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OOP - Factory Pattern


benanamen

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I ran across the following example of a Factory Pattern. My question is, why would you have all this Factory code when you could just call one of the extended button classes that you need?

<?php
abstract class Button {
    protected $_html;

    public function getHtml()
    {
        return $this->_html;
    }
}

class ImageButton extends Button {
    protected $_html = "..."; //This should be whatever HTML you want for your image-based button
}

class InputButton extends Button {
    protected $_html = "..."; //This should be whatever HTML you want for your normal button (<input type="button"... />);
}

class FlashButton extends Button {
    protected $_html = "..."; //This should be whatever HTML you want for your flash-based button
}

class ButtonFactory
{
    public static function createButton($type)
    {
        $baseClass = 'Button';
        $targetClass = ucfirst($type).$baseClass;

        if (class_exists($targetClass) && is_subclass_of($targetClass, $baseClass)) {
            return new $targetClass;
        } else {
            throw new Exception("The button type '$type' is not recognized.");
        }
    }
}

$buttons = array('image','input','flash');
foreach($buttons as $b) {
    echo ButtonFactory::createButton($b)->getHtml();
} 
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The factory is an abstraction layer which provides more flexibility than instantiating objects directly.

 

When you just put hard-coded Button subclasses into your code, you're restricted to those exact classes once and forever. You cannot switch to a different implementation. It's also your job to prepare everything for the object instantiation, which can be a lot of effort for complex classes (not in this trivial example, of course).

 

The Factory Pattern is smarter than that:

  • The factory decides which class is instantiated. You can plug in a different class at any time by subclassing the factory.
  • Choosing the right implementation can also be more sophisticated than mapping strings to classes. The factory is free to choose the optimum based on any parameter.
  • The entire preparation procedure is delegated to the factory. This can greatly simplify the code.

However, the Factory Pattern is now often replaced with Dependency Injection.

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