php组合模式的一个例子相关的问题

Question

<?php
abstract class MenuComponent
{
    abstract function add(MenuComponent $component);
    abstract function remove(MenuComponent $component);
    abstract function getName();
    abstract function displayOperation();

}

class MenuComposite extends MenuComponent
{
    private $_items = array(); 
    private $_name = null;

    function __construct($name)
    {
        $this->_name = $name;
    }

    function remove(MenuComponent $component){

    }

    function add(MenuComponent $component)
    {
        $this->_items[$component->getName()] = $component;
    }

    function getName(){
        return $this->_name;
    }

     public function displayOperation() {  
        static $align = '|';  

            $align .='';  
        
        echo $this->_name, " <br/>";  
        foreach($this->_items as $name=> $item) {  
            echo $align;  
            $item->displayOperation();  
        }  
    }  
}

class Client
{
    static function displayMenu()
    {
        $subMenu1 = new MenuComposite('submenu1');
        $subMenu1->add($subMenu1);
        $subMenu1->displayOperation();
    }
}

Client::displayMenu();

输出是无限循环的
submenu1
|submenu1
|submenu1
...

为什么会这样，第一次递归调用displayOperation（）后foreach里面应该是空，就停止了，为什么会无限循环？

Answer 1

add 的实现 $this->_items[$component->getName()] = $component;

注意$subMenu1->add($subMenu1);

实例$subMenu1的属性$_items["submenu1"]是他自己本身 $subMenu1

然后无穷下去...

Answer 2

类属性$this->item一直是你实例化传进去的对象，所以，这个displayOperation()方法就一直循环下去了