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 from a child class.

Event Handlers

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

• a global PHP function specified as a string (without parentheses), e.g., 'trim';
• an object method specified as an array of an object and a method name as a string (without parentheses), e.g., [$object, 'methodName'];
• a static class method specified as an array of a class name and a method name as a string (without parentheses), e.g., ['ClassName', 'methodName'];
• an anonymous function, e.g., function ($event) { ... }.

The signature of an event handler is:

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

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

• event name;
• event sender: the object whose trigger() method was called;
• custom data: the data that is provided when attaching the event handler (to be explained next).

Attaching Event Handlers

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

$foo = new Foo;

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

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

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

// this 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 as the 3rd argument to "on"
$foo->on(Foo::EVENT_HELLO, 'function_name', 'abc');

function function_name($event) {
    echo $event->data;
}

Event Handler Order

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

$foo->on(Foo::EVENT_HELLO, function ($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, you may call yii\base\Component::on(), passing false for the fourth parameter $append:

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

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);
    }
}

With the above code, any calls to bar() 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 represents the hello event. This approach has three benefits. First, it prevents typos. Second, it can make events recognizable for IDE auto-completion support. Third, you can tell what events are supported in a class by simply checking its 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 to 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 all handlers attached to the named event.

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, it is assumed that 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

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

For example, an Active Record object will trigger an 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 use 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 be invoked whenever an instance of ActiveRecord, or one of its child classes, triggers the EVENT_AFTER_INSERT event. In the handler, you can get the object that triggered the event through $event->sender.

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

You may trigger a 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 ($event) {
    var_dump($event->sender);  // displays "null"
});

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

Note that, in this case, $event->sender is null instead of an object instance.

Note: Because a class-level handler will respond to an event triggered by any instance of that class, or any child classes, 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);

Events using interfaces

There is even more abstract way to deal with events. You can create a separated interface for the special event and implement it in classes, where you need it.

For example, we can create the following interface:

namespace app\interfaces;

interface DanceEventInterface
{
    const EVENT_DANCE = 'dance';
}

And two classes, that implement it:

class Dog extends Component implements DanceEventInterface
{
    public function meetBuddy()
    {
        echo "Woof!";
        $this->trigger(DanceEventInterface::EVENT_DANCE);
    }
}

class Developer extends Component implements DanceEventInterface
{
    public function testsPassed()
    {
        echo "Yay!";
        $this->trigger(DanceEventInterface::EVENT_DANCE);
    }
}

To handle the EVENT_DANCE, triggered by any of these classes, call Event::on() and pass the interface class name as the first argument:

Event::on('app\interfaces\DanceEventInterface', DanceEventInterface::EVENT_DANCE, function ($event) {
    Yii::trace(get_class($event->sender) . ' just danced'); // Will log that Dog or Developer danced
});

You can trigger the event of those classes:

// trigger event for Dog class
Event::trigger(Dog::className(), DanceEventInterface::EVENT_DANCE);

// trigger event for Developer class
Event::trigger(Developer::className(), DanceEventInterface::EVENT_DANCE);

But please notice, that you can not trigger all the classes, that implement the interface:

// DOES NOT WORK. Classes that implement this interface will NOT be triggered.
Event::trigger('app\interfaces\DanceEventInterface', DanceEventInterface::EVENT_DANCE);

To detach event handler, call Event::off(). For example:

// detaches $handler
Event::off('app\interfaces\DanceEventInterface', DanceEventInterface::EVENT_DANCE, $handler);

// detaches all handlers of DanceEventInterface::EVENT_DANCE
Event::off('app\interfaces\DanceEventInterface', DanceEventInterface::EVENT_DANCE);

Global Events

Yii supports a so-called global event, which is actually a trick based on the event mechanism described above. The global event requires a globally accessible Singleton, such as the application instance itself.

To create the global event, an event sender calls the Singleton's trigger() method to trigger the event, instead of calling the sender's own trigger() method. Similarly, the event handlers are attached to the event on 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 using global events is that you do not need an 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").