yii2-bootstrap/docs/guide/behaviors.md

Behaviors

A behavior (also knows as mixin) can be used to enhance the functionality of an existing component without modifying the component's code. In particular, a behavior can "inject" its own methods and properties into the component, making them directly accessible via the component itslef. A behavior can also respond to events triggered in the component, thus intercepting the normal code execution. Unlike PHP's traits, behaviors can be attached to classes at runtime.

Using behaviors

A behavior can be attached to any class that extends from Component. In order to attach a behavior to a class, the component class must implement the behaviors method. As an example, Yii provides the AutoTimestamp behavior for automatically updating timestamp fields when saving an Active Record model:

class User extends ActiveRecord
{
	// ...

	public function behaviors()
	{
		return [
			'timestamp' => [
				'class' => 'yii\behaviors\AutoTimestamp',
				'attributes' => [
					ActiveRecord::EVENT_BEFORE_INSERT => ['create_time', 'update_time'],
					ActiveRecord::EVENT_BEFORE_UPDATE => 'update_time',
				],
			],
		];
	}
}

In the above, the class value is a string representing the fully qualified behavior class name. All of the other key-value pairs represent corresponding public properties of the AutoTimestamp class, thereby customizing how the behavior functions.

Creating your own behaviors

NEEDS UPDATING FOR Yii 2

To create your own behavior, you must define a class that implements the IBehavior interface. This can be accomplished by extending CBehavior. More specifically, you can extend CModelBehavior or CActiveRecordBehavior for behaviors to be used specifically with models or with Active Record models.

class MyBehavior extends CActiveRecordBehavior
{
}

To make your behavior customizable, like AutoTimestamp, add public properties:

class MyBehavior extends CActiveRecordBehavior
{
	public $attr;
}

Now, when the behavior is used, you can set the attribute to which you'd want the behavior to be applied:

class User extends ActiveRecord
{
	// ...

	public function behaviors()
	{
		return [
			'mybehavior' => [
				'class' => 'ext\mybehavior\MyBehavior',
				'attr' => 'member_type'
				],
			],
		];
	}
}

Behaviors are normally written to take action when certain model-related events occur, such as beforeSave or afterFind. You can write your behaviors to have the corresponding method. Within the method, you can access the model instance through $this->getOwner():

class MyBehavior extends CActiveRecordBehavior
{
	public $attr;
	public function beforeSave() {
		$model = $this->getOwner();
		// Use $model->$attr
	}
}