Übersicht über die Attribute
(PHP 8)
PHP-Attribute bieten strukturierte, maschinenlesbare Metadaten für Klassen,
Methoden, Funktionen, Parameter, Eigenschaften und Konstanten. Sie können
zur Laufzeit über die
Reflection-API inspiziert werden und
ermöglichen dynamisches Verhalten ohne Änderung des Codes. Attribute bieten
eine deklarative Möglichkeit, den Code mit Metadaten zu kommentieren.
Attribute ermöglichen es, die Implementierung eines Merkmals von seiner
Verwendung zu trennen. Während Schnittstellen die Struktur definieren,
indem sie Methoden vorschreiben, liefern Attribute Metadaten über mehrere
Elemente, einschließlich Methoden, Funktionen, Eigenschaften und
Konstanten. Im Gegensatz zu Schnittstellen, die die Implementierung von
Methoden vorschreiben, kommentieren Attribute den Code, ohne dessen
Struktur zu verändern.
Attribute können optionale Schnittstellenmethoden ergänzen oder ersetzen,
indem sie Metadaten anstelle einer vorgeschriebenen Struktur bereitstellen.
Ein Beispiel könnte eine ActionHandler-Schnittstelle
sein, die eine Operation in einer Anwendung repräsentiert. Einige
Implementierungen könnten einen Konfigurationsschritt erfordern, während
andere dies nicht tun.
Anstatt alle Klassen, die ActionHandler implementieren,
zu zwingen, eine setUp()-Methode zu definieren, kann ein
Attribut die Setup-Anforderungen spezifizieren. Dieser Ansatz erhöht die
Flexibilität, da Attribute bei Bedarf mehrfach verwendet werden können.
Beispiel #1 Implementierung optionaler Methoden einer Schnittstelle mit Hilfe von Attributen
<?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("Die Datei existiert nicht");
}
}
#[SetUp]
public function targetDirectoryExists()
{
if (!file_exists($this->targetDirectory)) {
mkdir($this->targetDirectory);
} elseif (!is_dir($this->targetDirectory)) {
throw new RuntimeException("Das Zielverzeichnis $this->targetDirectory ist kein Verzeichnis");
}
}
public function execute()
{
copy($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));
