flyweight.md

Flyweight Design Pattern

The Flyweight pattern allows sharing common state across multiple objects to reduce memory usage.

How to apply

Idea is to create and share digital books with someone. If it doesn't exists already create a new one or share the existing one.

interface BookFlyweight
{
    public function render(string $info): void;
}

class SharedBook implements BookFlyweight
{
    public function __construct(private string $title) {}

    public function render(string $info): void
    {
        echo "Book: {$this->title} is {$info}";
    }
}

class BookFactory
{
    private array $books = [];

    public function getBook(string $title): BookFlyweight
    {
        if (!isset($this->books[$title])) {
            $this->books[$title] = new SharedBook($title);
        }

        return $this->books[$title];
    }
}

Usage:

$factory = new BookFactory();

$book1 = $factory->getBook("The Lord of the Rings");
$book1->render("shared with A");

$book2 = $factory->getBook("The Martian");
$book2->render("shared with B");

$book3 = $factory->getBook("The Lord of the Rings");
$book3->render("shared with C");

Output:

Book: The Lord of the Rings is shared with A
Book: The Martian is shared with B
Book: The Lord of the Rings is shared with C

Analysis:

$book3 is uses an existing instance which created by $book1, instead of creating a new one. We can verify this with the snippet below:

if ($book1 === $book3) {
    echo "Both are the same instance";
}

We should get:

Both are the same instance