Image_Processing_Queue

	class Image_Processing_Queue {

		/**
		 * Singleton
		 *
		 * @var Image_Processing_Queue|null
		 */
		protected static $instance = null;

		/**
		 * Whether or not we're updating the backup sizes
		 *
		 * @var bool
		 */
		private $is_updating_backup_sizes = false;

		/**
		 * Instance of the background process class
		 *
		 * @var IPQ_Process|null
		 */
		public $process = null;

		/**
		 * Singleton
		 *
		 * @return Image_Processing_Queue|null
		 */
		public static function instance() {
			if ( is_null( self::$instance ) ) {
				self::$instance = new self();
			}
			return self::$instance;
		}

		/**
		 * Image_Processing_Queue constructor.
		 */
		public function __construct() {
			$this->process = new IPQ_Process();
			add_filter( 'update_post_metadata', array( $this, 'filter_update_post_metadata' ), 10, 5 );
		}

		/**
		 * Filter the post meta data for backup sizes
		 *
		 * Unfortunately WordPress core is lacking hooks in its image resizing functions so we are reduced
		 * to this hackery to detect when images are resized and previous versions are relegated to backup sizes.
		 *
		 * @param bool   $check
		 * @param int    $object_id
		 * @param string $meta_key
		 * @param mixed  $meta_value
		 * @param mixed  $prev_value
		 * @return bool
		 */
		public function filter_update_post_metadata( $check, $object_id, $meta_key, $meta_value, $prev_value ) {
			if ( '_wp_attachment_backup_sizes' !== $meta_key ) {
				return $check;
			}

			$current_value = get_post_meta( $object_id, $meta_key, true );

			if ( ! $current_value ) {
				$current_value = array();
			}

			$diff = array_diff_key( $meta_value, $current_value );

			if ( ! $diff ) {
				return $check;
			}

			$key = key( $diff );
			$suffix = substr( $key, strrpos( $key, '-' ) + 1 );

			$image_meta = self::get_image_meta( $object_id );

			foreach ( $image_meta['sizes'] as $size_name => $size ) {
				if ( 0 !== strpos( $size_name, 'ipq-' ) ) {
					continue;
				}

				$meta_value[ $size_name . '-' . $suffix ] = $size;
				unset( $image_meta['sizes'][ $size_name ] );
			}

			if ( ! $this->is_updating_backup_sizes ) {
				$this->is_updating_backup_sizes = true;
				update_post_meta( $object_id, '_wp_attachment_backup_sizes', $meta_value );
				wp_update_attachment_metadata( $object_id, $image_meta );
				return true;
			}

			$this->is_updating_backup_sizes = false;

			return $check;
		}

		/**
		 * Check if the image sizes exist and push them to the queue if not.
		 *
		 * @param int   $post_id
		 * @param array $sizes
		 */
		protected function process_image( $post_id, $sizes ) {
			$new_item = false;

			foreach ( $sizes as $size ) {
				if ( self::does_size_already_exist_for_image( $post_id, $size ) ) {
					continue;
				}

				if ( self::is_size_larger_than_original( $post_id, $size ) ) {
					continue;
				}

				$item = array(
					'post_id' => $post_id,
					'width'   => $size[0],
					'height'  => $size[1],
					'crop'    => $size[2],
				);
				$this->process->push_to_queue( $item );
				$new_item = true;
			}

			if ( $new_item ) {
				$this->process->save()->dispatch();
			}
		}

		/**
		 * Get image HTML for a specific context in a theme, specifying the exact sizes
		 * for the image. The first image size is always used as the `src` and the other
		 * sizes are used in the `srcset` if they're the same aspect ratio as the original
		 * image. If any of the image sizes don't currently exist, they are queued for
		 * creation by a background process. Example:
		 *
		 * echo ipq_get_theme_image( 1353, array(
		 *         array( 600, 400, false ),
		 *         array( 1280, 720, false ),
		 *         array( 1600, 1067, false ),
		 *     ),
		 * 	   array(
		 *         'class' => 'header-banner'
		 *     )
		 * );
		 *
		 * @param int    $post_id Image attachment ID.
		 * @param array  $sizes   Array of arrays of sizes in the format array(width,height,crop).
		 * @param string $attr    Optional. Attributes for the image markup. Default empty.
		 * @return string HTML img element or empty string on failure.
		 */
		public function get_image( $post_id, $sizes, $attr = '' ) {
			$this->process_image( $post_id, $sizes );

			return wp_get_attachment_image( $post_id, array( $sizes[0][0], $sizes[0][1] ), false, $attr );
		}

		/**
		 * Get image URL for a specific context in a theme, specifying the exact size
		 * for the image. If the image size does not currently exist, it is queued for
		 * creation by a background process. Example:
		 *
		 * echo ipq_get_theme_image_url( 1353, array( 600, 400, false ) );
		 *
		 * @param int   $post_id
		 * @param array $size
		 *
		 * @return string
		 */
		public function get_image_url( $post_id, $size ) {
			$this->process_image( $post_id, array( $size ) );

			$size = self::get_size_name( $size );
			$src  = wp_get_attachment_image_src( $post_id, $size );

			if ( isset( $src[0] ) ) {
				return $src[0];
			}

			return '';
		}

		/**
		 * Get array index name for image size.
		 *
		 * @param array $size array in format array(width,height,crop).
		 * @return string Image size name.
		 */
		public static function get_size_name( $size ) {
			$crop = $size[2] ? 'true' : 'false';
			return 'ipq-' . $size[0] . 'x' . $size[1] . '-' . $crop;
		}

		/**
		 * Get an image's file path.
		 *
		 * @param int $post_id ID of the image post.
		 * @return false|string
		 */
		public static function get_image_path( $post_id ) {
			return get_attached_file( $post_id );
		}

		/**
		 * Get an image's post meta data.
		 *
		 * @param int $post_id ID of the image post.
		 * @return mixed Post meta field. False on failure.
		 */
		public static function get_image_meta( $post_id ) {
			return wp_get_attachment_metadata( $post_id );
		}

		/**
		 * Update meta data for an image
		 *
		 * @param int   $post_id Image ID.
		 * @param array $data    Image data.
		 * @return bool|int False if $post is invalid.
		 */
		public static function update_image_meta( $post_id, $data ) {
			return wp_update_attachment_metadata( $post_id, $data );
		}

		/**
		 * Checks if an image size already exists for an image
		 *
		 * @param int   $post_id Image ID.
		 * @param array $size array in format array(width,height,crop).
		 * @return bool
		 */
		public static function does_size_already_exist_for_image( $post_id, $size ) {
			$image_meta = self::get_image_meta( $post_id );
			$size_name  = self::get_size_name( $size );

			return isset( $image_meta['sizes'][ $size_name ] );
		}

		/**
		 * Check if an image size is larger than the original.
		 *
		 * @param int   $post_id Image ID.
		 * @param array $size array in format array(width,height,crop).
		 *
		 * @return bool
		 */
		public static function is_size_larger_than_original( $post_id, $size ) {
			$image_meta = self::get_image_meta( $post_id );

			if ( ! isset( $image_meta['width'] ) || ! isset( $image_meta['height'] ) ) {
				return true;
			}

			if ( $size[0] > $image_meta['width'] || $size[1] > $image_meta['height'] ) {
				return true;
			}

			return false;
		}
	}