yii2/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 itself. 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 yii\base\Component. In order to attach a behavior to a class, the component class must implement the behaviors method. As an example, Yii provides the yii\behaviors\AutoTimestamp behavior for automatically updating timestamp fields when saving an yii\db\ActiveRecord model:

class User extends ActiveRecord
{
	// ...

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

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 yii\behaviors\AutoTimestamp class, thereby customizing how the behavior functions.

Creating your own behaviors

To create your own behavior, you must define a class that extends yii\base\Behavior.

namespace app\components;

use yii\base\Behavior;

class MyBehavior extends Behavior
{
}

To make it customizable, like yii\behaviors\AutoTimestamp, add public properties:

namespace app\components;

use yii\base\Behavior;

class MyBehavior extends Behavior
{
	public $attr;
}

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

namespace app\models;

use yii\db\ActiveRecord;

class User extends ActiveRecord
{
	// ...

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

Behaviors are normally written to take action when certain events occur. Below we're implementing events method to assign event handlers:

namespace app\components;

use yii\base\Behavior;
use yii\db\ActiveRecord;

class MyBehavior extends Behavior
{
	public $attr;

	public function events()
	{
		return [
			ActiveRecord::EVENT_BEFORE_INSERT => 'beforeInsert',
			ActiveRecord::EVENT_BEFORE_UPDATE => 'beforeUpdate',
		];
	}

	public function beforeInsert() {
		$model = $this->owner;
		// Use $model->$attr
	}

	public function beforeUpdate() {
		$model = $this->owner;
		// Use $model->$attr
	}
}