2.3. Composite

2.3.1. Purpose

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

2.3.2. Examples

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

2.3.3. UML Diagram

2.3.4. Code

You can also find this code on GitHub

RenderableInterface.php

<?php
 
namespace DesignPatterns\Structural\Composite;
 
interface RenderableInterface
{
    public function render(): string;
}

Form.php

<?php
 
namespace DesignPatterns\Structural\Composite;
 
/**
 * The composite node MUST extend the component contract. This is mandatory for building
 * a tree of components.
 */
class Form implements RenderableInterface
{
    /**
     * @var RenderableInterface[]
     */
    private $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
     *
     * @return string
     */
    public function render(): string
    {
        $formCode = '<form>';
 
        foreach ($this->elements as $element) {
            $formCode .= $element->render();
        }
 
        $formCode .= '</form>';
 
        return $formCode;
    }
 
    /**
     * @param RenderableInterface $element
     */
    public function addElement(RenderableInterface $element)
    {
        $this->elements[] = $element;
    }
}

InputElement.php

<?php
 
namespace DesignPatterns\Structural\Composite;
 
class InputElement implements RenderableInterface
{
    public function render(): string
    {
        return '<input type="text" />';
    }
}

TextElement.php

<?php
 
namespace DesignPatterns\Structural\Composite;
 
class TextElement implements RenderableInterface
{
    /**
     * @var string
     */
    private $text;
 
    public function __construct(string $text)
    {
        $this->text = $text;
    }
 
    public function render(): string
    {
        return $this->text;
    }
}

2.3.5. Test

Tests/CompositeTest.php

<?php
 
namespace DesignPatterns\Structural\Composite\Tests;
 
use DesignPatterns\Structural\Composite;
use PHPUnit\Framework\TestCase;
 
class CompositeTest extends TestCase
{
    public function testRender()
    {
        $form = new Composite\Form();
        $form->addElement(new Composite\TextElement('Email:'));
        $form->addElement(new Composite\InputElement());
        $embed = new Composite\Form();
        $embed->addElement(new Composite\TextElement('Password:'));
        $embed->addElement(new Composite\InputElement());
        $form->addElement($embed);
 
        // This is just an example, in a real world scenario it is important to remember that web browsers do not
        // currently support nested forms
 
        $this->assertEquals(
            '<form>Email:<input type="text" /><form>Password:<input type="text" /></form></form>',
            $form->render()
        );
    }
}