Is it possible to extend a class dynamically?

Question

I have a class which I need to use to extend different classes (up to hundreds) depending on criteria. Is there a way in PHP to extend a class by a dynamic class name?

I assume it would require a method to specify extension with instantiation.

Ideas?

Answer 1

While it's still not possible and not exactly your answer i needed the same thing and didn't wanted to use eval, monkey-patching etc. So i used a default class by extending it in conditions.

Of course it means if you have 100 classes to extend you need to add 100 conditions with another extending operation but for me this looked like the right way.

<?php
if(class_exists('SolutionClass')) {
    class DynamicParent extends SolutionClass {}
} else {
    class DynamicParent extends DefaultSolutionClass {}
}

class ProblemChild extends DynamicParent {}
?>

Answer 2

Yes. I like the answer with eval, but a lot of people afraid of any eval in their code, so here's one with no eval:

<?php //MyClass.php 
namespace my\namespace;
function get_dynamic_parent() {
    return 'any\other\namespace\ExtendedClass';// return what you need
}
class_alias(get_dynamic_parent(), 'my\namespace\DynamicParent');

class MyClass extends DynamicParent {}

Answer 3

It is possible to create dynamic inheritance in PHP using the power of the magic __call function. It takes a little bit of infrastructure code to work but isn't too daunting.

Disclaimer

You really ought to think at least twice before using this technique as it's actually kind of bad thing to do.

The only reason I'm using this technique is because I don't want to have to either create the interface definitions or setup the dependency injections when creating templates for a site. I want to just be able to just define a couple of function 'blocks' in a template and then have inheritance automatically use the correct 'block'.

Implementation

The steps required are:

• The child class now extends a 'DynamicExtender' class. This class intercepts any calls made by the child class, to methods that don't exist in the child class and redirects them to the parent instance.

• Each 'ParentClass' is extended to a 'ProxyParentClass'. For every accessible method in the parent class, there exists an equivalent method in the 'ProxyParentClass'. Each of those methods in the 'ProxyParentClass' checks to see if the method exists in the ChildClass and calls the childs version of the function if it exists, otherwise it calls the version from ParentClass

• When the DynamicExtender class is constructed you pass in what parent class you require, the DynamicExtender creates a new instance of that class, and sets itself as the child of the ParentClass.

So, now when we create the child object we can specify the parent class required and the DynamicExtender will create it for us, and it will appear as if the child class is extended from the class we requested at run-time rather than it being hard-coded.

This may be easier to understand as a couple of images:

Fixed inheritance is fixed

Dynamic inheritance with proxies

Demo implementation

The code for this solution is available on Github and a slightly fuller explanation of how this can be used here , but the code for the above image is:

//An interface that defines the method that must be implemented by any renderer.
interface Render {
    public function render();
}


/**
 * Class DynamicExtender
 */
class DynamicExtender implements Render {

    var $parentInstance = null;

    /**
     * Construct a class with it's parent class chosen dynamically.
     *
     * @param $parentClassName The parent class to extend.
     */
    public function __construct($parentClassName) {
        $parentClassName = "Proxied".$parentClassName;

        //Check that the requested parent class implements the interface 'Render'
        //to prevent surprises later.
        if (is_subclass_of($parentClassName, 'Render') == false) {
            throw new Exception("Requested parent class $parentClassName does not implement Render, so cannot extend it.");
        }

        $this->parentInstance = new $parentClassName($this);
    }

    /**
     * Magic __call method is triggered whenever the child class tries to call a method that doesn't
     * exist in the child class. This is the case whenever the child class tries to call a method of
     * the parent class. We then redirect the method call to the parentInstance.
     *
     * @param $name
     * @param array $arguments
     * @return mixed
     * @throws PHPTemplateException
     */
    public function __call($name, array $arguments) {
        if ($this->parentInstance == null) {
            throw new Exception("parentInstance is null in Proxied class in renderInternal.");
        }

        return call_user_func_array([$this->parentInstance, $name], $arguments);
    }

    /**
     * Render method needs to be defined to satisfy the 'implements Render' but it
     * also just delegates the function to the parentInstance.
     * @throws Exception
     */
    function render() {
        $this->parentInstance->render();
    }
}



/**
 * Class PageLayout
 *
 * Implements render with a full HTML layout.
 */
class PageLayout implements Render {

    //renders the whole page.
    public function render() {
        $this->renderHeader();
        $this->renderMainContent();
        $this->renderFooter();
    }

    //Start HTML page
    function renderHeader() {
        echo "<html><head></head><body>";
        echo "<h2>Welcome to a test server!</h2>";

        echo "<span id='mainContent'>";
    }

    //Renders the main page content. This method should be overridden for each page
    function renderMainContent(){
        echo "Main content goes here.";
    }

    //End the HTML page, including Javascript
    function renderFooter(){
        echo "</span>";
        echo "<div style='margin-top: 20px'>Dynamic Extension Danack@basereality.com</div>";
        echo "</body>";
        echo "<script type='text/javascript' src='jquery-1.9.1.js' ></script>";
        echo "<script type='text/javascript' src='content.js' ></script>";
        echo "</html>";
    }

    //Just to prove we're extending dynamically.
    function getLayoutType() {
        return get_class($this);
    }
}

/**
 * Class ProxiedPageLayout
 *
 * Implements render for rendering some content surrounded by the opening and closing HTML
 * tags, along with the Javascript required for a page.
 */
class ProxiedPageLayout extends PageLayout {

    /**
     * The child instance which has extended this class.
     */
    var $childInstance = null;

    /**
     * Construct a ProxiedPageLayout. The child class must be passed in so that any methods
     * implemented by the child class can override the same method in this class.
     * @param $childInstance
     */
    function __construct($childInstance){
        $this->childInstance = $childInstance;
    }

    /**
     * Check if method exists in child class or just call the version in PageLayout
     */
    function renderHeader() {
        if (method_exists ($this->childInstance, 'renderHeader') == true) {
            return $this->childInstance->renderHeader();
        }
        parent::renderHeader();
    }

    /**
     * Check if method exists in child class or just call the version in PageLayout
     */
    function renderMainContent(){
        if (method_exists ($this->childInstance, 'renderMainContent') == true) {
            return $this->childInstance->renderMainContent();
        }
        parent::renderMainContent();
    }

    /**
     * Check if method exists in child class or just call the version in PageLayout
     */
    function renderFooter(){
        if (method_exists ($this->childInstance, 'renderFooter') == true) {
            return $this->childInstance->renderFooter();
        }
        parent::renderFooter();
    }
}


/**
 * Class AjaxLayout
 *
 * Implements render for just rendering a panel to replace the existing content.
 */
class AjaxLayout implements Render {

    //Render the Ajax request.
    public function render() {
        $this->renderMainContent();
    }

    //Renders the main page content. This method should be overridden for each page
    function renderMainContent(){
        echo "Main content goes here.";
    }

    //Just to prove we're extending dynamically.
    function getLayoutType() {
        return get_class($this);
    }
}

/**
 * Class ProxiedAjaxLayout
 *
 * Proxied version of AjaxLayout. All public functions must be overridden with a version that tests
 * whether the method exists in the child class.
 */
class ProxiedAjaxLayout extends AjaxLayout {

    /**
     * The child instance which has extended this class.
     */
    var $childInstance = null;

    /**
     * Construct a ProxiedAjaxLayout. The child class must be passed in so that any methods
     * implemented by the child class can override the same method in this class.
     * @param $childInstance
     */
    function __construct($childInstance){
        $this->childInstance = $childInstance;
    }

    /**
     * Check if method exists in child class or just call the version in AjaxLayout
     */
    function renderMainContent() {
        if (method_exists ($this->childInstance, 'renderMainContent') == true) {
            return $this->childInstance->renderMainContent();
        }
        parent::renderMainContent();
    }
}



/**
 * Class ImageDisplay
 *
 * Renders some images on a page or Ajax request.
 */
class ImageDisplay extends DynamicExtender {

    private $images = array(
        "6E6F0115.jpg",
        "6E6F0294.jpg",
        "6E6F0327.jpg",
        "6E6F0416.jpg",
        "6E6F0926.jpg",
        "6E6F1061.jpg",
        "6E6F1151.jpg",
        "IMG_4353_4_5_6_7_8.jpg",
        "IMG_4509.jpg",
        "IMG_4785.jpg",
        "IMG_4888.jpg",
        "MK3L5774.jpg",
        "MK3L5858.jpg",
        "MK3L5899.jpg",
        "MK3L5913.jpg",
        "MK3L7764.jpg",
        "MK3L8562.jpg",
    );

    //Renders the images on a page, along with a refresh button
    function renderMainContent() {
        $totalImages = count($this->images);
        $imagesToShow = 4;
        $startImage = rand(0, $totalImages - $imagesToShow);

        //Code inspection will not be available for 'getLayoutType' as it
        //doesn't exist statically in the class hierarchy
        echo "Parent class is of type: ".$this->getLayoutType()."<br/>";

        for($x=0 ; $x<$imagesToShow ; $x++) {
            echo "<img src='images/".$this->images[$startImage + $x]."'/>";
        }

        echo "<br/>&nbsp;<br/>";
        echo "<span onclick='loadImagesDynamic();' style='border: 2px solid #000000; padding: 4px:'>Click to refresh images</span>";
    }
}


$parentClassName = 'PageLayout';

if (isset($_REQUEST['panel']) && $_REQUEST['panel']) {
    //YAY! Dynamically set the parent class.
    $parentClassName = 'AjaxLayout';
}

$page = new ImageDisplay($parentClassName);

$page->render();

Answer 4

I don't think it's possible to dynamically extend a class (however if I'm wrong I'd love to see how it's done). Have you thought about using the Composite pattern ( http://en.wikipedia.org/wiki/Composite_pattern , http://devzone.zend.com/article/7 )? You could dynamically composite another class (even multiple classes - this is often used as a work around to multiple inheritance) to 'inject' the methods/properties of your parent class into the child class.

Answer 5

Couldn't you just use an eval?

<?php
function dynamic_class_name() {
    if(time() % 60)
        return "Class_A";
    if(time() % 60 == 0)
        return "Class_B";
}
eval(
    "class MyRealClass extends " . dynamic_class_name() . " {" . 
    # some code string here, possibly read from a file
    . "}"
);
?>

Answer 6

I have solved my same type of problem. The first parameter defines the original class name and the second parameter defines the new class name of class_alias function. Then we can use this function in if and else condition.

if(1==1){
  class_alias('A', 'C');
}
else{
  class_alias('B', 'C');
}

class Apple extends C{
      ...
}

Apple class extends to virtual class "C" which can be defined as class "A" Or "B" depend on if and else condition.

For More information you can check this link https://www.php.net/manual/en/function.class-alias.php

Answer 7

1. Get All declared_classes
2. Position, where the class will be declare.

class myClass { public $parentVar; function __construct() { $all_classes = get_declared_classes(); // all classes $parent = $parent[count($parent) -2]; //-2 is the position $this->parentVar = new $parent(); } }

Answer 8

I have an idea so simple, you can try

class A {} 
class B {}
$dynamicClassName = "A";
eval("class DynamicParent extends $dynamicClassName {}");

class C extends DynamicParent{
   // extends success
   // Testing
   function __construct(){
        echo get_parent_class('DynamicParent'); exit; //A :)
   }
}

Answer 9

I had to do this with a processor class that extends one of two abstract classes.

The working solution looks like this:

if (class_exists('MODX\Revolution\Processors\Processor')) {
    abstract class DynamicProcessorParent extends 
        MODX\Revolution\Processors\Processor {}
} else {
    abstract class DynamicProcessorParent extends modProcessor {}
}

class NfSendEmailProcessor extends DynamicProcessorParent {
  /* Concrete class */
}

If the abstract parent classes contain abstract methods, they don't need to be implemented in either of the dynamic parent classes.

If you're working on a large project, you probably don't want to use DynamicParent as the class name unless the classes are namespaced. You'll need something more specific to avoid collisions.