Attributes overview

    (PHP 8)

    Attributes offer the ability to add structured, machine-readable metadata information on declarations in code: Classes, methods, functions, parameters, properties and class constants can be the target of an attribute. The metadata defined by attributes can then be inspected at runtime using the Reflection APIs. Attributes could therefore be thought of as a configuration language embedded directly into code.

    With attributes the generic implementation of a feature and its concrete use in an application can be decoupled. In a way it is comparable to interfaces and their implementations. But where interfaces and implementations are about code, attributes are about annotating extra information and configuration. Interfaces can be implemented by classes, yet attributes can also be declared on methods, functions, parameters, properties and class constants. As such they are more flexible than interfaces.

    A simple example of attribute usage is to convert an interface that has optional methods to use attributes. Let's assume an ActionHandler interface representing an operation in an application, where some implementations of an action handler require setup and others do not. Instead of requiring all classes that implement ActionHandler to implement a method setUp(), an attribute can be used. One benefit of this approach is that we can use the attribute several times.

    Example #1 Implementing optional methods of an interface with Attributes

    <?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("File does not exist");
    }
    }

    #[    SetUp]
    public function     targetDirectoryExists()
    {
    if (!    file_exists($this->targetDirectory)) {
    mkdir($this->targetDirectory);
    } elseif (!    is_dir($this->targetDirectory)) {
    throw new     RuntimeException("Target directory $this->targetDirectory is not a directory");
    }
    }

    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
    2 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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