Basic concepts of Yii

Component and Object

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

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

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

Object Configuration

The yii\base\Object class introduces a uniform way of configuring objects. Any descendant class of yii\base\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 example, the last parameter of the constructor must take a configuration array which contains name-value pairs that will be used to initialize the object's properties at the end of the constructor. You can override the init() method to do initialization work after the configuration is applied.

By following this convention, you will be able to create and configure new objects 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 an @ symbol 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 the base URL for the currently running web application. Path aliases are supported in most places in the Yii core code. For example, FileCache::cachePath can accept both a path alias and a normal directory path.

Path aliases are also closely related to class namespaces. It is recommended that a path alias be defined for each root namespace so that Yii's class autoloader can be used 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.

The following aliases are predefined by the core framework:

@yii - framework directory.
@app - base path of currently running application.
@runtime - runtime directory.
@vendor - Composer vendor directory.
@webroot - web root directory of currently running web application.
@web - base URL of currently running web application.

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 true for Composer-loaded packages as well as Yii extensions.

Yii's autoloader works according to PSR-4. That means namespaces, classes, interfaces and traits must correspond to file system paths and file names accordinly, except for root namespace paths that are defined by an alias.

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

Custom aliases may be added using the following code:

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

Additional autoloaders may be registered using PHP's standard spl_autoload_register.

Helper classes

Helper classes typically contain static methods only and are used as follows:

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

There are several classes provided by framework:

ArrayHelper
Console
FileHelper
Html
HtmlPurifier
Image
Inflector
Json
Markdown
Security
StringHelper
Url
VarDumper

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