yii2 i18n学习

举例说明常见的翻译：Yii::t('app','Login')；
追踪源码：BaseYii.php 文件 ，Yii::t($category, $message, $params = [], $language = null) 这个方法实际上是 \yii\i18n\I18N::translate()的一个捷径方法

  /**
      * Translates a message to the specified language.
      *
      * This is a shortcut method of [[\yii\i18n\I18N::translate()]].
      *
      * The translation will be conducted according to the message category and the target language will be used.
      *
      * You can add parameters to a translation message that will be substituted（取代） with the corresponding（相应的） value after
      * translation. The format for this is to use curly brackets around the parameter name as you can see in the following example:
      *
      * ```php
      * $username = 'Alexander';
      * echo \Yii::t('app', 'Hello, {username}!', ['username' => $username]);
      * ```
      *
      * Further formatting of message parameters is supported using the [PHP intl extensions](http://www.php.net/manual/en/intro.intl.php)
      * message formatter. See [[\yii\i18n\I18N::translate()]] for more details.
      *
      * @param string $category the message category.
      * @param string $message the message to be translated.
      * @param array $params the parameters that will be used to replace the corresponding placeholders in the message.
      * @param string $language the language code (e.g. `en-US`, `en`). If this is null, the current
      * [[\yii\base\Application::language|application language]] will be used.
      * @return string the translated message.
      */
     public static function t($category, $message, $params = [], $language = null)
     {
         if (static::$app !== null) {
             return static::$app->getI18n()->translate($category, $message, $params, $language ?: static::$app->language);
         } else {
             $p = [];
             foreach ((array) $params as $name => $value) {
                 $p['{' . $name . '}'] = $value;
             }

             return ($p === []) ? $message : strtr($message, $p);
         }
     }

查看 \yii\i18n\I18N::translate()方法：

 /**
      * Translates a message to the specified language.
      *
      * After translation the message will be formatted using [[MessageFormatter]] if it contains
      * ICU message format and `$params` are not empty.
      *
      * @param string $category the message category.
      * @param string $message the message to be translated.
      * @param array $params the parameters that will be used to replace the corresponding placeholders in the message.
      * @param string $language the language code (e.g. `en-US`, `en`).
      * @return string the translated and formatted message.
      */
     public function translate($category, $message, $params, $language)
     {
         // 返回MessageSource the message source for the given category.
         $messageSource = $this->getMessageSource($category);
         //返回翻译的内容
         $translation = $messageSource->translate($category, $message, $language);
         //格式话翻译的内容
         if ($translation === false) {
             return $this->format($message, $params, $messageSource->sourceLanguage);
         } else {
             return $this->format($translation, $params, $language);
         }
     }

其中 $messageSource = $this->getMessageSource($category);//根据配置文件内容和 给的参数$category 返回继承MessageSource 相应子类的实例对象， 本例根据配置文件返回PhpMessageSource对象

 <?php
 return [
     'vendorPath' => dirname(dirname(__DIR__)) . '/vendor',
     'components' => [
         'cache' => [
             'class' => 'yii\caching\FileCache',
         ],
         'i18n'=>[
             'translations' => [
                 'app*'=> [
                     'class' => 'yii\i18n\PhpMessageSource',
                     'basePath'=>'@common/messages',
                     'fileMap'=>[
                         'app'=>'app.php',
                     ],
                 ],
             ],
         ],
     ],
 ];

$messageSource = $this->getMessageSource($category); 返回继承MessageSource 相应子类的实例对象

  /**
      * Returns the message source for the given category.
      * @param string $category the category name.
      * @return MessageSource the message source for the given category.
      * @throws InvalidConfigException if there is no message source available for the specified category.
      */
     //根据配置文件内容和 给的参数$category 返回继承MessageSource 相应子类的实例对象， 本例根据配置文件返回PhpMessageSource对象
     public function getMessageSource($category)
     {
         if (isset($this->translations[$category])) {
             $source = $this->translations[$category];
             if ($source instanceof MessageSource) {
                 return $source;
             } else {
                 return $this->translations[$category] = Yii::createObject($source);
             }
         } else {
             // try wildcard matching
             foreach ($this->translations as $pattern => $source) {
                 if (strpos($pattern, '*') > 0 && strpos($category, rtrim($pattern, '*')) === 0) {
                     if ($source instanceof MessageSource) {
                         return $source;
                     } else {
                         return $this->translations[$category] = $this->translations[$pattern] = Yii::createObject($source);
                     }
                 }
             }
             // match '*' in the last
             if (isset($this->translations['*'])) {
                 $source = $this->translations['*'];
                 if ($source instanceof MessageSource) {
                     return $source;
                 } else {
                     return $this->translations[$category] = $this->translations['*'] = Yii::createObject($source);
                 }
             }
         }

         throw new InvalidConfigException("Unable to locate message source for category '$category'.");
     }

再根据实例对象的translate方法返回翻译的内容，  $translation = $messageSource->translate($category, $message, $language);

MessageSource  类的 translate方法如下：

   /**
      * Translates a message to the specified language.
      *
      * Note that unless [[forceTranslation]] is true, if the target language
      * is the same as the [[sourceLanguage|source language]], the message
      * will NOT be translated.
      *
      * If a translation is not found, a [[EVENT_MISSING_TRANSLATION|missingTranslation]] event will be triggered.
      *
      * @param string $category the message category
      * @param string $message the message to be translated
      * @param string $language the target language
      * @return string|boolean the translated message or false if translation wasn't found or isn't required
      */
     // 翻译一段信息为指定的语言文件下的内容
     public function translate($category, $message, $language)
     {
         if ($this->forceTranslation || $language !== $this->sourceLanguage) {

             return $this->translateMessage($category, $message, $language);
         } else {
             return false;
         }
     }

$this->translateMessage($category, $message, $language);

  /**
      * Translates the specified message.
      * If the message is not found, a [[EVENT_MISSING_TRANSLATION|missingTranslation]] event will be triggered.
      * If there is an event handler, it may provide a [[MissingTranslationEvent::$translatedMessage|fallback translation]].
      * If no fallback translation is provided this method will return `false`.
      * @param string $category the category that the message belongs to.
      * @param string $message the message to be translated.
      * @param string $language the target language.
      * @return string|boolean the translated message or false if translation wasn't found.
      */
     protected function translateMessage($category, $message, $language)
     {
         $key = $language . '/' . $category;
         if (!isset($this->_messages[$key])) {
             // $language = 'zh-CN'  $category = 'app'
             // $this->loadMessages($category, $language) 为合并后的指定类别语言的翻译数组 key => value
             $this->_messages[$key] = $this->loadMessages($category, $language);
         }
         if (isset($this->_messages[$key][$message]) && $this->_messages[$key][$message] !== '') {
             return $this->_messages[$key][$message];
         } elseif ($this->hasEventHandlers(self::EVENT_MISSING_TRANSLATION)) {
             $event = new MissingTranslationEvent([
                 'category' => $category,
                 'message' => $message,
                 'language' => $language,
             ]);
             $this->trigger(self::EVENT_MISSING_TRANSLATION, $event);
             if ($event->translatedMessage !== null) {
                 return $this->_messages[$key][$message] = $event->translatedMessage;
             }
         }

         return $this->_messages[$key][$message] = false;
     }

根据配置文件生成的 MessageSource子类相应对象的loadMessages方法，生成翻译的文件数组内容，在根据内容查找相应的以$message为键的对应值：
return $this->_messages[$key][$message];

本例根据PhpMessageSource对象的translateMessage方法为例说明：

   /**
      * Loads the message translation for the specified $language and $category.
      * If translation for specific locale code such as `en-US` isn't found it
      * tries more generic `en`. When both are present, the `en-US` messages will be merged
      * over `en`. See [[loadFallbackMessages]] for details.
      * If the $language is less specific than [[sourceLanguage]], the method will try to
      * load the messages for [[sourceLanguage]]. For example: [[sourceLanguage]] is `en-GB`,
      * $language is `en`. The method will load the messages for `en` and merge them over `en-GB`.
      *
      * @param string $category the message category
      * @param string $language the target language
      * @return array the loaded messages. The keys are original messages, and the values are the translated messages.
      * @see loadFallbackMessages
      * @see sourceLanguage
      */
     protected function loadMessages($category, $language)
     {
         //返回指定类别和语言的文件路径
         $messageFile = $this->getMessageFilePath($category, $language);
         // 根据文件路径，加载翻译文件，为数组键值对
         $messages = $this->loadMessagesFromFile($messageFile);

         $fallbackLanguage = substr($language, 0, 2);
         $fallbackSourceLanguage = substr($this->sourceLanguage, 0, 2);

         if ($language !== $fallbackLanguage) {
             // $language = 'zh-CN'    $fallbackLanguage = 'zh'  合并语言zh-CN和zh文件为新的$messages
             $messages = $this->loadFallbackMessages($category, $fallbackLanguage, $messages, $messageFile);
         } elseif ($language === $fallbackSourceLanguage) {
             //  $language = 'zh'    $fallbackLanguage = 'zh' $this->sourceLanguage = 'en' 合并zh 文件和en 文件，当zh文件翻译key的value信息没有时用 en 文件的相应的value 替代，合并为新的翻译数组内容 $messages
             $messages = $this->loadFallbackMessages($category, $this->sourceLanguage, $messages, $messageFile);
         } else {
             if ($messages === null) {
                 Yii::error("The message file for category '$category' does not exist: $messageFile", __METHOD__);
             }
         }

         return (array) $messages;
     }

 //返回指定类别和语言的文件路径
$messageFile = $this->getMessageFilePath($category, $language);

  /**
      * Returns message file path for the specified language and category.
      *
      * @param string $category the message category
      * @param string $language the target language
      * @return string path to message file
      */
     //返回指定的语言和类别文件路径
     protected function getMessageFilePath($category, $language)
     {
         //语言类别文件路径
         $messageFile = Yii::getAlias($this->basePath) . "/$language/";
         if (isset($this->fileMap[$category])) {
             $messageFile .= $this->fileMap[$category];
         } else {
             $messageFile .= str_replace('\\', '/', $category) . '.php';
         }

         return $messageFile;
     }

// 根据文件路径，加载翻译文件，为数组键值对
$messages = $this->loadMessagesFromFile($messageFile);

  /**
      * Loads the message translation for the specified language and category or returns null if file doesn't exist.
      *
      * @param string $messageFile path to message file
      * @return array|null array of messages or null if file not found
      */
     //加载翻译文件，翻译文件为数组文件
     protected function loadMessagesFromFile($messageFile)
     {
         if (is_file($messageFile)) {
             $messages = include($messageFile);
             if (!is_array($messages)) {
                 $messages = [];
             }

             return $messages;
         } else {
             return null;
         }
     }

//根据不同的条件,从其他文件中补充语言文件的缺少的key 的值value 
$messages = $this->loadFallbackMessages($category, $this->sourceLanguage, $messages, $messageFile);

 /**
      * The method is normally called by [[loadMessages]] to load the fallback messages for the language.
      * Method tries to load the $category messages for the $fallbackLanguage and adds them to the $messages array.
      *
      * @param string $category the message category
      * @param string $fallbackLanguage the target fallback language
      * @param array $messages the array of previously loaded translation messages.
      * The keys are original messages, and the values are the translated messages.
      * @param string $originalMessageFile the path to the file with messages. Used to log an error message
      * in case when no translations were found.
      * @return array the loaded messages. The keys are original messages, and the values are the translated messages.
      * @since 2.0.7
      */
     protected function loadFallbackMessages($category, $fallbackLanguage, $messages, $originalMessageFile)
     {
         // 返回的信息文件路径
         $fallbackMessageFile = $this->getMessageFilePath($category, $fallbackLanguage);
         //根据文件路径信息，加载文件的内容
         $fallbackMessages = $this->loadMessagesFromFile($fallbackMessageFile);

         if (
             $messages === null && $fallbackMessages === null
             && $fallbackLanguage !== $this->sourceLanguage
             && $fallbackLanguage !== substr($this->sourceLanguage, 0, 2)
         ) {
             Yii::error("The message file for category '$category' does not exist: $originalMessageFile "
                 . "Fallback file does not exist as well: $fallbackMessageFile", __METHOD__);
         } elseif (empty($messages)) {
             return $fallbackMessages;
         } elseif (!empty($fallbackMessages)) {
             // 当$message数组key 值为空时，且$fallbackMessages对应的key的翻译内容存在是，把$fallbackMessages的内容翻译数组赋值给 $messages[$key]
             foreach ($fallbackMessages as $key => $value) {
                 if (!empty($value) && empty($messages[$key])) {
                     $messages[$key] = $fallbackMessages[$key];
                 }
             }
         }

         return (array) $messages;
     }

最后返回$messages:

return (array) $messages;

根据提供的参数（$message）把$message作为key 查找 $messages 数组中对应的key的值