2.10. Proxy

2.10.1. Doel

Om te communiceren met iets dat duur of onmogelijk te dupliceren is.

2.10.2. Bijvoorbeeld

• Doctrine2 gebruikt proxy’s om raamwerkmagie (bijv. Luie initialisatie) daarin te implementeren, terwijl de gebruiker nog steeds met zijn eigen entiteitsklassen werkt en de proxy’s nooit zal gebruiken of aanraken

2.10.3. UML Diagram

2.10.4. Code

Je kan deze broncode terugvinden op GitHub

BankAccount.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Structural\Proxy;

interface BankAccount
{
    public function deposit(int $amount);

    public function getBalance(): int;
}

HeavyBankAccount.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Structural\Proxy;

class HeavyBankAccount implements BankAccount
{
    /**
     * @var int[]
     */
    private array $transactions = [];

    public function deposit(int $amount)
    {
        $this->transactions[] = $amount;
    }

    public function getBalance(): int
    {
        // this is the heavy part, imagine all the transactions even from
        // years and decades ago must be fetched from a database or web service
        // and the balance must be calculated from it

        return array_sum($this->transactions);
    }
}

BankAccountProxy.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Structural\Proxy;

class BankAccountProxy extends HeavyBankAccount implements BankAccount
{
    private ?int $balance = null;

    public function getBalance(): int
    {
        // because calculating balance is so expensive,
        // the usage of BankAccount::getBalance() is delayed until it really is needed
        // and will not be calculated again for this instance

        if ($this->balance === null) {
            $this->balance = parent::getBalance();
        }

        return $this->balance;
    }
}

2.10.5. Теst

ProxyTest.php

<?php

declare(strict_types=1);

namespace DesignPatterns\Structural\Proxy\Tests;

use DesignPatterns\Structural\Proxy\BankAccountProxy;
use PHPUnit\Framework\TestCase;

class ProxyTest extends TestCase
{
    public function testProxyWillOnlyExecuteExpensiveGetBalanceOnce()
    {
        $bankAccount = new BankAccountProxy();
        $bankAccount->deposit(30);

        // this time balance is being calculated
        $this->assertSame(30, $bankAccount->getBalance());

        // inheritance allows for BankAccountProxy to behave to an outsider exactly like ServerBankAccount
        $bankAccount->deposit(50);

        // this time the previously calculated balance is returned again without re-calculating it
        $this->assertSame(30, $bankAccount->getBalance());
    }
}