yii2/docs/guide/concept-behaviors.md

Behaviors

Behaviors are instances of yii\base\Behavior or its child class. Behaviors, also known as mixins, allow you to enhance the functionality of an existing yii\base\Component class without the need of changing its class inheritance. When a behavior is attached to a component, it will "inject" its methods and properties into the component, and you can access these methods and properties as if they are defined by the component class. Moreover, a behavior can respond to the events triggered by the component so that it can customize or adapt the normal code execution of the component.

Using Behaviors

To use a behavior, you first need to attach it to a yii\base\Component. We will describe how to attach a behavior in the next subsection.

Once a behavior is attached to a component, its usage is straightforward.

You can access a public member variable or a property defined by a getter and/or a setter of the behavior through the component it is attached to, like the following,

// "prop1" is a property defined in the behavior class
echo $component->prop1;
$component->prop1 = $value;

You can also call a public method of the behavior similarly,

// bar() is a public method defined in the behavior class
$component->bar();

As you can see, although $component does not define prop1 and bar(), they can be used as if they are part of the component definition.

If two behaviors define the same property or method and they are both attached to the same component, the behavior that is attached to the component first will take precedence when the property or method is being accessed.

A behavior may be associated with a name when it is attached to a component. If this is the case, you may access the behavior object using the name, like the following,

$behavior = $component->getBehavior('myBehavior');

You may also get all behaviors attached to a component:

$behaviors = $component->getBehaviors();

Attaching Behaviors

You can attach a behavior to a yii\base\Component either statically or dynamically. The former is more commonly used in practice.

To attach a behavior statically, override the yii\base\Component::behaviors() method of the component class that it is being attached. For example,

namespace app\models;

use yii\db\ActiveRecord;
use app\components\MyBehavior;

class User extends ActiveRecord
{
    public function behaviors()
    {
        return [
            // anonymous behavior, behavior class name only
            MyBehavior::className(),

            // named behavior, behavior class name only
            'myBehavior2' => MyBehavior::className(),

            // anonymous behavior, configuration array
            [
                'class' => MyBehavior::className(),
                'prop1' => 'value1',
                'prop2' => 'value2',
            ],

            // named behavior, configuration array
            'myBehavior4' => [
                'class' => MyBehavior::className(),
                'prop1' => 'value1',
                'prop2' => 'value2',
            ]
        ];
    }
}

The yii\base\Component::behaviors() method should return a list of behavior configurations. Each behavior configuration can be either a behavior class name or a configuration array.

You may associate a name with a behavior by specifying the array key corresponding to the behavior configuration. In this case, the behavior is called a named behavior. In the above example, there are two named behaviors: myBehavior2 and myBehavior4. If a behavior is not associated with a name, it is called an anonymous behavior.

To attach a behavior dynamically, call the yii\base\Component::attachBehavior() method of the component that it is attached to. For example,

use app\components\MyBehavior;

// attach a behavior object
$component->attachBehavior('myBehavior1', new MyBehavior);

// attach a behavior class
$component->attachBehavior('myBehavior2', MyBehavior::className());

// attach a configuration array
$component->attachBehavior('myBehavior3', [
    'class' => MyBehavior::className(),
    'prop1' => 'value1',
    'prop2' => 'value2',
]);

You may attach multiple behaviors at once by using the yii\base\Component::attachBehaviors() method. For example,

$component->attachBehaviors([
    'myBehavior1' => new MyBehavior,  // a named behavior
    MyBehavior::className(),          // an anonymous behavior
]);

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

Detaching Behaviors

To detach a behavior, you can call yii\base\Component::detachBehavior() with the name associated with the behavior:

$component->detachBehavior('myBehavior1');

You may also detach all behaviors:

$component->detachBehaviors();

Defining Behaviors

To define a behavior, create a class by extending from yii\base\Behavior or its child class. For example,

namespace app\components;

use yii\base\Model;
use yii\base\Behavior;

class MyBehavior extends Behavior
{
    public $prop1;

    private $_prop2;

    public function getProp2()
    {
        return $this->_prop2;
    }

    public function setProp2($value)
    {
        $this->_prop2 = $value;
    }

    public function foo()
    {
        // ...
    }
}

The above code defines the behavior class app\components\MyBehavior which will provide two properties prop1 and prop2, and one method foo() to the component it is attached to. Note that property prop2 is defined via the getter getProp2() and the setter setProp2(). This is so because yii\base\Object is an ancestor class of yii\base\Behavior, which supports defining properties by getters/setters.

Within a behavior, you can access the component that the behavior is attached to through the yii\base\Behavior::owner property.

If a behavior needs to respond to the events triggered by the component it is attached to, it should override the yii\base\Behavior::events() method. For example,

namespace app\components;

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

class MyBehavior extends Behavior
{
    // ...

    public function events()
    {
        return [
            ActiveRecord::EVENT_BEFORE_VALIDATE => 'beforeValidate',
        ];
    }

    public function beforeValidate($event)
    {
        // ...
    }
}

The yii\base\Behavior::events() method should return a list of events and their corresponding handlers. The above example declares that the yii\db\ActiveRecord::EVENT_BEFORE_VALIDATE event and its handler beforeValidate(). When specifying an event handler, you may use one of the following formats:

• a string that refers to the name of a method of the behavior class, like the example above;
• an array of an object or class name, and a method name, e.g., [$object, 'methodName'];
• an anonymous function.

The signature of an event handler should be as follows, where $event refers to the event parameter. Please refer to the Events section for more details about events.

function ($event) {
}

Using TimestampBehavior

To wrap up, let's take a look at yii\behaviors\TimestampBehavior - a behavior that supports automatically updating the timestamp attributes of an yii\db\ActiveRecord when it is being saved.

First, attach this behavior to the yii\db\ActiveRecord class that you plan to use.

namespace app\models\User;

use yii\db\ActiveRecord;
use yii\behaviors\TimestampBehavior;

class User extends ActiveRecord
{
    // ...

    public function behaviors()
    {
        return [
            [
                'class' => TimestampBehavior::className(),
                'attributes' => [
                    ActiveRecord::EVENT_BEFORE_INSERT => ['created_at', 'updated_at'],
                    ActiveRecord::EVENT_BEFORE_UPDATE => ['updated_at'],
                ],
            ],
        ];
    }
}

The behavior configuration above specifies that

• when the record is being inserted, the behavior should assign the current timestamp to the created_at and updated_at attributes;
• when the record is being updated, the behavior should assign the current timestamp to the updated_at attribute.

Now if you have a User object and try to save it, you will find its created_at and updated_at are automatically filled with the current timestamp:

$user = new User;
$user->email = 'test@example.com';
$user->save();
echo $user->created_at;  // shows the current timestamp

The yii\behaviors\TimestampBehavior also offers a useful method yii\behaviors\TimestampBehavior::touch() which will assign the current timestamp to a specified attribute and save it to the database:

$user->touch('login_time');

Comparison with Traits

While behaviors are similar to traits in that they both "inject" their properties and methods to the primary class, they differ in many aspects. As explained below, they both have pros and cons. They are more like complements rather than replacements to each other.

Pros for Behaviors

Behavior classes, like normal classes, support inheritance. Traits, on the other hand, can be considered as language-supported copy and paste. They do not support inheritance.

Behaviors can be attached and detached to a component dynamically without requiring you to modify the component class. To use a trait, you must modify the class using it.

Behaviors are configurable while traits are not.

Behaviors can customize the code execution of a component by responding to its events.

When there is name conflict among different behaviors attached to the same component, the conflict is automatically resolved by respecting the behavior that is attached to the component first. Name conflict caused by different traits requires you to manually resolve it by renaming the affected properties or methods.

Pros for Traits

Traits are much more efficient than behaviors because behaviors are objects which take both time and memory.

IDEs are more friendly to traits as they are language construct.