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

2.3.3. UML Diagram

Alt Composite UML Diagram

2.3.4. Code

You can also find this code on GitHub

Renderable.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Composite;

interface Renderable
{
    public function render(): string;
}

Form.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Composite;

/**
 * The composite node MUST extend the component contract. This is mandatory for building
 * a tree of components.
 */
class Form implements Renderable
{
    /**
     * @var Renderable[]
     */
    private array $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
     */
    public function render(): string
    {
        $formCode = '<form>';

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

        $formCode .= '</form>';

        return $formCode;
    }

    public function addElement(Renderable $element)
    {
        $this->elements[] = $element;
    }
}

InputElement.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Composite;

class InputElement implements Renderable
{
    public function render(): string
    {
        return '<input type="text" />';
    }
}

TextElement.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Composite;

class TextElement implements Renderable
{
    private string $text;

    public function __construct(string $text)
    {
        $this->text = $text;
    }

    public function render(): string
    {
        return $this->text;
    }
}

2.3.5. Test

Tests/CompositeTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Composite\Tests;

use DesignPatterns\Structural\Composite\Form;
use DesignPatterns\Structural\Composite\TextElement;
use DesignPatterns\Structural\Composite\InputElement;
use PHPUnit\Framework\TestCase;

class CompositeTest extends TestCase
{
    public function testRender()
    {
        $form = new Form();
        $form->addElement(new TextElement('Email:'));
        $form->addElement(new InputElement());
        $embed = new Form();
        $embed->addElement(new TextElement('Password:'));
        $embed->addElement(new 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->assertSame(
            '<form>Email:<input type="text" /><form>Password:<input type="text" /></form></form>',
            $form->render()
        );
    }
}