2.10. Proxy

2.10.1. Purpose

To interface to anything that is expensive or impossible to duplicate.

2.10.2. Examples

  • Doctrine2 uses proxies to implement framework magic (e.g. lazy initialization) in them, while the user still works with his own entity classes and will never use nor touch the proxies

2.10.3. UML Diagram

Alt Proxy UML Diagram

2.10.4. Code

You can also find this code on GitHub

BankAccount.php

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<?php declare(strict_types=1);

namespace DesignPatterns\Structural\Proxy;

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

    public function getBalance(): int;
}

HeavyBankAccount.php

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<?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 (int) array_sum($this->transactions);
    }
}

BankAccountProxy.php

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<?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. Test

ProxyTest.php

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<?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());
    }
}