1. 組成

Yii2的logger主要分為三個(gè)部分：

• Logger 負(fù)責(zé)日志級(jí)別恳守，記錄格式等等的配置和管理；
• Dispatcher 負(fù)責(zé)日志的收集和對(duì)target的管理催烘；
• target 負(fù)責(zé)執(zhí)行寫入的操作颗圣，可以是寫文件聘萨，寫數(shù)據(jù)庫等等芜飘。
  （我是覺得代碼寫得不是很好茧妒，Dispatcher作為L(zhǎng)ogger的成員及老，而Logger也作為Dispatcher的成員，我覺得耦合性太強(qiáng)了食铐，monolog相對(duì)就好很多，有空再寫這個(gè)源碼的分析）

2. 部分源碼

1. 先看application類里面注冊(cè)的核心組件

\yii\base\Application::coreComponents
   /**
     * Returns the configuration of core application components.
     * @see set()
     */
    public function coreComponents()
    {
        return [
            'log' => ['class' => 'yii\log\Dispatcher'],
            'view' => ['class' => 'yii\web\View'],
            'formatter' => ['class' => 'yii\i18n\Formatter'],
            'i18n' => ['class' => 'yii\i18n\I18N'],
            'mailer' => ['class' => 'yii\swiftmailer\Mailer'],
            'urlManager' => ['class' => 'yii\web\UrlManager'],
            'assetManager' => ['class' => 'yii\web\AssetManager'],
            'security' => ['class' => 'yii\base\Security'],
        ];
    }

• 2 初始化log組件

- 0. \yii\base\Application::getLog
- 1. \yii\log\Dispatcher::__construct 初始化logger
 /**
     * @inheritdoc
     */
    public function __construct($config = [])
    {
        // ensure logger gets set before any other config option
        if (isset($config['logger'])) {
            $this->setLogger($config['logger']);
            unset($config['logger']);
        }
        // connect logger and dispatcher
        $this->getLogger(); // 重要

        parent::__construct($config);
    }
- 2. \yii\log\Dispatcher::getLogger 創(chuàng)建一個(gè)logger單例
    /**
     * Gets the connected logger.
     * If not set, [[\Yii::getLogger()]] will be used.
     * @property Logger the logger. If not set, [[\Yii::getLogger()]] will be used.
     * @return Logger the logger.
     */
    public function getLogger()
    {
        if ($this->_logger === null) {
            $this->setLogger(Yii::getLogger());
        }
        return $this->_logger;
    }


- 3. 從上一步的getLogger追代碼到：\yii\log\Dispatcher::setLogger
    /**
     * Sets the connected logger.
     * @param Logger $value the logger.
     */
    public function setLogger($value)
    {
        $this->_logger = $value;
        $this->_logger->dispatcher = $this; //這一步重要寞秃，把logger里的dispatcher關(guān)聯(lián)了自己
    }


- 4. \yii\log\Dispatcher::init 初始化注冊(cè)的target
 /**
     * @inheritdoc
     */
    public function init()
    {
        parent::init();

        foreach ($this->targets as $name => $target) {
            if (!$target instanceof Target) {
                $this->targets[$name] = Yii::createObject($target);
            }
        }
    }

關(guān)鍵步驟就是以上了。

• 3. 打日志朗涩，調(diào)用 Yii::error.

- 1.調(diào)用 Yii::error其實(shí)就是調(diào)用\yii\BaseYii::error
   /**
     * Logs an error message.
     * An error message is typically logged when an unrecoverable error occurs
     * during the execution of an application.
     * @param string $message the message to be logged.
     * @param string $category the category of the message.
     */
    public static function error($message, $category = 'application')
    {
        static::getLogger()->log($message, Logger::LEVEL_ERROR, $category);
    }
- 2.再看getLogger
   /**
     * @return Logger message logger
     */
    public static function getLogger()
    {
        if (self::$_logger !== null) {
            return self::$_logger;
        } else {
            return self::$_logger = static::createObject('yii\log\Logger'); //返回單例
        }
    }
- 3. 注意到初始化組件的時(shí)候谢床， \yii\log\Dispatcher::getLogger調(diào)用的就是上面的 getLogger()厘线，
所以皆的，這是同一個(gè)對(duì)象來的，打日志就可以用logger里面的dispatcher來管理target了费薄，target就是執(zhí)行寫日志的功能。