Visão geral dos atributos
(PHP 8)
Os atributos PHP fornecem metadados estruturados e legíveis por máquina para classes, métodos,
funções, parâmetros, propriedades e constantes. Eles podem ser inspecionados no momento da execução
por meio da API Reflection, permitindo comportamento
dinâmico sem modificar o código. Os atributos fornecem uma maneira declarativa de anotar
código com metadados.
Atributos permitem o desacoplamento da implementação de um recurso de seu uso. Enquanto
interfaces definem estrutura ao impor métodos, atributos fornecem metadados em vários
elementos, incluindo métodos, funções, propriedades e constantes. Diferentemente de interfaces,
que impõem implementações de métodos, atributos anotam código sem alterar sua estrutura.
Atributos podem complementar ou substituir métodos de interface opcionais fornecendo metadados em vez de
estrutura imposta. Considere uma interface ActionHandler que representa uma
operação em um aplicativo. Algumas implementações podem exigir uma etapa de configuração, enquanto outras não.
Em vez de forçar todas as classes que implementam ActionHandler a definir um
método setUp(), um atributo pode indicar requisitos de configuração. Essa abordagem
aumenta a flexibilidade, permitindo que os atributos sejam aplicados várias vezes quando necessário.
Exemplo #1 Implementando métodos opcionais de uma interface com Atributos
<?php
interface ActionHandler
{
public function execute();
}
#[Attribute]
class SetUp {}
class CopyFile implements ActionHandler
{
public string $fileName;
public string $targetDirectory;
#[SetUp]
public function fileExists()
{
if (!file_exists($this->fileName)) {
throw new RuntimeException("Arquivo não existe");
}
}
#[SetUp]
public function targetDirectoryExists()
{
if (!file_exists($this->targetDirectory)) {
mkdir($this->targetDirectory);
} elseif (!is_dir($this->targetDirectory)) {
throw new RuntimeException("Diretório de destino $this->targetDirectory não é um diretório");
}
}
public function execute()
{
copy($this->fileName, $this->targetDirectory . '/' . basename($this->fileName));
}
}
function executeAction(ActionHandler $actionHandler)
{
$reflection = new ReflectionObject($actionHandler);
foreach ($reflection->getMethods() as $method) {
$attributes = $method->getAttributes(SetUp::class);
if (count($attributes) > 0) {
$methodName = $method->getName();
$actionHandler->$methodName();
}
}
$actionHandler->execute();
}
$copyAction = new CopyFile();
$copyAction->fileName = "/tmp/foo.jpg";
$copyAction->targetDirectory = "/home/user";
executeAction($copyAction);
Harshdeep ¶3 years ago
While the example displays us what we can accomplish with attributes, it should be kept in mind that the main idea behind attributes is to attach static metadata to code (methods, properties, etc.). This metadata often includes concepts such as "markers" and "configuration". For example, you can write a serializer using reflection that only serializes marked properties (with optional configuration, such as field name in serialized file). This is reminiscent of serializers written for C# applications.That said, full reflection and attributes go hand in hand. If your use case is satisfied by inheritance or interfaces, prefer that. The most common use case for attributes is when you have no prior information about the provided object/class.<?phpinterface JsonSerializable{ public function toJson() : array;}?>versus, using attributes,<?php#[Attribute]class JsonSerialize { public function __constructor(public ?string $fieldName = null) {}}class VersionedObject{ #[JsonSerialize] public const version = '0.0.1';}public class UserLandClass extends VersionedObject{ #[JsonSerialize('call it Jackson')] public string $myValue;}?>The example above is a little extra convoluted with the existence of the VersionedObject class as I wished to display that with attribute mark ups, you do not need to care how the base class manages its attributes (no call to parent in overriden method).
Florian Krmer ¶2 years ago
I've tried Harshdeeps example and it didn't run out of the box and I think it is not complete, so I wrote a complete and working naive example regarding attribute based serialization.<?phpdeclare(strict_types=1);#[Attribute(Attribute::TARGET_CLASS_CONSTANT|Attribute::TARGET_PROPERTY)]class JsonSerialize{ public function __construct(public ?string $fieldName = null) {}}class VersionedObject{ #[JsonSerialize] public const version = '0.0.1';}class UserLandClass extends VersionedObject{ protected string $notSerialized = 'nope'; #[JsonSerialize('foobar')] public string $myValue = ''; #[JsonSerialize('companyName')] public string $company = ''; #[JsonSerialize('userLandClass')] protected ?UserLandClass $test; public function __construct(?UserLandClass $userLandClass = null) { $this->test = $userLandClass; }}class AttributeBasedJsonSerializer { protected const ATTRIBUTE_NAME = 'JsonSerialize'; public function serialize($object) { $data = $this->extract($object); return json_encode($data, JSON_THROW_ON_ERROR); } protected function reflectProperties(array $data, ReflectionClass $reflectionClass, object $object) { $reflectionProperties = $reflectionClass->getProperties(); foreach ($reflectionProperties as $reflectionProperty) { $attributes = $reflectionProperty->getAttributes(static::ATTRIBUTE_NAME); foreach ($attributes as $attribute) { $instance = $attribute->newInstance(); $name = $instance->fieldName ?? $reflectionProperty->getName(); $value = $reflectionProperty->getValue($object); if (is_object($value)) { $value = $this->extract($value); } $data[$name] = $value; } } return $data; } protected function reflectConstants(array $data, ReflectionClass $reflectionClass) { $reflectionConstants = $reflectionClass->getReflectionConstants(); foreach ($reflectionConstants as $reflectionConstant) { $attributes = $reflectionConstant->getAttributes(static::ATTRIBUTE_NAME); foreach ($attributes as $attribute) { $instance = $attribute->newInstance(); $name = $instance->fieldName ?? $reflectionConstant->getName(); $value = $reflectionConstant->getValue(); if (is_object($value)) { $value = $this->extract($value); } $data[$name] = $value; } } return $data; } protected function extract(object $object) { $data = []; $reflectionClass = new ReflectionClass($object); $data = $this->reflectProperties($data, $reflectionClass, $object); $data = $this->reflectConstants($data, $reflectionClass); return $data; }}$userLandClass = new UserLandClass();$userLandClass->company = 'some company name';$userLandClass->myValue = 'my value';$userLandClass2 = new UserLandClass($userLandClass);$userLandClass2->company = 'second';$userLandClass2->myValue = 'my second value';$serializer = new AttributeBasedJsonSerializer();$json = $serializer->serialize($userLandClass2);var_dump(json_decode($json, true));
