Use fieldmanager to create meta boxes on a subpage from a term

class-fieldmanager-context-termpage.php

<?php

if ( class_exists( 'Fieldmanager_Context_Term' ) ) {

	/**
	 * Use fieldmanager to create meta boxes on a subpage from a term
	 */
	class Fieldmanager_Context_Termpage extends Fieldmanager_Context_Term {

		/**
		 * @var string|null $submit_button_label
		 */
		protected $submit_button_label = null;

		/**
		 * Add a context to a fieldmanager
		 * @param string $title Currently both menu and page titles; should add an option later.
		 * @param string|string[] $taxonomies
		 * @param string $parent
		 * @param Fieldmanager_Field $fm
		 */
		public function __construct( $title, $taxonomies, $parent = '', $fm = null ) {
			// Populate the list of taxonomies for which to add this meta box with the given settings
			if ( ! is_array( $taxonomies ) ) {
				$taxonomies = array( $taxonomies );
			}

			// Set the class variables
			$this->title = $title;
			$this->taxonomies = $taxonomies;
			$this->show_on_add = false;
			$this->show_on_edit = false;
			$this->parent = $parent;
			$this->fm = $fm;
			
			// If this should use FM Term meta, uncomment the next line. Otherwise, this will
			// use core's term meta.
			// $this->use_fm_meta = true;

			// Iterate through the taxonomies and add the fields to the requested forms
			// Also add handlers for saving the fields and which forms to validate (if enabled)
			foreach ( $taxonomies as $taxonomy ) {
				add_action( "{$taxonomy}_edit_form_fields", array( $this, 'show_term_subpage_link' ), 10, 2 );
				add_filter( "{$taxonomy}_row_actions", array( $this, 'show_term_row_link' ), 10, 2 );

				// Also handle removing data when a term is deleted
				add_action( 'delete_term', array( $this, 'delete_term_fields'), 10, 4 );

			}

			add_action( 'admin_menu', array( $this, 'register_submenu_page' ) );

			add_action( 'admin_init', array( $this, 'handle_submenu_save' ) );
		}

		/**
		 * Register the taxonomy submenu pages with WordPress.
		 */
		public function register_submenu_page() {
			foreach ( $this->taxonomies as $taxonomy ) {
				$tax_obj = get_taxonomy( $taxonomy );
				add_submenu_page( 'edit-tags.php', $this->title, $this->title, $tax_obj->cap->manage_terms, $this->get_menu_page_slug( $taxonomy ), array( $this, 'render_submenu_page' ) );
			}
		}

		/**
		 * Get the submenu page slug for a taxonomy for this context.
		 *
		 * @param string $taxonomy
		 * @return string
		 */
		public function get_menu_page_slug( $taxonomy ) {
			return "{$taxonomy}_submenu_{$this->fm->name}";
		}

		/**
		 * Get the URL to edit the fields for a term.
		 *
		 * @param int $term_id
		 * @param string $taxonomy
		 * @return string
		 */
		public function url( $term_id, $taxonomy ) {
			return admin_url( "edit-tags.php?page={$this->get_menu_page_slug( $taxonomy )}&term_id={$term_id}&taxonomy={$taxonomy}" );
		}

		/**
		 * Output the markup for the term submenu form.
		 */
		public function render_submenu_page() {
			if ( empty( $_GET['term_id'] ) || empty( $_GET['taxonomy'] ) || empty( $_GET['page'] ) ) {
				return;
			}

			$msg = empty( $_GET['msg'] ) ? null : sanitize_text_field( $_GET['msg'] );
			$term_id = absint( $_GET['term_id'] );
			$taxonomy = sanitize_title( $_GET['taxonomy'] );
			$term = get_term( $term_id, $taxonomy );
			?>
			<div class="wrap">
				<?php if ( 'success' === $msg ) : ?>
					<div class="updated success"><p><?php esc_html_e( 'Options updated', 'fieldmanager' ); ?></p></div>
				<?php endif ?>

				<h2><?php echo esc_html( $this->title ) ?>: <?php echo esc_html( $term->name ); ?></h2>

				<p>
					<a href="<?php echo esc_url( get_edit_term_link( $term_id, $taxonomy ) ); ?>">
						<?php echo esc_html( sprintf( __( 'Edit %s', 'fieldmanager' ), $term->name ) ); ?>
					</a>
				</p>

				<form method="POST" id="<?php echo esc_attr( $this->uniqid ) ?>">
					<input type="hidden" name="fm-term-subpage" value="<?php echo sanitize_title( $this->get_menu_page_slug( $taxonomy ) ) ?>" />
					<input type="hidden" name="term_id" value="<?php echo absint( $term_id ); ?>" />
					<input type="hidden" name="taxonomy" value="<?php echo esc_attr( $taxonomy ); ?>" />

					<table class="form-table">
						<tbody>
							<?php $this->edit_term_fields( $term, $taxonomy ); ?>
						</tbody>
					</table>

					<?php submit_button( $this->submit_button_label, 'primary', 'fm-submit' ) ?>
				</form>
			</div>
			<?php
		}

		/**
		 * Save a term submenu page's values.
		 */
		public function handle_submenu_save() {
			if ( empty( $_POST['term_id'] ) || empty( $_GET['taxonomy'] ) || empty( $_POST['fm-term-subpage'] ) ) {
				return;
			}

			// Make sure that our nonce field arrived intact.
			if ( ! $this->is_valid_nonce() ) {
				return;
			}

			$term_id = absint( $_POST['term_id'] );
			$taxonomy = sanitize_title( $_POST['taxonomy'] );

			$this->save_term_fields( $term_id, null, $taxonomy );

			// TODO: Other functions will require a bit of refactoring for us to be able to properly conditionally show this message.
			wp_redirect( esc_url_raw( add_query_arg( array( 'msg' => 'success' ), $this->url( $term_id, $taxonomy ) ) ) );
			exit;
		}

		/**
		 * Add the link to the submenu page in the edit term form.
		 *
		 * @param WP_Term $term
		 * @param string $taxonomy
		 */
		public function show_term_subpage_link( $term, $taxonomy ) {
			$label = empty( $this->title ) ? __( 'Fields', 'fieldmanager' ) : $this->title;
			?>
			<tr class="form-field">
				<th scope="row" valign="top"></th>
				<td>
					<a href="<?php echo esc_url( $this->url( $term->term_id, $taxonomy ) ); ?>">
						<?php echo esc_html( sprintf( __( 'Edit %s', 'fieldmanager' ), $label ) ); ?>
					</a>
				</td>
			</tr>
			<?php
		}

		/**
		 * Add the link to the row actions in the list of terms.
		 *
		 * @param array $actions
		 * @param WP_Term $term
		 * @return array
		 */
		public function show_term_row_link( $actions, $term ) {
			$tax_obj = get_taxonomy( $term->taxonomy );
			if ( current_user_can( $tax_obj->cap->manage_terms, $term->term_id ) ) {
				$actions[ "edit_{$this->fm->name}" ] = '<a href="' . esc_url( $this->url( $term->term_id, $term->taxonomy ) ) . '">' . esc_html( sprintf( __( 'Edit %s', 'fieldmanager' ), $this->title ) ) . '</a>';
			}
			return $actions;
		}

		/**
		 * Helper to register an admin bar link for a termpage.
		 *
		 * @param string $label Label to display on the button
		 * @param string|array $taxonomies Taxonomies to display the button on
		 * @param string $fm_name Name field of the real FM instance.
		 */
		public static function register_admin_bar_link( $label, $taxonomies, $fm_name ) {
			if ( ! is_admin() ) {
				// We just need a minimal pseudo-context for the menu buttons.
				// Not very DRY when invoking it, but should be suitable for now.
				add_action( 'init', array(
					new Fieldmanager_Context_Termpage(
						$label,
						$taxonomies,
						'',
						new Fieldmanager_TextField( array( 'name' => $fm_name ) )
					),
					'add_to_admin_bar',
				) );
			}
		}

		/**
		 * Tell this context to add a button to the admin bar. After you
		 * instantiate this class, call this method to "enable" this featured.
		 */
		public function add_to_admin_bar() {
			add_action( 'admin_bar_menu', array( $this, 'admin_bar_button' ), 999 );
			add_action( 'wp_footer', array( $this, 'add_admin_bar_css' ), 999 );
			add_action( 'admin_footer', array( $this, 'add_admin_bar_css' ), 999 );
		}

		/**
		 * Add a link to the termpage if we're looking at a term.
		 *
		 * @param WP_Admin_Bar $wp_admin_bar Admin bar instance.
		 */
		public function admin_bar_button( $wp_admin_bar ) {
			if ( is_tax() || is_tag() || is_category() ) {
				$term = get_queried_object();
			}

			if ( ! isset( $term->term_id ) || ! in_array( $term->taxonomy, $this->taxonomies ) ) {
				return;
			}

			$tax_obj = get_taxonomy( $term->taxonomy );
			if ( current_user_can( $tax_obj->cap->manage_terms, $term->term_id ) ) {
				$wp_admin_bar->add_node( array(
					'id'    => 'termpage-' . sanitize_title( $this->fm->name ),
					'title' => $this->title,
					'href'  => $this->url( $term->term_id, $term->taxonomy ),
				) );
			}
		}

		/**
		 * Add dashicon to termpage link in the Admin Bar.
		 */
		public function add_admin_bar_css() {
			if ( is_user_logged_in() ) :
				?>
				<style>
				#wp-admin-bar-termpage-<?php echo sanitize_title( $this->fm->name ) ?> a:before { font-family: 'dashicons'; content: "\f111" !important; margin-top: 2px; }
				</style>
			<?php
			endif;
		}

	}

}