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Marco Pivetta

Ocramius

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Ocramius

WHY?

The Proxy Pattern
in PHP

What is a Proxy?

A proxy is a class functioning as an interface to something else

(carefully pasted from wikipedia)

Simple put

A Proxy implements a Subject's interface

A proxy replaces subject thanks to the LSP

Client consumes SubjectInterface

Client consumes RealSubject

Client consumes SubjectProxy

Simplified:

SubjectProxy extending RealSubject

class SubjectProxy extends RealSubject { /* ... */ }

Though you should use interfaces!

What do Proxies do?

Some definitions here:

And here:

And a lot of confusion over the internets :(

From the GoF

Remote proxy

Virtual proxy

Protection proxy

Smart reference

I'm a software architect

Let's get practical!

Remote Proxy

Remote Object example

class Tweet {
    protected $data;

    public function __construct(array $data) {
        $this->data = $data;
    }

    public function getText() {
        return $this->data['text'];
    }
}

class TweetProxy extends Tweet {
    protected $api;
    protected $id;

    public function __construct(TwitterApi $api, $id) {
        $this->api = $api;
        $this->id  = $id;
    }

    public function getText() {
        return $this->api->getTweetText($this->id);
    }
}

$tweet = new Tweet(array('text' => 'Proxies in PHP!'));
var_dump($tweet->getText()); // Proxies in PHP!

$api = new TwitterApi(/* yadda */);

$remoteTweet = new TweetProxy($api, 280643708968386560);
var_dump($remoteTweet->getText()); // Tweet text!

$remoteTweet = new TweetProxy($api, 280643708968386561);
var_dump($remoteTweet->getText()); // Another text!

remote object pros/cons

Share objects across multiple systems

No local memory usage

Can act as an adapter for a completely different remote object

Complex setup

Fails on adapter failure

As slow as the protocol

Lazy Loading

For very expensive objects:

• DB Connections
• Files (I/O in general)
• Large objects (memory)
• Objects with long instantiation time (hashes/parsed values/etc)

(A pattern, not a proxy type)

Lazy Loading example

class Customer {
    public function __construct(DbConnection $db, $id) {
        $this->db = $db;
        $this->id = $id;
    }

    public function getName() {
        $this->init();
        return $this->data['name'];
    }

    private function init() {
        if (!isset($this->data)) {
            $this->data = $this->db->load($this->id);
        }
    }
}

Problems with Lazy Loading

Instantiation logic mixed with class logic

Dependency to loader objects in the object

Not really optimized, and unreadable if optimized

Harder to test

Lazy loading via Proxies

Virtual Proxy
An object that looks like the RealSubject, but just holds a lazy reference to it.

Ghost Object
An object that looks like the RealSubject, but with all properties not being set and being lazy loaded.

Virtual Proxy

class Image {
    protected $image;
    public function __construct($path) {
        $this->image = imagecreatefromjpeg($path);
    }

    public function getSize() {
        return array(
            imagesx($this->image),
            imagesy($this->image),
        );
    }
}

class ImageVirtualProxy extends Image {
    protected $wrapped;
    public function __construct($path) {
        $this->path = $path;
    }

    public function getSize() {
        $this->init();
        return $this->wrapped->getSize();
    }

    private function init() {
        if ( ! $this->wrapped) {
            $this->wrapped = new Image($this->path);
        }
    }
}

$img1 = new ImageProxy('/path/to/image1.jpg');
var_dump(memory_get_usage()); // ~200Kb
$img2 = new ImageProxy('/path/to/image2.jpg');
var_dump(memory_get_usage()); // ~200Kb
$img3 = new ImageProxy('/path/to/image3.jpg');
var_dump(memory_get_usage()); // ~200Kb

$size1 = $img1->getSize();
var_dump(memory_get_usage()); // ~4Mb
$size2 = $img2->getSize();
var_dump(memory_get_usage()); // ~8Mb

Virtual Proxy pros/cons

Abstracts initialization logic away

Improved memory/performance impact

Low overhead

Easy to implement

Not optimal for data that is always loaded

Lazy loading means lazy failing

Actually, not the same object as RealSubject (different identity)

Ghost Object

An object whose properties are the same of the proxied object, but null.

Accessing any method causes loading of the properties.

Useful when the identity of the object has to be preserved

Doctrine Proxies are generated this way.

Ghost Object (simplified) example

class ImageGhostProxy extends Image {
    protected $initialized = false;
    public function __construct($path) {
        $this->path = $path;
    }
    public function getSize() {
        $this->init();
        return parent::getSize();
    }
    private function init() {
        if ( ! $this->initialized) {
            $image             = new Image($this->path);
            $this->image       = $image->image;
            $this->initialized = true;
        }
    }
}

Ghost Object pros/cons

Same identity as RealSubject

Abstracts initialization logic away

Improved memory/performance impact

Low overhead

Harder to implement

May require reflection to access parent class's properties

Not optimal for data that is always loaded

Lazy loading means lazy failing

Protection proxy

A Protection Proxy comes into play when you want to transparently limit access to an API through a set of rules (ACL/limits/custom logic).

It actually is a decorator to the proxied RealSubject.

class APIProtectionProxy extends API {
    protected $count = 0;
    public function __construct(API $api, $limit) {
        $this->api = $api; $this->limit = $limit;
    }

    public function doStuff() {
        $this->count();
        return $this->api->doStuff();
    }

    private function count() {
        if (++$this->count > $this->limit) {
            throw new RemoteApiLimit('STAHP!');
        }
    }
}

$api = new APIProtectionProxy(new API(/* ... */), 50);

while (true) {
    $api->doStuff(); // RemoteApiLimit exception!
}

Protection Proxy pros/cons

Limiting access to an API does not require changes in the API

Modifies proxied object behavior

Smart reference

A Smart reference executes additional operations when the RealSubject is accessed

Smart reference is actually DecoratorPattern

Good for AOP

Smart reference use cases

• Caching slow method execution
• Logging
• Event triggering
• Mocking
• AOP in general

Smart reference example

Let's cache a complex/slow method call

    /** @AOP\Cache(ttl=3600) */
    public function doHeavyStuff() {
        // [...]
    }

Could become:

class APICachingProxy extends API {
    public function __construct(API $api, Cache $cache) {
        $this->api = $api; $this->cache = $cache;
    }

    public function doHeavyStuff() {
        if ($cached = $this->cache->get('doHeavyStuff')) {
            return $cached;
        }

        $result = $this->api->doHeavyStuff();

        $this->cache->set('doHeavyStuff', $result, 3600);

        return $result;
    }
}

Implementing a Proxy in PHP

Poor coder's proxy implementation:

class MyProxy {
    // ...

    public function __call($method, $args) {
        return call_user_func_array(
            $this->someWrappedStuff,
            $args
        );
    }
}

class BankAccount { /* ... */ }

function pay(BankAccount $account) { /* ... */ }

class PoorProxy {
    public function __construct($wrapped) {
        $this->wrapped = $wrapped;
    }

    public function __call($method, $args) {
        return call_user_func_array(
            $this->wrapped,
            $args
        );
    }
}

$account = new PoorProxy(new BankAccount());

pay($account); // KABOOM!

Implementing a
(generic) Proxy

1. MUST respect the LSP!

2. Not so trivial!

PHP is a maze!

• magic getters
• magic setters
• serialization
• cloning
• public properties

A lot of stuff to consider!

Proxies are NOT nice:

<?php
namespace Proxy\__PM__;

class A extends \A
{
    private $valueHolder;
    private $initializer;

    public function getFoo()
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, 'getFoo', array(), $this->initializer);

        return $this->valueHolder->getFoo();
    }

    public function __construct($initializer)
    {
        unset($this->foo);

        $this->initializer = $initializer;
    }

    public function __get($name)
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__get', array('name' => $name), $this->initializer);

        return $this->valueHolder->$name;
    }

    public function __set($name, $value)
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__set', array('name' => $name, 'value' => $value), $this->initializer);

        $this->valueHolder->$name = $value;
    }

    public function __isset($name)
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__isset', array('name' => $name), $this->initializer);

        return isset($this->valueHolder->$name);
    }

    public function __unset($name)
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__unset', array('name' => $name), $this->initializer);

        unset($this->valueHolder->$name);
    }

    public function __clone()
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__clone', array(), $this->initializer);

        $this->valueHolder = clone $this->valueHolder;
    }

    public function __sleep()
    {
        $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, '__sleep', array(), $this->initializer);

        return array('valueHolder');
    }

    public function __wakeup()
    {
        unset($this->foo);
    }

    public function setProxyInitializer(\Closure $initializer = null)
    {
        $this->initializer = $initializer;
    }

    public function getProxyInitializer()
    {
        return $this->initializer;
    }

    public function initializeProxy()
    {
        return $this->initializer && $this->initializer->__invoke($this->valueHolder, $this, 'initializeProxy', array(), $this->initializer);
    }

    public function isProxyInitialized()
    {
        return null !== $this->valueHolder;
    }

    public function getWrappedValueHolderValue()
        {
        return $this->valueHolder;
    }
}

Introducing ProxyManager

https://github.com/Ocramius/ProxyManager

Avoid bringing out the generated garbage yourself!

(Currently) deals with Virtual Proxies, Smart References, Null Objects, Ghost Objects and Remote Objects

Virtual Proxy with ProxyManager

namespace My\Slow;

class Foo
{
    public function __construct()
    {
        sleep(10);
    }

    public function doFoo()
    {
        echo 'foo!';
    }
}

require_once 'vendor/autoload.php';
use ProxyManager\Factory\LazyLoadingValueHolderFactory;

$factory = new LazyLoadingValueHolderFactory();

$proxy = $factory->createProxy(
    'My\Slow\Foo',
    function (& $wrappedObject, $proxy) {
        $wrappedObject = new My\Slow\Object();

        $proxy->setProxyInitializer(null);
    }
);

$proxy->doFoo();

Smart Reference with ProxyManager

require_once 'vendor/autoload.php';
use ProxyManager\Factory\AccessInterceptorValueHolderFactory;

$factory = new AccessInterceptorValueHolderFactory();
$db = $factory->createProxy(
    new \My\Db\Connection(),
    ['query' => function () {
        echo "Query being executed!\n";
    }],
    ['query' => function () {
        echo "Query completed!\n";
    }]
);

$db->query();

Yes, I'm no good at naming classes - really.

Integrates with
Symfony2
and
Zend Framework 2

Virtual (Service) Proxies + Zend Framework 2

return [
    'service_manager' => [
        'delegators' => [
            'MyServiceName' => ['LazyServiceFactory']
        ],
    ],
    'lazy_services' => [
        'map' => ['MyServiceName' => 'My\Service\ClassName']
    ]
]

Virtual (Service) Proxies + Symfony 2

<services>
    <service
        id="MyServiceName"
        class="My\Service\ClassName"
        lazy="true"
    />
</services>

Some useful libraries

Doctrine's Proxy Generator

ProxyManager

Flow (Typo3)

schmittjoh/cg-library

Proxies and AOP

lisachenko/go-aop-php

Uses proxies for AOP purposes

Very interesting usage of autoloaders!

And before we get to the dirty stuff...

Questions?

Ocramius/proxy-pattern-in-php

Ocramius

Proxying Fluent Interfaces

Breaks Virtual Proxies, Protection Proxies, and Smart References

More important:

Possible broken Encapsulation!

class BankAccount {
    protected $amount = 0;

    public function pay($amount) {
        $this->amount -= $amount;

        return $this;
    }
}

$account = new BankAccount();

$account->pay(100)->pay(200)->pay(300);

Caution!

Dirty hacks!

class BankAccountProxy extends BankAccount {
    public function __construct(BankAccount $wrapped) {
        $this->amount = & $wrapped->amount;
    }

    public function pay($amount) {
        echo 'Paid ' . $amount . "!\n";

        return parent::pay($amount);
    }
}

$account = new BankAccountProxy(new BankAccount());

$account->pay(100)->pay(200)->pay(300);

Paid 100
Paid 200
Paid 300

Public properties proxying

(without property accessors!)

class Customer {
    public $name;
    public $surname;
}

Caution!

Dirty hacks!

class Customer {
    public $name;
    public $surname;
}

class CustomerProxy extends Customer {
    public function __construct(Customer $customer) {
        unset($this->name, $this->surname);
        $this->customer = $customer;
    }

    public function __set($name, $value) {
        $this->customer->$name = $value;
    }

    public function __get($name) {
        return $this->customer->$name;
    }

    // __isset, __unset
}

Don't do this at home!

Thank you!

Ocramius/proxy-pattern-in-php