2.3. Composite

2.3.1. Purpose

To treat a group of objects the same way as a single instance of the object.

2.3.2. Examples

  • a form class instance handles all its form elements like a single instance of the form, when render() is called, it subsequently runs through all its child elements and calls render() on them
  • Zend_Config: a tree of configuration options, each one is a Zend_Config object itself

2.3.3. UML Diagram

Alt Composite UML Diagram

2.3.4. Code

You can also find these code on GitHub

FormElement.php

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<?php

namespace DesignPatterns\Structural\Composite;

/**
 * Class FormElement
 */
abstract class FormElement
{
    /**
     * renders the elements' code
     *
     * @param int $indent
     *
     * @return mixed
     */
    abstract public function render($indent = 0);
}

Form.php

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<?php

namespace DesignPatterns\Structural\Composite;

/**
 * The composite node MUST extend the component contract. This is mandatory for building
 * a tree of components.
 */
class Form extends FormElement
{
    /**
     * @var array|FormElement[]
     */
    protected $elements;

    /**
     * runs through all elements and calls render() on them, then returns the complete representation
     * of the form
     *
     * from the outside, one will not see this and the form will act like a single object instance
     *
     * @param int $indent
     *
     * @return string
     */
    public function render($indent = 0)
    {
        $formCode = '';

        foreach ($this->elements as $element) {
            $formCode .= $element->render($indent + 1) . PHP_EOL;
        }

        return $formCode;
    }

    /**
     * @param FormElement $element
     */
    public function addElement(FormElement $element)
    {
        $this->elements[] = $element;
    }
}

InputElement.php

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<?php

namespace DesignPatterns\Structural\Composite;

/**
 * Class InputElement
 */
class InputElement extends FormElement
{
    /**
     * renders the input element HTML
     *
     * @param int $indent
     *
     * @return mixed|string
     */
    public function render($indent = 0)
    {
        return str_repeat('  ', $indent) . '<input type="text" />';
    }
}

TextElement.php

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<?php

namespace DesignPatterns\Structural\Composite;

/**
 * Class TextElement
 */
class TextElement extends FormElement
{
    /**
     * renders the text element
     *
     * @param int $indent
     *
     * @return mixed|string
     */
    public function render($indent = 0)
    {
        return str_repeat('  ', $indent) . 'this is a text element';
    }
}

2.3.5. Test

Tests/CompositeTest.php

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<?php

namespace DesignPatterns\Structural\Composite\Tests;

use DesignPatterns\Structural\Composite;

/**
 * FormTest tests the composite pattern on Form
 */
class CompositeTest extends \PHPUnit_Framework_TestCase
{

    public function testRender()
    {
        $form = new Composite\Form();
        $form->addElement(new Composite\TextElement());
        $form->addElement(new Composite\InputElement());
        $embed = new Composite\Form();
        $embed->addElement(new Composite\TextElement());
        $embed->addElement(new Composite\InputElement());
        $form->addElement($embed);  // here we have a embedded form (like SF2 does)

        $this->assertRegExp('#^\s{4}#m', $form->render());
    }

    /**
     * The all point of this pattern, a Composite must inherit from the node
     * if you want to builld trees
     */
    public function testFormImplementsFormEelement()
    {
        $className = 'DesignPatterns\Structural\Composite\Form';
        $abstractName = 'DesignPatterns\Structural\Composite\FormElement';
        $this->assertTrue(is_subclass_of($className, $abstractName));
    }
}