浅析PHP中Collection类的设计_php技巧

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：
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：
代码如下:
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：
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：
代码如下:
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);