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95 lines
5.4 KiB
Markdown
95 lines
5.4 KiB
Markdown
Class Autoloading
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=================
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Yii relies on the [class autoloading mechanism](https://www.php.net/manual/en/language.oop5.autoload.php)
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to locate and include all required class files. It provides a high-performance class autoloader that is compliant with the
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[PSR-4 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader.md).
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The autoloader is installed when you include the `Yii.php` file.
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> Note: For simplicity of description, in this section we will only talk about autoloading of classes. However, keep in
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mind that the content we are describing here applies to autoloading of interfaces and traits as well.
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Using the Yii Autoloader <span id="using-yii-autoloader"></span>
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------------------------
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To make use of the Yii class autoloader, you should follow two simple rules when creating and naming your classes:
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* Each class must be under a [namespace](https://www.php.net/manual/en/language.namespaces.php) (e.g. `foo\bar\MyClass`)
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* Each class must be saved in an individual file whose path is determined by the following algorithm:
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```php
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// $className is a fully qualified class name without the leading backslash
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$classFile = Yii::getAlias('@' . str_replace('\\', '/', $className) . '.php');
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```
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For example, if a class name and namespace is `foo\bar\MyClass`, the [alias](concept-aliases.md) for the corresponding class file path
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would be `@foo/bar/MyClass.php`. In order for this alias to be resolvable into a file path,
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either `@foo` or `@foo/bar` must be a [root alias](concept-aliases.md#defining-aliases).
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When using the [Basic Project Template](start-installation.md), you may put your classes under the top-level
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namespace `app` so that they can be autoloaded by Yii without the need of defining a new alias. This is because
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`@app` is a [predefined alias](concept-aliases.md#predefined-aliases), and a class name like `app\components\MyClass`
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can be resolved into the class file `AppBasePath/components/MyClass.php`, according to the algorithm just described.
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In the [Advanced Project Template](https://github.com/yiisoft/yii2-app-advanced/blob/master/docs/guide/README.md), each tier has its own root alias. For example,
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the front-end tier has a root alias `@frontend`, while the back-end tier root alias is `@backend`. As a result,
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you may put the front-end classes under the namespace `frontend` while the back-end classes are under `backend`. This will
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allow these classes to be autoloaded by the Yii autoloader.
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To add a custom namespace to the autoloader you need to define an alias for the base directory of the namespace using [[Yii::setAlias()]].
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For example to load classes in the `foo` namespace that are located in the `path/to/foo` directory you will call `Yii::setAlias('@foo', 'path/to/foo')`.
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Class Map <span id="class-map"></span>
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---------
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The Yii class autoloader supports the *class map* feature, which maps class names to the corresponding class file paths.
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When the autoloader is loading a class, it will first check if the class is found in the map. If so, the corresponding
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file path will be included directly without further checks. This makes class autoloading super fast. In fact,
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all core Yii classes are autoloaded this way.
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You may add a class to the class map, stored in `Yii::$classMap`, using:
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```php
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Yii::$classMap['foo\bar\MyClass'] = 'path/to/MyClass.php';
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```
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[Aliases](concept-aliases.md) can be used to specify class file paths. You should set the class map in the
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[bootstrapping](runtime-bootstrapping.md) process so that the map is ready before your classes are used.
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Using Other Autoloaders <span id="using-other-autoloaders"></span>
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-----------------------
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Because Yii embraces Composer as a package dependency manager, it is recommended that you also install
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the Composer autoloader. If you are using 3rd-party libraries that have their own autoloaders, you should
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also install those.
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When using the Yii autoloader together with other autoloaders, you should include the `Yii.php` file
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*after* all other autoloaders are installed. This will make the Yii autoloader the first one responding to
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any class autoloading request. For example, the following code is extracted from
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the [entry script](structure-entry-scripts.md) of the [Basic Project Template](start-installation.md). The first
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line installs the Composer autoloader, while the second line installs the Yii autoloader:
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```php
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require __DIR__ . '/../vendor/autoload.php';
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require __DIR__ . '/../vendor/yiisoft/yii2/Yii.php';
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```
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You may use the Composer autoloader alone without the Yii autoloader. However, by doing so, the performance
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of your class autoloading may be degraded, and you must follow the rules set by Composer in order for your classes
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to be autoloadable.
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> Info: If you do not want to use the Yii autoloader, you must create your own version of the `Yii.php` file
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and include it in your [entry script](structure-entry-scripts.md).
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Autoloading Extension Classes <span id="autoloading-extension-classes"></span>
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-----------------------------
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The Yii autoloader is capable of autoloading [extension](structure-extensions.md) classes. The sole requirement
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is that an extension specifies the `autoload` section correctly in its `composer.json` file. Please refer to the
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[Composer documentation](https://getcomposer.org/doc/04-schema.md#autoload) for more details about specifying `autoload`.
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In case you do not use the Yii autoloader, the Composer autoloader can still autoload extension classes for you.
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