Fixes #1452: Added Module::getInstance() to allow accessing the module instance from anywhere within the module

docs/guide/structure-modules.md

@@ -3,8 +3,9 @@ Modules
 
 Modules are self-contained software units that consist of [models](structure-models.md), [views](structure-views.md),
 [controllers](structure-controllers.md), and other supporting components. End users can access the controllers
-of a module when it is installed in [application](structure-applications.md). Modules differ from
-[applications](structure-applications.md) in that the former cannot be deployed alone and must reside within the latter.
+of a module when it is installed in [application](structure-applications.md). For these reasons, modules are
+often viewed as mini-applications. Modules differ from [applications](structure-applications.md) in that
+modules cannot be deployed alone and must reside within applications.
 
 
 ## Creating Modules <a name="creating-modules"></a>
@@ -135,7 +136,7 @@ the [[yii\base\Application::modules|modules]] property of the application. The f
 ```
 
 The [[yii\base\Application::modules|modules]] property takes an array of module configurations. Each array key
-represents a module ID which uniquely identifies the module among all modules in the application, and the corresponding
+represents a *module ID* which uniquely identifies the module among all modules in the application, and the corresponding
 array value is a [configuration](concept-configurations.md) for creating the module.
 
 
@@ -152,8 +153,24 @@ controller in the `forum` module.
 
 ### Accessing Modules <a name="accessing-modules"></a>
 
-A module is instantiated when one of its controllers is accessed by end users. You may access the instance of a module
-using the approaches shown in the following example:
+Within a module, you may often need to get the instance of the [module class](#module-classes) so that you can
+access the module ID, module parameters, module components, etc. You can do so by using the following statement:
+
+```php
+$module = MyModuleClass::getInstance();
+```
+
+where `MyModuleClass` refers to the name of the module class that you are interested in. The `getInstance()` method
+will return the currently requested instance of the module class. If the module is not requested, the method will
+return null. Note that You do not want to manually create a new instance of the module class because it will be
+different from the one created by Yii in response to a request.
+
+> Info: When developing a module, you should not assume the module will use a fixed ID. This is because a module
+  can be associated with an arbitrary ID when used in an application or within another module. In order to get
+  the module ID, you should use the above approach to get the module instance first, and then get the ID via
+  `$module->id`.
+
+You may also access the instance of a module using the following approaches:
 
 ```php
 // get the module whose ID is "forum"
@@ -163,8 +180,8 @@ $module = \Yii::$app->getModule('forum');
 $module = \Yii::$app->controller->module;
 ```
 
-The first approach is only useful in application code which has knowledge about the module ID, while the second approach
-is best used by the code within the module.
+The first approach is only useful when you know the module ID, while the second approach is best used when you
+know about the controllers being requested.
 
 Once getting hold of a module instance, you can access parameters or components registered with the module. For example,
 
@@ -230,5 +247,5 @@ a set of closely related features. Each such feature group can be developed as a
 maintained by a specific developer or team.
 
 Modules are also a good way of reusing code at the feature group level. Some commonly used features, such as
-user management, comment management, can all be developed in terms of modules so that they can be resued easily
+user management, comment management, can all be developed in terms of modules so that they can be reused easily
 in future projects.

framework/CHANGELOG.md

@@ -66,6 +66,7 @@ Yii Framework 2 Change Log
 - Bug: Fixed the bug that requesting protected or private action methods would cause 500 error instead of 404 (qiangxue)
 - Bug: Fixed Object of class Imagick could not be converted to string in CaptchaAction (eXprojects, cebe)
 - Enh #422: Added Support for BIT(M) data type default values in Schema (cebe)
+- Enh #1452: Added `Module::getInstance()` to allow accessing the module instance from anywhere within the module (qiangxue)
 - Enh #2264: `CookieCollection::has()` will return false for expired or removed cookies (qiangxue)
 - Enh #2435: `yii\db\IntegrityException` is now thrown on database integrity errors instead of general `yii\db\Exception` (samdark)
 - Enh #2558: Enhanced support for memcached by adding `yii\caching\MemCache::persistentId` and `yii\caching\MemCache::options` (qiangxue)
@@ -110,12 +111,14 @@ Yii Framework 2 Change Log
 - Enh #3774: Added `FileValidator::checkExtensionByMimeType` to support validating file types against file mime-types (Ragazzo)
 - Enh #3801: Base migration controller `yii\console\controllers\BaseMigrateController` extracted (klimov-paul)
 - Enh #3811: Now Gii model generator makes autocomplete for model class field (mitalcoi)
+- Enh #3926: `yii\widgets\Breadcrumbs::$links`. Allows individual link to have its own `template` (creocoder, umneeq)
 - Enh #3939: `\yii\Inflector::slug()` improvements (samdark)
     - Added protected `\yii\Inflector::transliterate()` that could be replaced with custom translit implementation.
     - Added proper tests for both intl-based slug and PHP fallback.
     - Removed character maps for non-latin languages.
     - Improved overall slug results.
     - Added note about the fact that intl is required for non-latin languages to requirements checker.
+- Enh #4028: Added ability to `yii\widgets\Menu` to encode each item's label separately (creocoder, umneeq)
 - Enh: Added support for using sub-queries when building a DB query with `IN` condition (qiangxue)
 - Enh: Supported adding a new response formatter without the need to reconfigure existing formatters (qiangxue)
 - Enh: Added `yii\web\UrlManager::addRules()` to simplify adding new URL rules (qiangxue)
@@ -129,8 +132,6 @@ Yii Framework 2 Change Log
 - Enh: Added param `hideOnSinglePage` to `yii\widgets\LinkPager` (arturf)
 - Enh: Added support for array attributes in `in` validator (creocoder)
 - Enh: Improved `yii\helpers\Inflector::slug` to support more cases for Russian, Hebrew and special characters (samdark)
-- Enh #3926: `yii\widgets\Breadcrumbs::$links`. Allows individual link to have its own `template` (creocoder, umneeq)
-- Enh #4028: Added ability to `yii\widgets\Menu` to encode each item's label separately (creocoder, umneeq)
 - Chg #2287: Split `yii\db\ColumnSchema::typecast()` into two methods `phpTypecast()` and `dbTypecast()` to allow specifying PDO type explicitly (cebe)
 - Chg #2898: `yii\console\controllers\AssetController` is now using hashes instead of timestamps (samdark)
 - Chg #2913: RBAC `DbManager` is now initialized via migration (samdark)

framework/base/Application.php

@@ -189,6 +189,7 @@ abstract class Application extends Module
     public function __construct($config = [])
     {
         Yii::$app = $this;
+        $this->setInstance($this);
 
         $this->state = self::STATE_BEGIN;
 

framework/base/Module.php

@@ -122,7 +122,10 @@ class Module extends ServiceLocator
      * @var array child modules of this module
      */
     private $_modules = [];
-
+    /**
+     * @var array list of currently requested modules indexed by their class names
+     */
+    private static $_instances = [];
 
     /**
      * Constructor.
@@ -137,6 +140,32 @@ class Module extends ServiceLocator
         parent::__construct($config);
     }
 
+    /**
+     * Returns the currently requested instance of this module class.
+     * If the module class is not currently requested, null will be returned.
+     * This method is provided so that you access the module instance from anywhere within the module.
+     * @return static|null the currently requested instance of this module class, or null if the module class is not requested.
+     */
+    public static function getInstance()
+    {
+        $class = get_called_class();
+        return isset(self::$_instances[$class]) ? self::$_instances[$class] : null;
+    }
+
+    /**
+     * Sets the currently requested instance of this module class.
+     * @param Module|null $instance the currently requested instance of this module class.
+     * If it is null, the instance of the calling class will be removed, if any.
+     */
+    public static function setInstance($instance)
+    {
+        if ($instance === null) {
+            unset(self::$_instances[get_class()]);
+        } else {
+            self::$_instances[get_class($instance)] = $instance;
+        }
+    }
+
     /**
      * Initializes the module.
      *
@@ -326,8 +355,10 @@ class Module extends ServiceLocator
                 if (is_array($this->_modules[$id]) && !isset($this->_modules[$id]['class'])) {
                     $this->_modules[$id]['class'] = 'yii\base\Module';
                 }
-
-                return $this->_modules[$id] = Yii::createObject($this->_modules[$id], [$id, $this]);
+                /** @var $module Module */
+                $module = Yii::createObject($this->_modules[$id], [$id, $this]);
+                $module->setInstance($module);
+                return $this->_modules[$id] = $module;
             }
         }