Object composition by Proxy

[Selkirk]

Standard PHP has a mechanism for specifying a reference to an object and a method to call it:

$ref = array(&$obj, "methodName");

$params = array("hello", "world");

call_user_object_array($ref, $params); 

In PHP 5, the overloading method could be used with this to create a proxy object which re-routed method calls:

// Untested

class ProxyReference {



    var $Obj;

    var $Ref;

    var $File;

    var $Class;

    var $Method;



    function Proxy($File, $Class, $Method) {

        $this->File = $File;

        $this->Class = $Class;

        $this->Method = $Method;

    }



    function __call($method, $params, &$return) {

        if (!is_object($this->Obj)) {

            require_once $this->File;

            $this->Obj =& new $this->Class(); //syntax correct?

            $this->Ref = array(&$this->Obj, $this->Method);

        }

        call_user_object_array($this->Ref, $params);

    }

} 

This ProxyReference class has the feature of not instantiating the object called unless a call is actually made. This is almost like a generic decorator.

This could be used to transparently compose objects:

class SomeClass {

    var $Obj

    function SomeClass($ReferenceObj) {

        $this->Obj =& $ReferenceObj;

    }



    function RarelyCalledMethod($Parameter) {

        $this->Obj->DoSomething($Parameter);

    }

} 

So, correct me if I am wrong, but it is possible in PHP5 for the SomeClass Object to be completely unaware of the fact that the object it is using is a proxy for another object. The SomeClass object is complete unaware that the real object it is communicating with has not been loaded.

SomeClass makes a normal method call. The call_user_object_array business is encapsulated in the ProxyReference class.

My question is, Is it possible to do this in PHP 4? If not, how close can you get?

One use I see for this is in creating a GenericDecorator class. This might only be possible in PHP 5. The advantage of a GenericDecorator (performance considerations aside for a moment) is that you could decorate any object without having to declare its entire interface. Only the part you wished to decorate. Thus, when you changed the object being decorated, you might not necessarily have to change the decorator classes. A good dependency limiter, resulting is less code for the same thing.

The use that I am interested in is creating on demand references in PHP 4. I want to minimize the effort involved with invoking an on-demand reference by encapsulating the call as much as possible. In other words, I should not have to copy/paste/modify a block of code like the implementation of the __call method in the ProxyReference object for every method invocation on the object in the RarelyCalledMethod invocation.

The context is creating objects that persist across several HTTP requests. I would like to construct the object as if it were going to persist across several requests. This object is composed of several other objects. Only a small subset of the objects in the assembly will actually be called on each HTTP requests. Over the course of the sequence of requests, most of the objects will be called. Since the object assembly can't persist across requests, I want to limit the cost of re-instantiating it each time, by not actually instantiating the portions of the assembly which are not not used. Yet, I want my construction code to be unaware of which portions of the assembly will or won't be used during each request. (The same construction code should get executed for each request.)

I am not sure if I have been very clear about what I am looking for, but I was hoping for some ideas.

[FelhoBacsi]

Hi!

Originally Posted by Selkirk

My question is, Is it possible to do this in PHP 4? If not, how close can you get?

http://hu.php.net/manual/en/ref.overload.php
This is not good for you?

Felho

[ghurtado]

If you dont want to use the overloading extension, lastcraft posted this possible implementation for Decorators in PHP4, not too long ago:
http://www.sitepoint.com/forums/show...ight=decorator

[HarryF]

Think I can see what you're trying to do and very powerful it would be if it can be done - suddenly it's worth persisting large numbers of objects.

Off the top of my head, one way to do it (and perhaps the only way) is to generate proxy classes (active generation) although a factory will be needed to get references to them. Something like;

<?php

class ProxyFactory {

    function & getProxy($Class,$File) {

        $this->Class = $Class;

        $this->File = $File;



        $this->checkProxyExists();



        include_once $File_Proxy;



        $ProxyClass = $Class."_Proxy";

        return new $ProxyClass();

    }



    function checkProxyExists() {

        // Something cleverer needed here, to check times / versions

        if (!file_exists($this->File) ) {

            $this->generateProxy();

        }

    }



    function generateProxy() {

        include_once $this->File;

        $Methods = get_class_methods($this->Class);



        // Insert code here the creates class and defines stub methods

    }

}

?>

<?php

// Here's the generated proxy code

class SomeClass_Proxy {

    var $__realInstance = NULL;

    function & __getRealInstance() {

       include_once "SomeClass.php";

       if ( ! $this->__realInstance ) {

           $this->__realInstance = & new SomeClass();

       }

       return $this->__realInstance;

    }



    function RarelyCalledMethod() {

       $args = & func_get_args();

       $obj =  & $this->__getRealInstance();

       $ref = array(&$obj,'RarelyCalledMethod');

       return call_user_func_array($ref,$args);

    }

}

?>

Not 100% sure that meets all the requirements but getting there perhaps. The first time a proxy is ever needed, it will have the overhead of included the real class. After that, it should be a fairly lightweight class and "lazy instantiates" the real class it's proxying for.

One issue might be whether the proxy RarelyUsedMethod should return a reference or copy of the result it gets from calling the real RarelyUsedMethod. For that I don't know.

[lastcraft]

Hi...

Originally Posted by Selkirk

My question is, Is it possible to do this in PHP 4? If not, how close can you get?

This works on PHP 4.3.0 (my current home version)...

<?php

class LazyProxy {

    var $_wrapped;

    var $_parameters;

    var $_class;

    

    function LazyProxy() {

        $this->_wrapped = false;

        $this->_parameters = func_get_args();

        $this->_class = array_shift($this->_parameters);

    }

    

    function &_instantiate() {

        $parameter_names = array();

        for ($i = 0; $i < count($this->_parameters); $i++) {

            $parameter_names[$i] = "\$this->_parameters[$i]";

        }

        $parameter_code = implode(', ', $parameter_names);

        $code = "\$instance = &new " . $this->_class . "($parameter_code);";

        eval($code);

        return $instance;

    }

    

    function __call($method, $parameters, &$result) {

        if ($this->_wrapped === false) {

            $this->_wrapped = &$this->_instantiate();

        }

        if (method_exists($this->_wrapped, $method)) {

            $result = call_user_func_array(

                    array($this->_wrapped, $method),

                    $parameters);

            return true;

        }

        return false;

    }

}

overload('LazyProxy');

?>

Here is the rather messy test case...

<?php

define('SIMPLE_TEST', 'simpletest/');

require_once(SIMPLE_TEST . 'unit_tester.php');

require_once(SIMPLE_TEST . 'reporter.php');

require_once('lazy_proxy.php');



class Prefixer {

    var $_prefix;

    

    function Prefixer($prefix) {

        $this->_prefix = $prefix;

        $this->countInstances(true);

    }

    function prefix($message) {

        return $this->_prefix . $message;

    }

    function countInstances($another = false) {

        static $count = 0;

        if ($another) {

            $count++;

        }

        return $count;

    }

}



class TestOfLazyProxy extends UnitTestCase {

    function TestOfLazyProxy() {

        $this->UnitTestCase();

    }

    function testProxyWrapsClass() {

        $start = Prefixer::countInstances();

        $proxy = &new LazyProxy('Prefixer', 'Hello ');

        $this->assertEqual($start, Prefixer::countInstances());

        $this->assertEqual($proxy->prefix('Jeff'), 'Hello Jeff');

        $this->assertEqual($start + 1, Prefixer::countInstances());

        $this->assertEqual($proxy->prefix('Harry'), 'Hello Harry');

        $this->assertEqual($start + 1, Prefixer::countInstances());

    }

}



$test = &new TestOfLazyProxy();

$test->run(new HtmlReporter());

?>

I am dropping references all over the place here. Without proper reflection I have the same problem as with code generation. i.e. func_get_args() copies parameters.

yours, Marcus

[sweatje]

Originally Posted by lastcraft

I am dropping references all over the place here. Without proper reflection I have the same problem as with code generation. i.e. func_get_args() copies parameters.

Well...it is ugly but the:

$decorated_object->method( array(&$object) ); 

should be a way around that.

[HarryF]

Think the problem there is the "overload extension" only became a default part of PHP with 4.3.0 (I think).

Also there was a weird bug with __call() - if you declare it, suddenly your own class methods cease to be able to return values. For example;

<?php

class Test {

    var $test;

    function Test () {

        $this->test='Hello World';

    }



    function getTest () {

        return $this->test;

    }



    function __call () {



    }

}



// Declare the class as overloaded

overload ("Test");



$test=new Test;



echo ( $test->getTest() ); // This returns nothing

echo ( $test->test ); // This works.

?>

Can't say I'd want to rely on the PHP4 implementation.

[HarryF]

Before I lose the link, there's a nice article here: An introduction to object prevalence, and Prevayler. Only vaguely related, as implementing it in PHP would probably only be worthwhile if you're using Shared memory.

[Selkirk]

Well, after some thought, I realized another way to express what I am looking to do. I want to be able to reference a class and have that reference include the information for finding that class. (similar to autoload) This is like in Java where a class name in com.x.x.x.classname format combined with a class path is sufficient for loading that class.

I need to have a solution that works in PHP 4.1 or better.

Here is what I have so far:

function ResolveReference(&$Reference) {

    if (!is_object($Reference) && !is_null($Reference)) {

        if (is_string($Reference)) {

            if (is_integer($Pos = strpos($Reference, ':'))) {

                $File = substr($Reference, 0, $Pos);

                $Class = substr($Reference, $Pos + 1);

                require_once $File;

            } else {

                $Class = $Reference;

            }

        }

        $Reference = new $Class();  // =& doesn't work here.  Why?

    }

}



class SomeClass {

    var $Ref;

    function registerReference(&$Ref) {

        $this->Ref =& $Ref;

    }

    function someMethod() {

        ResolveReference($this->Ref);

        $this->Ref->someOtherMethod();

    }

} 

To Create a SomeClass and use a SomeOtherClass with it:

$Some =& new SomeClass();



// Alternatives

$Some->RegisterReference('SomeOtherClass');

$Some->RegisterReference('/path/file.php:SomeOtherClass');  

$Some->RegisterReference(new SomeOtherClass()); 

I have tested the ResolveReference function, not the SomeClass part of this.

The advantage of this method is that I can pass an object or a PHP 4.3+ LazyProxy, or a simple classname, or a classname with a file specification.

The disadvantage is that the ResolveReference helper function has to be called on any variable holding a reference before that reference can be used.

I love to find a better way to do this.

Next up, I would like to expand the concept of the reference to include parameters to the constructor. The problem I have is that there is no call_user_function_array equivalent that can create an object. Unless I am missing something.

Here is what I want to do. (Not sure which syntax is best, yet)

class SomeOtherClass {

    SomeOtherClass($templateFile) {

    ...

    }

}



$Some =& new SomeClass();



// Alternatives

$Some->RegisterReference(array('SomeOtherClass', 'template.html'));

$Some->RegisterReference(array('SomeOtherClass', array('template.html'))); 

[HarryF]

On the second problem; passing a variable number of arguments to a constructor, here's a tip off from the PHP manual under eval();

<?php

function classFactory($classname)

{

   $code = "return new {$classname}(";

   if (func_num_args() > 1) {

       $params = array_slice(func_get_args(),1);

       $c = count($params);

       for($i=0;$i<$c;++$i) {

           if ($i>0) {

               $code .= ',';

           }

           $code .= '$params[' . $i . ']';

       }

   }

   $code .= ');';

   return eval($code);

}



class Foo

{

   function Foo($arg1,$arg2,$arg3)

   {

       echo "I am Foo, I expect 3 arguments...\n";

       echo "arg1 = $arg1\n";

       echo "arg2 = $arg2\n";

       echo "arg3 = $arg3\n\n";

   }

}



class Bar

{

   function Bar($arg1)

   {

       echo "I am Bar, I expect 1 argument...\n";

       echo "arg1 = $arg1\n\n";

   }

}



$myFoo = classFactory('Foo',100,'moose',array(1,2,3));

$myBar = classFactory('Bar',4564);

?>
[/php]

[lastcraft]

Hi...

Originally Posted by HarryF

On the second problem; passing a variable number of arguments to a constructor, here's a tip off from the PHP manual

I used that in the LazyProxy::_instantiate() method above .

Jeff, can you express what you want as a test case? It sounds like a fun problem, but I am not exactly sure I understand everything.

yours, Marcus

[Selkirk]

I am prejudiced against eval.

Here is my Test case for the ResolveReference Function:

class UnaffectedObject {

    var $testVar = 'default';

}



class DeclaredInSameFile {

    var $testVar = 'default';

    function DeclaredInSameFile($Var = 'constructionDefault') {

        $this->testVar = $Var;

    }

}



class ReferenceTestCase extends UnitTestCase {

    function ReferenceTestCase($name = 'ReferenceTestCase') {

        $this->UnitTestCase($name);

    }



    function testNullReference() {

        $Ref = NULL;

        ResolveReference($Ref);

        $this->assertNull($Ref);

    }



    function testObjectUnaffected() {

        $Ref =& new UnaffectedObject();

        $Obj =& $Ref;

        $Obj->testVar = 'Changed';

        ResolveReference($Ref);

        $this->assertIsA($Ref, 'UnaffectedObject');

        $this->assertIdentical($Ref, $Obj);

        $this->assertEqual($Ref->testVar, 'Changed');

    }



    function testClassDeclaredInSameFile() {

        $Ref = 'DeclaredInSameFile';

        ResolveReference($Ref);

        $this->assertIsA($Ref, 'DeclaredInSameFile');

    }



    function testLoadClassFile() {

        $this->assertFalse(class_exists('LoadedReferenceClass'));

        $Ref = dirname(__FILE__) . '/reference.inc.php:LoadedReferenceClass';

        ResolveReference($Ref);

        $this->assertIsA($Ref, 'LoadedReferenceClass');

        $this->assertTrue(class_exists('LoadedReferenceClass'));

    }

    

    function testConstructor() {

        $Ref = array('DeclaredInSameFile', 'ConstructionParameter');

        ResolveReference($Ref);

        $this->assertIsA($Ref, 'DeclaredInSameFile');

        $this->assertEqual($Ref->testVar, 'ConstructionParameter');

    }

    

} 

Here is Reference.inc.php (part of the test case):

class LoadedReferenceClass {

    var $testVar = 'default';

} 

[Resolution]

Ok. As promised, Ghost.

 <?php 
class Ghost
{
 private $object;
 private $filterList = array();
 private $observerList = array(); //Exercise for you
 private $decoratorList = array(); //Exercise for you
 
 function Ghost($objectName)
 {
  $this -> object = new $objectName();
 }
 
 function __call($function, $args)
 {
  $data = $this -> object -> $function ($args);
  
  foreach ($this -> filterList[$function] as $objName => $obj)
  {
   echo "<BR>I am a filter named ".$obj -> filter($objName);
   $data = $obj -> filter($data);
  }
  return $data;
 }
 
 function addFilter($function, $obj)
 {
  $this -> filterList[$function][get_class($obj)] = $obj;
 }
}//End Ghost class
 
 
class Secret
{
 private $secret = "Cows really CAN jump over the moon!";
 
 function getSecret()
 {
  return $this -> secret;
 }
}//End Secret class
 
 
class ItalicFilter
{
 function filter($data)
 {
  return ("<I> $data </I>");
 }
}//End ItalicFilter class
 
 
class BoldFilter
{
 function filter($data)
 {
  return("<B> $data </B>");
 }
}//End BoldFilter class
 
 
 
$test = new Ghost('Secret');
$test -> addFilter("getSecret", new BoldFilter);
$test -> addFilter("getSecret", new ItalicFilter);
echo ("<BR>".$test -> getSecret());
?>

The neat thing about this "proxy"/"factory" pattern I call Ghost is that it can be very flexable. The class objects that were passed in can be lazy loaded or added as objects. One could also add observers and decorators just as easily!

This class might proove to be a great test root as well since Secret has no idea its being shadowed - though I would suggest adding a few safe guards in case the output is error code or not of same type as expected to be output. I would also suggest adding an interface to the Filters.. Ok, you could see that one right?

Anyhow, its strange I got this idea/concept while reading this post. I read up on Slots and Signals , AOP and AspectJ - took a nap woke up a few hours later with the idea you now see before you.

So, does this solution seem to help or did I just make yet another wheel?

Cheers,
Res

[Resolution]

Here's the output...

I am a filter named boldfilter
I am a filter named italicfilter
Cows really CAN jump over the moon!

[Selkirk]

Upon further testing, I've run into an unfortunate snag.

Earlier I had a list of alternative ways to call my RegisterReference function:

// Alternatives

$Some->RegisterReference('SomeOtherClass');

$Some->RegisterReference('/path/file.php:SomeOtherClass');  

$Some->RegisterReference(new SomeOtherClass()); 

It turns out that this cannot work based on my definition of RegisterReference:

    function registerReference(&$Ref) {

        $this->Ref =& $Ref;

    } 

Since the RegisterReference takes has a reference parameter, you cannot pass a constant value to this function.

It seems that I am forced to choose between taking a constant and loosing the ability to pass an already instantiated object (or Proxy Object) OR taking only an object and loosing the simplicity of being able to pass in a class name as a string. (which has some backward compatibillity issues for my purposes.)

Here is an example of how I wanted to use this capability in WACT:

require '../configure.inc.php';

require_once WACT_ROOT . '/controller/formpage.inc.php';



define('LOCAL_PATH', dirname(__FILE__));



$Form = new FormPage();

$Form->registerView(LOCAL_PATH . '/formview.inc.php:CategoryFormView');

$Form->registerValidator(LOCAL_PATH . '/validator.inc.php:CategoryValidator');

$Form->registerSubmitAction('preview', WACT_ROOT . '/controller/noaction.inc.php:NoAction');

$Form->registerSubmitAction('add', LOCAL_PATH . '/add.inc.php:AddCategory', PRE_VALID);

$Form->registerDefaultSubmitAction(LOCAL_PATH . '/add.inc.php:AddCategory', PRE_VALID); 

$Form->Run(); 

I also wanted to be able to support something like this alternative usage of the same thing (or the ability to mix and match from either style):

require '../configure.inc.php';

require_once WACT_ROOT . '/controller/formpage.inc.php';



require 'formview.inc.php';

require 'validator.inc.php';

require 'add.inc.php';



require WACT_ROOT . '/controller/noaction.inc.php';



$Form = new FormPage();

$Form->registerView(new CategoryFormView());

$Form->registerValidator(new CategoryValidator());

$Form->registerSubmitAction('preview', new NoAction());

$Form->registerSubmitAction('add',new AddCategory(), PRE_VALID);

$Form->registerDefaultSubmitAction(new AddCategory(), PRE_VALID); 

$Form->Run(); 

However, it seems that I can only have one or the other because of the reference parameter on the register functions. I don't like to remove the reference because then any objects that get passed in will get cloned, potentially leading to subtle bugs. I would almost rather remove the capability to pass in objects altogether.

[sweatje]

Here was a thread on using the array(&$obj) wrapper to allow for either a constant (simple string declaration) or a object by ref. It was not pretty, but it did seem functional

[Widow Maker]

Do you mean the following, where by PHP throws up an error ?

$hello = &new Hello( 'Hello World' );

...



class Hello {

function Hello( &$variable ) {

... } 

} 

Just wondering really, as if this is the case, does the function you mention in the link actually work okay ?

Thanks.

[sweatje]

Originally Posted by Widow Maker

Just wondering really, as if this is the case, does the function you mention in the link actually work okay?

In instances where I have needed it, it has worked fine. Consider:

class foo {

  var $p;

  function foo ($parm) {

    $this->p = $parm;

  }

}



class bar {

  var $foo;

  function bar ($foo_in) {

    if (!$foo =& obj_from_envelope($foo_in)) {

      $foo =& new foo($foo_in);

    }

    $this->foo =& $foo;

  }

} 

Now for bar, I can pass either a string to use as the parameter for creating a new foo, or an array containing a reference to an already created foo.

HTH

[Selkirk]

Thanks for finding that, sweatje. The envelope technique has eased my mind a bit about removing the reference from the parameter. There is no perfect solution in PHP 4, but at this this should be forward compatible with PHP 5.

I ran across this related blog entry today and now I wish I had used the same title for this thread:

Lazy Evaluation and Object Composition

[Widow Maker]

So it works then ? That is really cool - Okay for me to use the function SweatJe - Sure to find a use for it

[sweatje]

Originally Posted by Widow Maker

Okay for me to use the function SweatJe

I pretty much consider anything I put up on the boards fair game. Perhaps you could leave in a comment of

 * @author    Jason E. Sweat

in the function doc block, and I would be interested in hearing about any use in a open source project.

[Widow Maker]

No problem