Path Alias and Namespace

Yii uses path aliases extensively. A path alias is associated with a directory or file path. It is specified in dot syntax, similar to that of widely adopted namespace format:

RootAlias.path.to.target

where RootAlias is the alias of some existing directory.

By using YiiBase::getPathOfAlias(), an alias can be translated to its corresponding path. For example, system.web.CController would be translated as yii/framework/web/CController.

We can also use YiiBase::setPathOfAlias() to define new root path aliases.

Root Alias

For convenience, Yii predefines the following root aliases:

• system: refers to the Yii framework directory;
• zii: refers to the Zii library directory;
• application: refers to the application's base directory;
• webroot: refers to the directory containing the entry script file.
• ext: refers to the directory containing all third-party extensions.

Additionally, if an application uses modules, each module will have a predefined root alias that has the same name as the module ID and refers to the module's base path. For example, if an application uses a module whose ID is users, a root alias named users will be predefined.

Importing Classes

Using aliases, it is very convenient to include the definition of a class. For example, if we want to include the CController class, we can call the following:

Yii::import('system.web.CController');

The import method differs from include and require in that it is more efficient. The class definition being imported is actually not included until it is referenced for the first time (implemented via PHP autoloading mechanism). Importing the same namespace multiple times is also much faster than include_once and require_once.

Tip: When referring to a class defined by the Yii framework, we do not need to import or include it. All core Yii classes are pre-imported.

Using Class Map

Starting from version 1.1.5, Yii allows user classes to be pre-imported via a class mapping mechanism that is also used by core Yii classes. Pre-imported classes can be used anywhere in a Yii application without being explicitly imported or included. This feature is most useful for a framework or library that is built on top of Yii.

To pre-import a set of classes, the following code must be executed before CWebApplication::run() is invoked:

Yii::$classMap=array(
    'ClassName1' => 'path/to/ClassName1.php',
    'ClassName2' => 'path/to/ClassName2.php',
    ......
);

Importing Directories

We can also use the following syntax to import a whole directory so that the class files under the directory can be automatically included when needed.

Yii::import('system.web.*');

Besides import, aliases are also used in many other places to refer to classes. For example, an alias can be passed to Yii::createComponent() to create an instance of the corresponding class, even if the class file was not included previously.

Namespace

A namespace refers to a logical grouping of some class names so that they can be differentiated from other class names even if their names are the same. Do not confuse path alias with namespace. A path alias is merely a convenient way of naming a file or directory. It has nothing to do with a namespace.

Tip: Because PHP prior to 5.3.0 does not support namespace intrinsically, you cannot create instances of two classes who have the same name but with different definitions. For this reason, all Yii framework classes are prefixed with a letter 'C' (meaning 'class') so that they can be differentiated from user-defined classes. It is recommended that the prefix 'C' be reserved for Yii framework use only, and user-defined classes be prefixed with other letters.

Namespaced Classes

A namespaced class refers to a class declared within a non-global namespace. For example, the application\components\GoogleMap class is declared within the namespace application\components. Using namespaced classes requires PHP 5.3.0 or above.

Starting from version 1.1.5, it is possible to use a namespaced class without including it explicitly. For example, we can create a new instance of application\components\GoogleMap without including the corresponding class file explicitly. This is made possible with the enhanced Yii class autoloading mechanism.

In order to be able to autoload a namespaced class, the namespace must be named in a way similar to naming a path alias. For example, the class application\components\GoogleMap must be stored in a file that can be aliased as application.components.GoogleMap.