\base\Component::on() method. For example,

$foo = new Foo;

// the handler is a global function
$foo->on(Foo::EVENT_HELLO, 'function_name');

// the handler is an object method
$foo->on(Foo::EVENT_HELLO, [$object, 'methodName']);

// the handler is a static class method
$foo->on(Foo::EVENT_HELLO, ['app\components\Bar', 'methodName']);

// the handler is an anonymous function
$foo->on(Foo::EVENT_HELLO, function ($event) {
    // event handling logic
});

You may also attach event handlers through configurations. For more details, please refer to the Configurations section.

When attaching an event handler, you may provide additional data as the third parameter to yii\base\Component::on(). The data will be made available to the handler when the event is triggered and the handler is called. For example,

// The following code will display "abc" when the event is triggered
// because $event->data contains the data passed to "on"
$foo->on(Foo::EVENT_HELLO, function ($event) {
    echo $event->data;
}, 'abc');

You may attach one or multiple handlers to a single event. When an event is triggered, the attached handlers will be called in the order they are attached to the event. If a handler needs to stop the invocation of the handlers behind it, it may set the yii\base\Event::handled property of the $event parameter to be true, like the following,

$foo->on(Foo::EVENT_HELLO, function ($event) {
    $event->handled = true;
});

By default, a newly attached handler is appended to the existing handler queue for the event. As a result, the handler will be called in the last place when the event is triggered. To insert the new handler at the start of the handler queue so that the handler gets called first, y ou may call yii\base\Component::on() by passing the fourth parameter $append as false:

$foo->on(Foo::EVENT_HELLO, function ($event) {
    // ...
}, $data, false);

Detaching Event Handlers

To detach a handler from an event, call the yii\base\Component::off() method. For example,

// the handler is a global function
$foo->off(Foo::EVENT_HELLO, 'function_name');

// the handler is an object method
$foo->off(Foo::EVENT_HELLO, [$object, 'methodName']);

// the handler is a static class method
$foo->off(Foo::EVENT_HELLO, ['app\components\Bar', 'methodName']);

// the handler is an anonymous function
$foo->off(Foo::EVENT_HELLO, $anonymousFunction);

Note that in general you should not try to detach an anonymous function unless you store it somewhere when it is attached to the event. In the above example, we assume the anonymous function is stored as a variable $anonymousFunction.

To detach ALL handlers from an event, simply call yii\base\Component::off() without the second parameter:

$foo->off(Foo::EVENT_HELLO);

Class-Level Event Handlers

In the above subsections, we have described how to attach a handler to an event at instance level. Sometimes, you may want to respond to an event triggered by EVERY instance of a class instead of a specific instance. Instead of attaching an event handler to every instance, you may attach the handler at class level by calling the static method yii\base\Event::on().

For example, an Active Record object will trigger a yii\base\ActiveRecord::EVENT_AFTER_INSERT event whenever it inserts a new record into the database. In order to track insertions done by EVERY Active Record object, you may write the following code:

use Yii;
use yii\base\Event;
use yii\db\ActiveRecord;

Event::on(ActiveRecord::className(), ActiveRecord::EVENT_AFTER_INSERT, function ($event) {
    Yii::trace(get_class($event->sender) . ' is inserted');
});

The event handler will get invoked whenever an instance of yii\base\ActiveRecord or its child class triggers the yii\base\ActiveRecord::EVENT_AFTER_INSERT event. In the handler, you can get the object that triggers the event through $event->sender.

When an object triggers an event, it will first call instance-level handlers, followed by class-level handlers.

You may trigger an class-level event by calling the static method yii\base\Event::trigger(). A class-level event is not associated with a particular object. As a result, it will cause the invocation of class-level event handlers only. For example,

use yii\base\Event;

Event::on(Foo::className(), Foo::EVENT_HELLO, function ($event) {
    echo $event->sender;  // displays "app\models\Foo"
});

Event::trigger(Foo::className(), Foo::EVENT_HELLO);

Note that in this case, $event->sender refers to the name of the class triggering the event instead of an object instance.

Note: Because a class-level handler will respond to an event triggered by any instance of that class or its child class, you should use it carefully, especially if the class is a low-level base class, such as yii\base\Object.

To detach a class-level event handler, call yii\base\Event::off(). For example,

// detach $handler
Event::off(Foo::className(), Foo::EVENT_HELLO, $handler);

// detach all handlers of Foo::EVENT_HELLO
Event::off(Foo::className(), Foo::EVENT_HELLO);

Global Events

The so-called global event is actually a trick based on the event mechanism described above. It requires a globally accessible singleton, such as the application instance.

An event sender, instead of calling its own trigger() method, will call the singleton's trigger() method to trigger the event. Similarly, the event handlers are attached to the event of the singleton. For example,

use Yii;
use yii\base\Event;
use app\components\Foo;

Yii::$app->on('bar', function ($event) {
    echo get_class($event->sender);  // displays "app\components\Foo"
});

Yii::$app->trigger('bar', new Event(['sender' => new Foo]));

A benefit of global events is that you do not need the object when attaching a handler to the event which will be triggered by the object. Instead, the handler attachment and the event triggering are both done through the singleton (e.g. the application instance).

However, because the namespace of the global events is shared by all parties, you should name the global events wisely, such as introducing some sort of namespace (e.g. "frontend.mail.sent", "backend.mail.sent").