2.9. Flyweight

2.9.1. Purpose

To minimise memory usage, a Flyweight shares as much as possible memory with similar objects. It is needed when a large amount of objects is used that don’t differ much in state. A common practice is to hold state in external data structures and pass them to the flyweight object when needed.

2.9.2. UML Diagram

Alt Flyweight UML Diagram

2.9.3. Code

You can also find this code on GitHub

Text.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Flyweight;

/**
 * This is the interface that all flyweights need to implement
 */
interface Text
{
    public function render(string $extrinsicState): string;
}

Word.php

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<?php

namespace DesignPatterns\Structural\Flyweight;

class Word implements Text
{
    /**
     * @var string
     */
    private $name;

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

    public function render(string $font): string
    {
        return sprintf('Word %s with font %s', $this->name, $font);
    }
}

Character.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Flyweight;

/**
 * Implements the flyweight interface and adds storage for intrinsic state, if any.
 * Instances of concrete flyweights are shared by means of a factory.
 */
class Character implements Text
{
    /**
     * Any state stored by the concrete flyweight must be independent of its context.
     * For flyweights representing characters, this is usually the corresponding character code.
     *
     * @var string
     */
    private $name;

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

    public function render(string $font): string
    {
         // Clients supply the context-dependent information that the flyweight needs to draw itself
         // For flyweights representing characters, extrinsic state usually contains e.g. the font.

        return sprintf('Character %s with font %s', $this->name, $font);
    }
}

TextFactory.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Flyweight;

/**
 * A factory manages shared flyweights. Clients should not instantiate them directly,
 * but let the factory take care of returning existing objects or creating new ones.
 */
class TextFactory implements \Countable
{
    /**
     * @var Text[]
     */
    private $charPool = [];

    public function get(string $name): Text
    {
        if (!isset($this->charPool[$name])) {
            $this->charPool[$name] = $this->create($name);
        }

        return $this->charPool[$name];
    }

    private function create(string $name): Text
    {
        if (strlen($name) == 1) {
            return new Character($name);
        } else {
            return new Word($name);
        }
    }

    public function count(): int
    {
        return count($this->charPool);
    }
}

2.9.4. Test

Tests/FlyweightTest.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Flyweight\Tests;

use DesignPatterns\Structural\Flyweight\TextFactory;
use PHPUnit\Framework\TestCase;

class FlyweightTest extends TestCase
{
    private $characters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k',
        'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];
    private $fonts = ['Arial', 'Times New Roman', 'Verdana', 'Helvetica'];

    public function testFlyweight()
    {
        $factory = new TextFactory();

        for ($i = 0; $i <= 10; $i++) {
            foreach ($this->characters as $char) {
                foreach ($this->fonts as $font) {
                    $flyweight = $factory->get($char);
                    $rendered = $flyweight->render($font);

                    $this->assertSame(sprintf('Character %s with font %s', $char, $font), $rendered);
                }
            }
        }

        foreach ($this->fonts as $word) {
            $flyweight = $factory->get($word);
            $rendered = $flyweight->render('foobar');

            $this->assertSame(sprintf('Word %s with font foobar', $word), $rendered);
        }

        // Flyweight pattern ensures that instances are shared
        // instead of having hundreds of thousands of individual objects
        // there must be one instance for every char that has been reused for displaying in different fonts
        $this->assertCount(count($this->characters) + count($this->fonts), $factory);
    }
}