1. 设计模式
  2. 结构型模式
  3. 代理模式

代理模式

目的

为一些开销昂贵和无法复制的东西提供了一个访问接口。

使用场景

  • Doctrine2 内部使用代理来实现一些框架功能（如：延迟实例化），而用户仍然使用自己的实体类，不会使用也不会关心代理的具体细节。

UML 类图

代理模式类图

代码

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;
    }
}

测试

Tests/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());
    }
}
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