Behaviors

Behaviors are instances of yii\base\Behavior, or of a child class. Behaviors, also known as mixins, allow you to enhance the functionality of an existing component class without needing to change the class's inheritance. Attaching a behavior to a component "injects" the behavior's methods and properties into the component, making those methods and properties accessible as if they were defined in the component class itself. Moreover, a behavior can respond to the events triggered by the component, which allows behaviors to also customize the normal code execution of the component.

Defining Behaviors

To define a behavior, create a class that extends yii\base\Behavior, or extends a child class. For example:

namespace app\components;

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, with two properties prop1 and prop2 and one method foo(). Note that property prop2 is defined via the getter getProp2() and the setter setProp2(). This is the case because yii\base\Behavior extends yii\base\BaseObject and therefore supports defining properties via getters and setters.

Because this class is a behavior, when it is attached to a component, that component will then also have the prop1 and prop2 properties and the foo() method.

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

Note: In case yii\base\Behavior::__get() and/or yii\base\Behavior::__set() method of behavior is overridden you need to override yii\base\Behavior::canGetProperty() and/or yii\base\Behavior::canSetProperty() as well.

Handling Component Events

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 events() method should return a list of events and their corresponding handlers. The above example declares that the EVENT_BEFORE_VALIDATE event exists and defines 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 as a string (without parentheses), 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) {
}

Attaching Behaviors

You can attach a behavior to a component either statically or dynamically. The former is more common in practice.

To attach a behavior statically, override the behaviors() method of the component class to which the behavior is being attached. The behaviors() method should return a list of behavior configurations. Each behavior configuration can be either a behavior class name or a configuration array:

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',
            ]
        ];
    }
}

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 to which the behavior is being attached:

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 using the yii\base\Component::attachBehaviors() method:

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

You may also attach behaviors through configurations like the following:

[
    'as myBehavior2' => MyBehavior::className(),

    'as myBehavior3' => [
        'class' => MyBehavior::className(),
        'prop1' => 'value1',
        'prop2' => 'value2',
    ],
]

For more details, please refer to the Configurations section.

Using Behaviors

To use a behavior, first attach it to a component per the instructions above. 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:

// "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:

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

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

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 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:

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

You may also get all behaviors attached to a component:

$behaviors = $component->getBehaviors();

Detaching Behaviors

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

$component->detachBehavior('myBehavior1');

You may also detach all behaviors:

$component->detachBehaviors();

Using TimestampBehavior

To wrap up, let's take a look at yii\behaviors\TimestampBehavior. This behavior supports automatically updating the timestamp attributes of an Active Record model anytime the model is saved via insert(), update() or save() method.

First, attach this behavior to the Active Record 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'],
                ],
                // if you're using datetime instead of UNIX timestamp:
                // 'value' => new Expression('NOW()'),
            ],
        ];
    }
}

The behavior configuration above specifies that when the record is being:

• inserted, the behavior should assign the current UNIX timestamp to the created_at and updated_at attributes
• updated, the behavior should assign the current UNIX timestamp to the updated_at attribute

Note: For the above implementation to work with MySQL database, please declare the columns(created_at, updated_at) as int(11) for being UNIX timestamp.

With that code in place, 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 UNIX timestamp:

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

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

$user->touch('login_time');

Other behaviors

There are several built-in and external behaviors available:

• yii\behaviors\BlameableBehavior - automatically fills the specified attributes with the current user ID.
• yii\behaviors\SluggableBehavior - automatically fills the specified attribute with a value that can be used as a slug in a URL.
• yii\behaviors\AttributeBehavior - automatically assigns a specified value to one or multiple attributes of an ActiveRecord object when certain events happen.
• yii2tech\ar\softdelete\SoftDeleteBehavior - provides methods to soft-delete and soft-restore ActiveRecord i.e. set flag or status which marks record as deleted.
• yii2tech\ar\position\PositionBehavior - allows managing records order in an integer field by providing reordering methods.

Comparing Behaviors 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 to each other rather than alternatives.

Reasons to Use 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 modification of the component class. To use a trait, you must modify the code of 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 can be name conflicts among different behaviors attached to the same component, the conflicts are automatically resolved by prioritizing the behavior attached to the component first. Name conflicts caused by different traits requires manual resolution by renaming the affected properties or methods.

Reasons to Use Traits

Traits are much more efficient than behaviors as behaviors are objects that take both time and memory.

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