Integrate plugin update checker library

Added the Plugin Update Checker library under lib/plugin-update-checker and integrated it into WPSlug_Core for automatic update checks. This enables update notifications and management for the plugin via a remote JSON metadata file.

lib/plugin-update-checker/Puc/v5p3/Metadata.php

@@ -0,0 +1,162 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p3;
+
+use LogicException;
+use stdClass;
+use WP_Error;
+
+if ( !class_exists(Metadata::class, false) ):
+
+	/**
+	 * A base container for holding information about updates and plugin metadata.
+	 *
+	 * @author Janis Elsts
+	 * @copyright 2016
+	 * @access public
+	 */
+	abstract class Metadata {
+		/**
+		 * Additional dynamic properties, usually copied from the API response.
+		 *
+		 * @var array<string,mixed>
+		 */
+		protected $extraProperties = array();
+
+		/**
+		 * Create an instance of this class from a JSON document.
+		 *
+		 * @abstract
+		 * @param string $json
+		 * @return self
+		 */
+		public static function fromJson($json) {
+			throw new LogicException('The ' . __METHOD__ . ' method must be implemented by subclasses');
+		}
+
+		/**
+		 * @param string $json
+		 * @param self $target
+		 * @return bool
+		 */
+		protected static function createFromJson($json, $target) {
+			/** @var \StdClass $apiResponse */
+			$apiResponse = json_decode($json);
+			if ( empty($apiResponse) || !is_object($apiResponse) ){
+				$errorMessage = "Failed to parse update metadata. Try validating your .json file with https://jsonlint.com/";
+				do_action('puc_api_error', new WP_Error('puc-invalid-json', $errorMessage));
+				//phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_trigger_error -- For plugin developers.
+				trigger_error(esc_html($errorMessage), E_USER_NOTICE);
+				return false;
+			}
+
+			$valid = $target->validateMetadata($apiResponse);
+			if ( is_wp_error($valid) ){
+				do_action('puc_api_error', $valid);
+				//phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_trigger_error -- For plugin developers.
+				trigger_error(esc_html($valid->get_error_message()), E_USER_NOTICE);
+				return false;
+			}
+
+			foreach(get_object_vars($apiResponse) as $key => $value){
+				$target->$key = $value;
+			}
+
+			return true;
+		}
+
+		/**
+		 * No validation by default! Subclasses should check that the required fields are present.
+		 *
+		 * @param \StdClass $apiResponse
+		 * @return bool|\WP_Error
+		 */
+		protected function validateMetadata($apiResponse) {
+			return true;
+		}
+
+		/**
+		 * Create a new instance by copying the necessary fields from another object.
+		 *
+		 * @abstract
+		 * @param \StdClass|self $object The source object.
+		 * @return self The new copy.
+		 */
+		public static function fromObject($object) {
+			throw new LogicException('The ' . __METHOD__ . ' method must be implemented by subclasses');
+		}
+
+		/**
+		 * Create an instance of StdClass that can later be converted back to an
+		 * update or info container. Useful for serialization and caching, as it
+		 * avoids the "incomplete object" problem if the cached value is loaded
+		 * before this class.
+		 *
+		 * @return \StdClass
+		 */
+		public function toStdClass() {
+			$object = new stdClass();
+			$this->copyFields($this, $object);
+			return $object;
+		}
+
+		/**
+		 * Transform the metadata into the format used by WordPress core.
+		 *
+		 * @return object
+		 */
+		abstract public function toWpFormat();
+
+		/**
+		 * Copy known fields from one object to another.
+		 *
+		 * @param \StdClass|self $from
+		 * @param \StdClass|self $to
+		 */
+		protected function copyFields($from, $to) {
+			$fields = $this->getFieldNames();
+
+			if ( property_exists($from, 'slug') && !empty($from->slug) ) {
+				//Let plugins add extra fields without having to create subclasses.
+				$fields = apply_filters($this->getPrefixedFilter('retain_fields') . '-' . $from->slug, $fields);
+			}
+
+			foreach ($fields as $field) {
+				if ( property_exists($from, $field) ) {
+					$to->$field = $from->$field;
+				}
+			}
+		}
+
+		/**
+		 * @return string[]
+		 */
+		protected function getFieldNames() {
+			return array();
+		}
+
+		/**
+		 * @param string $tag
+		 * @return string
+		 */
+		protected function getPrefixedFilter($tag) {
+			return 'puc_' . $tag;
+		}
+
+		public function __set($name, $value) {
+			$this->extraProperties[$name] = $value;
+		}
+
+		public function __get($name) {
+			return isset($this->extraProperties[$name]) ? $this->extraProperties[$name] : null;
+		}
+
+		public function __isset($name) {
+			return isset($this->extraProperties[$name]);
+		}
+
+		public function __unset($name) {
+			unset($this->extraProperties[$name]);
+		}
+	}
+
+endif;