浅析PHP中Collection 类的设计

用.net开发已经很多年了，最近接触到php，发现php也很好玩。不过发现它里面没有集合collection类，只有数组，并且数组很强。这里我用数组来包装成一个集合collection，代码如下：

class collection{
    private $_members=array();

    public  function additem($obj,$key=null)
    {
        if($key)
        {
            if(isset($this->_members[$key]))
            {
                throw  new exception("key \"$key\" already in use!");
            }
            else
            {
                $this->_members[$key]=$obj;
            }
        }
        else
        {
            $this->_members[]=$obj;
        }
    }

    public function removeitem($key)
    {
        if(isset($this->_members[$key]))
        {
            unset($this->_members[$key]);
        }
        else
        {
            throw new exception("invalid key \"$key\"!");
        }
    }
    public function getitem($key)
    {
        if(isset($this->_members[$key]))
        {
            return $this->_members[$key];
        }
        else
        {
            throw new  exception("invalid key \"$key\"!");
        }
    }

    public function keys()
    {
        return array_keys($this->_members);
    }

    public function legth()
    {
        return sizeof($this->_members);
    }

    public function exists($key)
    {
        return (isset($this->_members[$key]));
    }
}

现在我们来测试一下这个集合是否好用。
我们首先建立一个集合元素类course：

class  course
{
    private $_id;
    private $_coursecode;
    private $_name;

  public function __construct($id,$coursecode,$name)
    {
        $this->_id=$id;
        $this->_coursecode=$coursecode;
        $this->_name=$name;
    }

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

    public function getid()
    {
        return $this->_id;
    }

    public function getcoursecode()
    {
        return $this->_coursecode;
    }

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

测试代码如下：
$courses=new collection();
$courses->additem(new course(1, "001", "语文"),1);
$courses->additem(new course(2, "002", "数学"),2);
$obj=$courses->getitem(1);
print $obj;
我想这个集合类应该可以满足我们平日开发的需求了吧。
可是我们现在。net里面有个对象延迟加载，举个例子来说吧，假如现在有student这个对象，它应该有很多course，但是我们希望在访问course之前course是不会加载的。也就是说在实例化student的时候course个数为0，当我们需要course的时候它才真正从数据库读取相应数据。就是需要我们把collection做成惰性实例化。
修改后的collection代码如下：

class collection {
  private $_members = array();    //collection members
  private $_onload;               //holder for callback function
  private $_isloaded = false;     //flag that indicates whether the callback
                                  //has been invoked
  public function additem($obj, $key = null) {
    $this->_checkcallback();      //_checkcallback is defined a little later

    if($key) {
      if(isset($this->_members[$key])) {
        throw new keyinuseexception("key \"$key\" already in use!");
      } else {
        $this->_members[$key] = $obj;
      }
    } else {
      $this->_members[] = $obj;
    }
  }
  public function removeitem($key) {
    $this->_checkcallback();

    if(isset($this->_members[$key])) {
      unset($this->_members[$key]);
    } else {
      throw new keyinvalidexception("invalid key \"$key\"!");
    } 
  }

  public function getitem($key) {
    $this->_checkcallback();

    if(isset($this->_members[$key])) {
      return $this->_members[$key];
    } else {
      throw new keyinvalidexception("invalid key \"$key\"!");
    }
  }
  public function keys() {
    $this->_checkcallback();
    return array_keys($this->_members);
  }
  public function length() {
    $this->_checkcallback();
    return sizeof($this->_members);
  }
  public function exists($key) {
    $this->_checkcallback();
    return (isset($this->_members[$key]));
  }
  /**
   * use this method to define a function to be
   * invoked prior to accessing the collection. 
   * the function should take a collection as a
   * its sole parameter.
   */
  public function setloadcallback($functionname, $objorclass = null) {
    if($objorclass) {
      $callback = array($objorclass, $functionname);
    } else {
      $callback = $functionname;
    }

    //make sure the function/method is valid
    if(!is_callable($callback, false, $callablename)) {
      throw new exception("$callablename is not callable " .
                          "as a parameter to onload");
      return false;
    }

    $this->_onload = $callback;
  }

  /**
   * check to see if a callback has been defined and if so,
   * whether or not it has already been called.  if not,
   * invoke the callback function.
   */
  private function _checkcallback() {
    if(isset($this->_onload) && !$this->_isloaded) {
      $this->_isloaded = true;
      call_user_func($this->_onload, $this);
    }
  }
}

所需的student如下：

class coursecollection extends collection {
 public function additem(course $obj,$key=null) {
        parent::additem($obj,$key);
    }
}
class student{
    private $_id;
    private $_name;
    public $course;

    public  function __construct($id,$name)
    {
        $this->_id=$id;
        $this->_name=$name;
        $this->course=new coursecollection();
        $this->course->setloadcallback('loadcourses',$this);
    }

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

    public function getid()
    {
        return $this->_id;
    }

    public function __tostring()
    {
        return $this->_name;
    }
    public function loadcourses(collection $col)
    {
        $col->additem(new course(1, "001", "语文"),1);
        $col->additem(new course(2, "002", "数学"),2);
    }
}

调用代码如下：
$student=new student(1, "majiang");
print $student->getname();
print $student->course->getitem(1);