17 March 2012

Decorator Pattern Implemented Properly In PHP

By Jurgens du Toit

While working on Backend-PHP I needed to do a proper implementation of the Decorator Pattern in PHP. Just googling PHP decorator pattern will come up with a number of simple solutions, but none of them are usable in a general, robust way that a framework requires. So I extended and tweaked the implementations a bit.

##TL;DR

Most decorator pattern implementations for PHP found on the web is broken. They need some extra functionality to work properly, especially for nested decorators. See the complete solution in the Backend-PHP code

##The Basics

I’m assuming you know how decorators work. If not, read up on it first! I’ve implemented a base Decorator class to handle the base functionality for Decorators. The simple implementations out there serve as a good base for an initial implementation:

class Decorator
{
    protected $object; //The object to decorate

    function __construct( $object)
    {
        $this->object = $object;
    }
}

This is enough to get you started, but you will need to wrap the original object’s properties and methods manually. Let’s define a couple of magic functions to do that automatically:

public function __call($method, $args)
    {
        if (is_callable($this->object, $method) {
            return call_user_func_array(array($this->object, $method), $args);
        }
        throw new Exception(
            'Undefined method - ' . get_class($this->object) . '::' . $method
        );
    }

    public function __get($property)
    {
        if (property_exists($this->object, $property)) {
            return $this->object->$property;
        }
        return null;
    }

    public function __set($property, $value)
    {
        $this->object->$property = $value;
        return $this;
    }

That should give you a working basic implementation.

##More Advanced

That will work as long as you don’t nest decorators, ie, decorate an already decorated object:

$object = new DecoratorOne(new DecoratorTwo(new SomeClass()));

Why? The magic functions will check the decorated object for the properties and methods, but in the case of DecoratorOne above, the decorated object will be another decorator, and you won’t get the expected behaviour. This happens because property_exists doesn’t trigger the __get (or __set) methods. is_callable does check for the __call function, but that doesn’t necessarily produce the expected results, especially if you stack decorators that modify method execution.

We solve this by adding two more functions:

###getOriginalObject

public function getOriginalObject()
{
    $object = $this->object;
    while ($object instanceof Decorator) {
        $object = $object->getOriginalObject();
    }
    return $object;
}

getOriginalObject will return the original, undecorated object. This is handy to get the correct class name or access to undecorated methods and properties:

echo get_class($decoratedClass) //Will give the name of the Decorator

echo get_class($decoratedClass->getOriginalObject()) //Will give the name of the original object

###isCallable

public function isCallable($method, $checkSelf = false)
{
    //Check the original object
    $object = $this->getOriginalObject();
    if (is_callable(array($object, $method))) {
        return $object;
    }
    //Check Decorators
    $object = $checkSelf ? $this : $this->object;
    while ($object instanceof Decorator) {
        if (is_callable(array($object, $method))) {
            return $object;
        }
        $object = $this->object;
    }
    return false;
}

isCallable will check the original object for the specified method, and if not found, it will check the other decorators for the method. The option exists to check itself as well.

##The Result

Now we can modify the magic functions like so:

public function __call($method, $args)
{
    if ($object = $this->isCallable($method)) {
        return call_user_func_array(array($object, $method), $args);
    }
    throw new Exception(
        'Undefined method - ' . get_class($this->getOriginalObject()) . '::' . $method
    );
}

public function __get($property)
{
    $object = $this->getOriginalObject();
    if (property_exists($object, $property)) {
        return $object->$property;
    }
    return null;
}

public function __set($property, $value)
{
    $object = $this->getOriginalObject();
    $object->$property = $value;
    return $this;
}

This will give you a robust Decorator Pattern implementation from which you can then create more Decorators. See the complete implementation in the Backend-PHP code

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Decorator Pattern Implemented Properly In PHP

By Jurgens du Toit

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