/**
  * 抽象基类
  */
  abstract class Unit
  {
    /**
    *获取名称
    */
    abstract public function getName();

  }

  /**
  * Cpu类
  */
  class Cpu extends Unit
  {
    public function getName()
    {
      return 'i am cpu';
    }
  }

  /**
  * 内存类
  */
  class Memory extends Unit
  {
    public function getName()
    {
      return 'i am memory';
    }
  }

  /**
  * 键盘类
  */
  class Keyboard extends Unit
  {
    public function getName()
    {
      return 'i am keyboard';
    }
  }

  /**
  * 计算机类
  */
  class Computer
  {
    protected $_items = [];

    public function add(Unit $unit)
    {
      $this->_items[] = $unit;
    }

    public function print()
    {
      // 循环打印各个组成部分
      foreach ($this->_items as $item) {
        $item->getName();
      }
    }
  }

  /**
  * 抽象基类
  */
  abstract class Unit
  {
    /**
    * 获取名称
    */
    abstract public function getName();

    /**
    * 用来接受访问者对象，回调访问者的visit方法
    * 非常关键的方法
    */
    public function accept(Visitor $visitor)
    {
      $method = visit . get_class($this);
      if (method_exists($visitor, $method)) {
        $visitor->$method($this);
      }
    }
  }

  /**
  * Cpu类
  */
  class Cpu extends Unit
  {
    public function getName()
    {
      return 'i am cpu';
    }
  }

  /**
  * Memory类
  */
  class Memory extends Unit
  {
    public function getName()
    {
      return 'i am memory';
    }
  }

  /**
  * Keyboard类
  */
  class Keyboard extends Unit
  {
    public function getName()
    {
      return 'i am keyboard';
    }
  }
  
  /**
  * Keyboard类
  */
  interface Visitor
  {
    public function visitCpu(Cpu $cpu);
    public function visitMemory(Memory $memory);
    public function visitKeyboard(Keyboard $keyboard);
  }

  /**
  * 
  */
  class PrintVisitor implements Visitor
  {
    public function visitCpu(Cpu $cpu)
    {
      echo "hello, " . $cpu->getName() . "\n";
    }

    public function visitMemory(Memory $memory)
    {
      echo "hello, " . $memory->getName() . "\n";
    }

    public function visitKeyboard(Keyboard $keyboard)
    {
      echo "hello, " . $keyboard->getName() . "\n";
    }
  }

  /**
  * 
  */
  class Computer
  {
    protected $_items = [];

    public function add(Unit $unit)
    {
      $this->_items[] = $unit;
    }
    
    /**
    * 调用各个组件的accept方法
    */
    public function accept(Visitor $visitor)
    {
      foreach ($this->_items as $item) {
        $item->accept($visitor);
      }
    }
  }

  $computer = new Computer();
  $computer->add(new Cpu());
  $computer->add(new Memory());
  $computer->add(new Keyboard());

  $printVisitor = new PrintVisitor();
  $computer->print($printVisitor);

  // 以上的代码将打印出  
  hello, i am cpu
  hello, i am memory
  hello, i am keyboard