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);
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User Contributed Notes 2 notes

up
42
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.

<?php
interface 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).
up
37
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.

<?php
declare(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));
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