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);