basics.md

Basic concepts of Yii

Component and Object

Classes of the Yii framework usually extend from one of the two base classes [[yii\base\Object]] and [[yii\base\Component]]. These classes provide useful features that are added automatically to all classes extending from them.

The [[yii\base\Object|Object]] class provides the configuration and property feature. The [[yii\base\Component|Component]] class extends from [[yii\base\Object|Object]] and adds event handling and behaviors.

[[yii\base\Object|Object]] is usually used for classes that represent basic data structures while [[yii\base\Component|Component]] is used for application components and other classes that implement higher logic.

Object Configuration

The [[yii\base\Object|Object]] class introduces a uniform way of configuring objects. Any descendant class of [[yii\base\Object|Object]] should declare its constructor (if needed) in the following way so that it can be properly configured:

class MyClass extends \yii\base\Object
{
    public function __construct($param1, $param2, $config = [])
    {
        // ... initialization before configuration is applied

        parent::__construct($config);
    }

    public function init()
    {
        parent::init();

        // ... initialization after configuration is applied
    }
}

In the above, the last parameter of the constructor must take a configuration array which contains name-value pairs for initializing the properties at the end of the constructor. You can override the init() method to do initialization work that should be done after the configuration is applied.

By following this convention, you will be able to create and configure a new object using a configuration array like the following:

$object = Yii::createObject([
    'class' => 'MyClass',
    'property1' => 'abc',
    'property2' => 'cde',
], $param1, $param2);

Path Aliases

Yii 2.0 expands the usage of path aliases to both file/directory paths and URLs. An alias must start with a @ character so that it can be differentiated from file/directory paths and URLs. For example, the alias @yii refers to the Yii installation directory while @web contains base URL for currently running web application. Path aliases are supported in most places in the Yii core code. For example, FileCache::cachePath can take both a path alias and a normal directory path.

Path alias is also closely related with class namespaces. It is recommended that a path alias be defined for each root namespace so that you can use Yii the class autoloader without any further configuration. For example, because @yii refers to the Yii installation directory, a class like yii\web\Request can be autoloaded by Yii. If you use a third party library such as Zend Framework, you may define a path alias @Zend which refers to its installation directory and Yii will be able to autoload any class in this library.

Autoloading

All classes, interfaces and traits are loaded automatically at the moment they are used. There's no need to use include or require. It is, as well, true for Composer-loaded packages and Yii extensions.

Autoloader works according to PSR-4. That means namespaces and class, interface and trait names should correspond to file system paths except root namespace path that is defined by an alias.

For example, if standard alias @app refers to /var/www/example.com/ then \app\models\User will be loaded from /var/www/example.com/app/models/User.php.

Custom alias may be added using the following code:

Yii::setAlias('@shared', realpath('~/src/shared'));

Additional autoloaders may be registered using standard PHP spl_autoload_register.

Helper classes

Helper class typically contains static methods only and used as follows:

use \yii\helpers\Html;
echo Html::encode('Test > test');

There are several classes provided by framework:

• ArrayHelper
• Console
• FileHelper
• Html
• HtmlPurifier
• Inflector
• Json
• Markdown
• Security
• StringHelper
• VarDumper