Quick notes about some OOP structure

attribute-entiy.php

<?php

interface EntityAttributeInterface {
    
    public function getName();

    public function setValue( AttributeValueInterface $value );
    
    public function setUnit( AttributeUnitInterface $unit );

    // getters dito
}

class PostAttribute implements EntityAttributeInterface {
    
    /**
     * the identifier of the attribute
     * @type string
     */
    private $name;
    
    private $post;
    
    private $value;
    
    private $unit;
    
    private $storage;
    
    public function __construct( $name, \WP_Post $post, AttributeStorageInterface $storage ) {
        
        $this->post = $post
    }
    
    public function update() {

        $data = array(
            'value' => $this->value->getValue(),
            'unit'  => $this->unit->getUnit()
        );

        $this->storage->update( $this->name, $data );
    }
    // implementing setter/getter from interface
}

/**
 * define a storage for attributes
 */
interface AttributeStorageInterface {
    
    public function update( $name, $data );
    
    public function get( $name );
    
    public function delete( $name );
}

class PostMetaAttributeStorage implements AttributeStorageInterface {
    
    private $post;
    
    public function __construct( \WP_Post ) {
        
        $this->post = $post;
    }
    
    public function update( $name, $data ) {
        
        update_post_meta( $this->post->ID, $name, $data );
    }
    
    //get(), delete() dito
}

/**
 * the value interface
 */
interface AttributeValueInterface {
    
    public function setValue( $value );
    
    public function getValue();
}
class NumericAttributeValue implements AttributeValueInterface {
    
    private $value = 0;

    public function setValue( $value ) {
        
        if ( is_numeric( $value  ) ) 
            $this->value = $value;
    }
    
    public function getValue() {
        
        return $this->value;
    }
}

/** 
 * the unit interface
 */
interface AttributeUnitInterface {
    
    public function setUnit( $unit );
    
    public function getUnit();
}

class WPTermAttributeUnit {
 
    private $unit_id = 0;

    /**
     * @param \stdClass $unit (WPTerm) 
     */
    public function setUnit( $unit ) {
        
        $this->unit_id = $unit->term_id;
    }

    pubic function getUnit() {

        return $this->unit;
    }
}

usage.php

<?php

// update a post attribute 'size' from a request
$post = get_post( $request[ 'post_ID' ] );
$size = new PostAttribute( 
    'size', 
    $post,
    new PostMetaAttributeStorage( $post ) 
);
$size_value = new NumericAttributeValue;
$size_value->setValue( $request[ 'size' ] );

$unit_term = get_term( /*...*/ );
$size_unit = new WPTermAttributeUnit( $unit_term );

$size->setValue( $size_value );
$size->setUnit( $size_unit );
$size->update();