PHP Velho Oeste 2024

La classe WeakMap

(PHP 8)

Introduction

Un WeakMap est un tableau associatif (ou dictionnaire) qui accepte des objets comme clés. Cependant, contrairement au autrement similaire SplObjectStorage, un objet dans une clé de WeakMap ne contribue pas au nombre de références de l'objet. En d'autres termes, si, à un moment donné, la seule référence restante à un objet est la clé d'un WeakMap, l'objet sera ramassé et supprimé du WeakMap. Son principal cas d'utilisation est la construction de caches de données dérivées d'un objet qui n'ont pas besoin d'être conservées plus longtemps que l'objet.

WeakMap implemente ArrayAccess, Iterator, et Countable, de sorte que, dans la plupart des cas, il peut être utilisé de la même manière qu'un tableau associatif.

Synopsis de la classe

final class WeakMap implements ArrayAccess, Countable, IteratorAggregate {
/* Méthodes */
public count(): int
public offsetExists(object $object): bool
public offsetGet(object $object): mixed
public offsetSet(object $object, mixed $value): void
public offsetUnset(object $object): void
}

Exemples

Exemple #1 Exemple d'utilisation d'un Weakmap

<?php
$wm
= new WeakMap();

$o = new stdClass;

class
A {
public function
__destruct() {
echo
"Dead!\n";
}
}

$wm[$o] = new A;

var_dump(count($wm));
echo
"Unsetting...\n";
unset(
$o);
echo
"Done\n";
var_dump(count($wm));

L'exemple ci-dessus va afficher :

int(1)
Unsetting...
Dead!
Done
int(0)

Sommaire

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User Contributed Notes 1 note

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Samu
3 months ago
PHP's implementation of WeakMap allows for iterating over the contents of the weak map, hence it's important to understand why it is sometimes dangerous and requires careful thought.

If the objects of the WeakMap are "managed" by other services such as Doctrine's EntityManager, it is never safe to assume that if the object still exists in the weak map, it is still managed by Doctrine and therefore safe to consume.

Doctrine might have already thrown that entity away but some unrelated piece of code might still hold a reference to it, hence it still existing in the map as well.

If you are placing managed objects into the WeakMap and later iterating over the WeakMap (e.g. after Doctrine flush), then for each such object you must verify that it is still valid in the context of the source of the object.

For example assigning a detached Doctrine entity to another entity's property would result in errors about non-persisted / non-managed entities being found in the hierarchy.
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