//获取数据表元数据

public function getTableSchema($name, $refresh = false)

{

    if (array_key_exists($name, $this->_tables) && !$refresh) {

        return $this->_tables[$name];

    }

    $db = $this->db;

    $realName = $this->getRawTableName($name);

    if ($db->enableSchemaCache && !in_array($name, $db->schemaCacheExclude, true)) {

        /* @var $cache Cache */

        $cache = is_string($db->schemaCache) ? Yii::$app->get($db->schemaCache, false) : $db->schemaCache;

        if ($cache instanceof Cache) {

            $key = $this->getCacheKey($name);

            if ($refresh || ($table = $cache->get($key)) === false) {

                //通过工厂方法loadTableSchema()去获取TableSchema实例

                $this->_tables[$name] = $table = $this->loadTableSchema($realName);

                if ($table !== null) {

                    $cache->set($key, $table, $db->schemaCacheDuration, new TagDependency([

                        'tags' => $this->getCacheTag(),

                    ]));

                }

            } else {

                $this->_tables[$name] = $table;

            }

            return $this->_tables[$name];

        }

    }

    //通过工厂方法loadTableSchema()去获取TableSchema实例

    return $this->_tables[$name] = $this->loadTableSchema($realName);

}

//获取TableSchema实例，让子类去实现

abstract protected function loadTableSchema($name);