MultilingualPress Add on as example to create a custom language switcher widget.

mlp-addon-custom-wigdet.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: MultilingualPress Add on as example to create a custom language switcher widget.
 * Description: This is a simple add-on for the MultilingualPress plugin to create a language switcher widget, that is only visible if there are relationship.
 * Author:      Inpsyde GmbH
 * Author URI:  http://inpsyde.com
 * Version:     2015-10-19
 * Text Domain: multilingualpressaddon
 * Domain Path: /languages
 * License:     GPLv2+
 * Network:
 */

defined( 'ABSPATH' ) or die();

if ( ! class_exists( 'Multilingual_Press' ) ) {
	return;
}

add_action( 'inpsyde_mlp_init', 'mlp_addon_widget_setup' );
/**
 * @param Inpsyde_Property_List_Interface $plugin_data Plugin data.
 *
 * @return void
 */
function mlp_addon_widget_setup( Inpsyde_Property_List_Interface $plugin_data ) {

	Mlp_Addon_Widget::insert_asset_instance( $plugin_data->get( 'assets' ) );
}

add_action( 'widgets_init', array( 'Mlp_Addon_Widget', 'widget_register' ) );
/**
 * Language switcher widget.
 */
class Mlp_Addon_Widget extends WP_Widget {

	/**
	 * The handle.
	 *
	 * @var string
	 */
	protected static $handle = 'mlp_addon_widget';

	/**
	 * Assets interface from MLP core.
	 *
	 * @var Mlp_Assets_Interface
	 */
	private static $assets;

	/**
	 * Register the widget and set up the description.
	 */
	public function __construct() {

		add_action( 'template_redirect', array( $this, 'require_style' ) );

		$widget_ops = array(
			'classname'   => self::$handle,
			'description' => __( 'Custom MultilingualPress Translations', 'multilingualpressaddon' ),
		);
		parent::__construct( 'Mlp_Addon_Widget', __( 'Language Switcher*', 'multilingualpressaddon' ), $widget_ops );
	}

	/**
	 * Load frontend CSS if the widget is active.
	 *
	 * @wp-hook template_redirect
	 *
	 * @return bool
	 */
	public function require_style() {

		if ( ! is_active_widget( FALSE, FALSE, self::$handle ) ) {
			return FALSE;
		}

		$theme_support = get_theme_support( 'multilingualpress' );
		if ( ! empty( $theme_support[ 0 ][ 'language_switcher_widget_style' ] ) ) {
			return FALSE;
		}

		self::$assets->provide( 'mlp_frontend_css' );

		return TRUE;
	}

	/**
	 * Display widget admin form.
	 *
	 * @param array $instance Widget settings
	 *
	 * @return void
	 */
	public function form( $instance ) {

		$instance = $this->adapt_settings( $instance );

		$title = isset( $instance[ 'widget_title' ] )
			? esc_attr( $instance[ 'widget_title' ] ) : '';
		?>
		<p>
			<?php $title_id = $this->get_field_id( 'mlp_widget_title' ); ?>
			<label for="<?php echo $title_id; ?>"><?php esc_html_e(
					'Title', 'multilingualpressaddon'
				); ?></label><br />
			<input class="widefat" type="text" id="<?php echo $title_id; ?>"
				name="<?php echo $this->get_field_name( 'mlp_widget_title' ); ?>" value="<?php echo $title; ?>">
		</p>
		<?php
	}

	/**
	 * Callback for widget update.
	 *
	 * @param array $new_instance New widget settings.
	 * @param array $old_instance Widget settings.
	 *
	 * @return array
	 */
	public function update( $new_instance, $old_instance ) {

		$instance = $old_instance;

		$instance[ 'widget_title' ] = esc_html( $new_instance[ 'mlp_widget_title' ] );

		return $instance;
	}

	public function get_mlp_translation() {

		$mlp_language_api = apply_filters( 'mlp_language_api', NULL );
		if ( is_a( $mlp_language_api, 'Mlp_Language_Api_Interface' ) ) {
			$args = [
				'site_id'      => get_current_blog_id(),
				'content_id'   => get_queried_object_id(),
				'type'         => 'post',
				'strict'       => TRUE,
				'search_term'  => '',
				'post_type'    => 'page',
				'include_base' => TRUE
			];
			/* @var Mlp_Language_Api_Interface $mlp_language_api */
			$translations = $mlp_language_api->get_translations( $args );
		}

		if ( ! empty( $translations ) ) {
			return TRUE;
		}

		return FALSE;
	}

	/**
	 * Frontend display.
	 *
	 * When a widget is restored from trash, the instance might be incomplete, hence the preparations.
	 *
	 * @param array $args     Widget arguments.
	 * @param array $instance Widget settings.
	 *
	 * @return void
	 */
	public function widget( $args, $instance ) {

		// Exit, if we are outside from singular sites, like post and page.
		if ( ! is_singular() ) {
			return;
		}

		// Exit, if we have no translations.
		if ( ! $this->get_mlp_translation() ) {
			return;
		}

		// Default arguments for the result.
		// @see multilingual-press/src/inc/common/Mlp_Helpers.php  Mlp_Helpers::show_linked_elements()
		$output_args = array(
			'link_text'         => 'native',
			'show_current_blog' => 0,
			'display_flag'      => 1,
		);
		$output      = Mlp_Helpers::show_linked_elements( $output_args );
		if ( ! $output ) {
			return;
		}

		$title = '';
		if ( isset( $instance[ 'widget_title' ] ) ) {
			$title = $instance[ 'widget_title' ];
		}
		/** This filter is documented in wp-includes/default-widgets.php */
		$title = apply_filters( 'widget_title', $title );

		echo $args[ 'before_widget' ];

		if ( ! empty( $instance[ 'widget_title' ] ) ) {
			echo $args[ 'before_title' ] . $title . $args[ 'after_title' ];
		}

		echo $output;

		echo $args[ 'after_widget' ];
	}

	/**
	 * Register the widget.
	 *
	 * @return void
	 */
	public static function widget_register() {

		register_widget( __CLASS__ );
	}

	/**
	 * Insert assets.
	 *
	 * @param Mlp_Assets_Interface $assets Assets.
	 *
	 * @return void
	 */
	public static function insert_asset_instance( Mlp_Assets_Interface $assets ) {

		self::$assets = $assets;
	}

}