lazy-loading-value-holder.md

Lazy Loading Value Holder Proxy

A lazy loading value holder proxy is a virtual proxy that wraps and lazily initializes a "real" instance of the proxied class.

What is lazy loading?

In pseudo-code, lazy loading looks like the following:

class MyObjectProxy
{
    private $wrapped;

    public function doFoo()
    {
        $this->init();

        return $this->wrapped->doFoo();
    }

    private function init()
    {
        if (null === $this->wrapped) {
            $this->wrapped = new MyObject();
        }
    }
}

This code is problematic and adds complexity that makes your unit tests' code even worse.

Also, this kind of usage often ends up in coupling your code with a particular Dependency Injection Container or a framework that fetches dependencies for you. That way, further complexity is introduced, and some problems related with service location raise, as explained in this article.

Lazy loading value holders abstract this logic for you, hiding your complex, slow, performance-impacting objects behind tiny wrappers that have their same API, and that get initialized at first usage.

When do I use a lazy value holder?

You usually need a lazy value holder in cases where the following applies:

• your object takes a lot of time and memory to be initialized (with all dependencies)
• your object is not always used, and the instantiation overhead is avoidable

Usage examples

ProxyManager provides a factory that eases instantiation of lazy loading value holders. To use it, follow these steps:

Firstly, define your object's logic without taking care of lazy loading:

namespace MyApp;

class HeavyComplexObject
{
    public function __construct()
    {
        // just write your business logic
        // don't worry about how heavy initialization of this will be!
    }

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

Then use the proxy manager to create a lazy version of the object (as a proxy):

namespace MyApp;

use ProxyManager\Factory\LazyLoadingValueHolderFactory;
use ProxyManager\Proxy\LazyLoadingInterface;

require_once __DIR__ . '/vendor/autoload.php';

$factory     = new LazyLoadingValueHolderFactory();
$initializer = function (& $wrappedObject, LazyLoadingInterface $proxy, $method, array $parameters, & $initializer) {
    $initializer   = null; // disable initialization
    $wrappedObject = new HeavyComplexObject(); // fill your object with values here

    return true; // confirm that initialization occurred correctly
};

$proxy = $factory->createProxy('MyApp\HeavyComplexObject', $initializer);

You can now use your object as before:

// this will just work as before
$proxy->doFoo(); // OK!

Lazy Initialization

As you can see, we use a closure to handle lazy initialization of the proxy instance at runtime. The initializer closure signature should be as following:

/**
 * @var object  $wrappedObject the instance (passed by reference) of the wrapped object,
 *                             set it to your real object
 * @var object  $proxy         the instance proxy that is being initialized
 * @var string  $method        the name of the method that triggered lazy initialization
 * @var array   $parameters    an ordered list of parameters passed to the method that
 *                             triggered initialization, indexed by parameter name
 * @var Closure $initializer   a reference to the property that is the initializer for the
 *                             proxy. Set it to null to disable further initialization
 *
 * @return bool true on success
 */
$initializer = function (& $wrappedObject, $proxy, $method, array $parameters, & $initializer) {};

The initializer closure should usually be coded like following:

$initializer = function (& $wrappedObject, $proxy, $method, array $parameters, & $initializer) {
    $newlyCreatedObject = new Foo(); // instantiation logic
    $newlyCreatedObject->setBar('baz'); // instantiation logic
    $newlyCreatedObject->setBat('bam'); // instantiation logic

    $wrappedObject = $newlyCreatedObject; // set wrapped object in the proxy
    $initializer   = null; // disable initializer

    return true; // report success
};

The ProxyManager\Factory\LazyLoadingValueHolderFactory produces proxies that implement both the ProxyManager\Proxy\ValueHolderInterface and the ProxyManager\Proxy\LazyLoadingInterface.

At any point in time, you can set a new initializer for the proxy:

$proxy->setProxyInitializer($initializer);

In your initializer, you currently MUST turn off any further initialization:

$proxy->setProxyInitializer(null);

or

$initializer = null; // if you use the initializer by reference

Triggering Initialization

A lazy loading proxy is initialized whenever you access any property or method of it. Any of the following interactions would trigger lazy initialization:

// calling a method
$proxy->someMethod();

// reading a property
echo $proxy->someProperty;

// writing a property
$proxy->someProperty = 'foo';

// checking for existence of a property
isset($proxy->someProperty);

// removing a property
unset($proxy->someProperty);

// cloning the entire proxy
clone $proxy;

// serializing the proxy
$unserialized = serialize(unserialize($proxy));

Remember to call $proxy->setProxyInitializer(null); to disable initialization of your proxy, or it will happen more than once.

Proxying interfaces

You can also generate proxies from an interface FQCN. When you proxy an interface, you will only be able to access the methods defined by the interface itself, even if the wrappedObject implements more methods. This will save some memory since the proxy will not contain useless inherited properties.

Known limitations

• methods using func_get_args(), func_get_arg() and func_num_arg() will not function properly for parameters that are not part of the proxied object interface: use variadic arguments instead.

Tuning performance for production

See Tuning ProxyManager for Production.