class ImageToolkitOperationBase

abstract class ImageToolkitOperationBase extends PluginBase implements ImageToolkitOperationInterface {
    
    /**
     * The image toolkit.
     *
     * @var \Drupal\Core\ImageToolkit\ImageToolkitInterface
     */
    protected $toolkit;
    
    /**
     * A logger instance.
     *
     * @var \Psr\Log\LoggerInterface
     */
    protected $logger;
    
    /**
     * Constructs an image toolkit operation plugin.
     *
     * @param array $configuration
     *   A configuration array containing information about the plugin instance.
     * @param string $plugin_id
     *   The plugin_id for the plugin instance.
     * @param array $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\ImageToolkit\ImageToolkitInterface $toolkit
     *   The image toolkit.
     * @param \Psr\Log\LoggerInterface $logger
     *   A logger instance.
     */
    public function __construct(array $configuration, $plugin_id, array $plugin_definition, ImageToolkitInterface $toolkit, LoggerInterface $logger) {
        parent::__construct($configuration, $plugin_id, $plugin_definition);
        $this->toolkit = $toolkit;
        $this->logger = $logger;
    }
    
    /**
     * Returns the image toolkit instance for this operation.
     *
     * Image toolkit implementers should provide a toolkit operation base class
     * that overrides this method to correctly document the return type of this
     * getter. This provides better DX (code checking and code completion) for
     * image toolkit operation developers.
     *
     * @return \Drupal\Core\ImageToolkit\ImageToolkitInterface
     */
    protected function getToolkit() {
        return $this->toolkit;
    }
    
    /**
     * Returns the definition of the operation arguments.
     *
     * Image toolkit operation implementers must implement this method to
     * "document" their operation, thus also if no arguments are expected.
     *
     * @return array
     *   An array whose keys are the names of the arguments (e.g. "width",
     *   "degrees") and each value is an associative array having the following
     *   keys:
     *   - description: A string with the argument description. This is used only
     *     internally for documentation purposes, so it does not need to be
     *     translatable.
     *   - required: (optional) A boolean indicating if this argument should be
     *     provided or not. Defaults to TRUE.
     *   - default: (optional) When the argument is set to "required" = FALSE,
     *     this must be set to a default value. Ignored for "required" = TRUE
     *     arguments.
     */
    protected abstract function arguments();
    
    /**
     * Checks for required arguments and adds optional argument defaults.
     *
     * Image toolkit operation implementers should not normally need to override
     * this method as they should place their own validation in validateArguments.
     *
     * @param array $arguments
     *   An associative array of arguments to be used by the toolkit operation.
     *
     * @return array
     *   The prepared arguments array.
     *
     * @throws \InvalidArgumentException.
     * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException.
     */
    protected function prepareArguments(array $arguments) {
        foreach ($this->arguments() as $id => $argument) {
            $argument += [
                'required' => TRUE,
            ];
            // Check if the argument is required and, if so, has been provided.
            if ($argument['required']) {
                if (!array_key_exists($id, $arguments)) {
                    // If the argument is required throw an exception.
                    throw new \InvalidArgumentException("Argument '{$id}' expected by plugin '{$this->getPluginId()}' but not passed");
                }
            }
            else {
                // Optional arguments require a 'default' value.
                // We check this even if the argument is provided by the caller, as we
                // want to fail fast here, i.e. at development time.
                if (!array_key_exists('default', $argument)) {
                    // The plugin did not define a default, so throw a plugin exception,
                    // not an invalid argument exception.
                    throw new InvalidPluginDefinitionException("Default for argument '{$id}' expected by plugin '{$this->getPluginId()}' but not defined");
                }
                // Use the default value if the argument is not passed in.
                if (!array_key_exists($id, $arguments)) {
                    $arguments[$id] = $argument['default'];
                }
            }
        }
        return $arguments;
    }
    
    /**
     * Validates the arguments.
     *
     * Image toolkit operation implementers should place any argument validation
     * in this method, throwing an InvalidArgumentException when an error is
     * encountered.
     *
     * Validation typically includes things like:
     * - Checking that width and height are not negative.
     * - Checking that a color value is indeed a color.
     *
     * But validation may also include correcting the arguments, e.g:
     * - Casting arguments to the correct type.
     * - Rounding pixel values to an integer.
     *
     * This base implementation just returns the array of arguments and thus does
     * not need to be called by overriding methods.
     *
     * @param array $arguments
     *   An associative array of arguments to be used by the toolkit operation.
     *
     * @return array
     *   The validated and corrected arguments array.
     *
     * @throws \InvalidArgumentException
     *   If one or more of the arguments are not valid.
     * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
     *   If the plugin does not define a default for an optional argument.
     */
    protected function validateArguments(array $arguments) {
        return $arguments;
    }
    
    /**
     * {@inheritdoc}
     */
    public final function apply(array $arguments) {
        $arguments = $this->prepareArguments($arguments);
        $arguments = $this->validateArguments($arguments);
        return $this->execute($arguments);
    }
    
    /**
     * Performs the actual manipulation on the image.
     *
     * Image toolkit operation implementers must implement this method. This
     * method is responsible for actually performing the operation on the image.
     * When this method gets called, the implementer may assume all arguments,
     * also the optional ones, to be available, validated and corrected.
     *
     * @param array $arguments
     *   An associative array of arguments to be used by the toolkit operation.
     *
     * @return bool
     *   TRUE if the operation was performed successfully, FALSE otherwise.
     */
    protected abstract function execute(array $arguments);

}