ObjectManager教程
    https://bsscommerce.com/confluence/quick-overview-of-object-manager-in-magento-2/

    1)构造方法 constructor
    \Magento\Framework\ObjectManagerInterface
    2)phtml中调用（不推荐仅限debug）
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();

    get()
    返回singleton object
    适用于单例共享组件、获取对象工厂
    create()
    返回new object
    适用于接口

    create vs get
    $_objectManager->create() creates new instance of the object, no-matter-what
    $_objectManager->get() first tries to find shared instance (already created), if it’s not found - it just creates new shared instance