WordPress函数：add_meta_box 将属性框添加到一个或多个屏幕

函数原型：

add_meta_box( string $id, string $title, callable $callback, string|array|WP_Screen $screen = null, string $context = 'advanced', string $priority = 'default', array $callback_args = null )

将属性框添加到一个或多个屏幕

参数说明：

$id 属性框ID。

$title 属性框的标题。

$callback 用于在属性框中填充所需内容。

$screen 属性框的一个或多个屏幕（如帖子类型，“link”或“comment”）。

$context 属性框应显示的屏幕中的上下文。可用的上下文因屏幕而异。编辑后的屏幕上下文包括“normal”、“side”和“advanced”。评论屏幕上下文包括“normal”和“side”。菜单元框（手风琴部分）都使用“side”上下文。全局默认值为“advanced”。

$priority 属性框应显示的上下文中的优先级。接受“high”、“core”、“normal”或“low”。默认值“default”。

$callback_args 应设置为box数组的$args属性的数据（这是传递给回调的第二个参数）。默认值：null

函数源码：

function add_meta_box( $id, $title, $callback, $screen = null, $context = 'advanced', $priority = 'default', $callback_args = null ) {
	global $wp_meta_boxes;

	if ( empty( $screen ) ) {
		$screen = get_current_screen();
	} elseif ( is_string( $screen ) ) {
		$screen = convert_to_screen( $screen );
	} elseif ( is_array( $screen ) ) {
		foreach ( $screen as $single_screen ) {
			add_meta_box( $id, $title, $callback, $single_screen, $context, $priority, $callback_args );
		}
	}

	if ( ! isset( $screen->id ) ) {
		return;
	}

	$page = $screen->id;

	if ( ! isset( $wp_meta_boxes ) ) {
		$wp_meta_boxes = array();
	}
	if ( ! isset( $wp_meta_boxes[ $page ] ) ) {
		$wp_meta_boxes[ $page ] = array();
	}
	if ( ! isset( $wp_meta_boxes[ $page ][ $context ] ) ) {
		$wp_meta_boxes[ $page ][ $context ] = array();
	}

	foreach ( array_keys( $wp_meta_boxes[ $page ] ) as $a_context ) {
		foreach ( array( 'high', 'core', 'default', 'low' ) as $a_priority ) {
			if ( ! isset( $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ] ) ) {
				continue;
			}

			// If a core box was previously removed, don't add.
			if ( ( 'core' === $priority || 'sorted' === $priority )
				&& false === $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ]
			) {
				return;
			}

			// If a core box was previously added by a plugin, don't add.
			if ( 'core' === $priority ) {
				/*
				 * If the box was added with default priority, give it core priority
				 * to maintain sort order.
				 */
				if ( 'default' === $a_priority ) {
					$wp_meta_boxes[ $page ][ $a_context ]['core'][ $id ] = $wp_meta_boxes[ $page ][ $a_context ]['default'][ $id ];
					unset( $wp_meta_boxes[ $page ][ $a_context ]['default'][ $id ] );
				}
				return;
			}

			// If no priority given and ID already present, use existing priority.
			if ( empty( $priority ) ) {
				$priority = $a_priority;
				/*
				 * Else, if we're adding to the sorted priority, we don't know the title
				 * or callback. Grab them from the previously added context/priority.
				 */
			} elseif ( 'sorted' === $priority ) {
				$title         = $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ]['title'];
				$callback      = $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ]['callback'];
				$callback_args = $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ]['args'];
			}

			// An ID can be in only one priority and one context.
			if ( $priority !== $a_priority || $context !== $a_context ) {
				unset( $wp_meta_boxes[ $page ][ $a_context ][ $a_priority ][ $id ] );
			}
		}
	}

	if ( empty( $priority ) ) {
		$priority = 'low';
	}

	if ( ! isset( $wp_meta_boxes[ $page ][ $context ][ $priority ] ) ) {
		$wp_meta_boxes[ $page ][ $context ][ $priority ] = array();
	}

	$wp_meta_boxes[ $page ][ $context ][ $priority ][ $id ] = array(
		'id'       => $id,
		'title'    => $title,
		'callback' => $callback,
		'args'     => $callback_args,
	);
}

使用举例：

/**
 * Calls the class on the post edit screen.
 */
function call_someClass() {
	new someClass();
}

if ( is_admin() ) {
	add_action( 'load-post.php',     'call_someClass' );
	add_action( 'load-post-new.php', 'call_someClass' );
}

/**
 * The Class.
 */
class someClass {

	/**
	 * Hook into the appropriate actions when the class is constructed.
	 */
	public function __construct() {
		add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
		add_action( 'save_post',      array( $this, 'save'         ) );
	}

	/**
	 * Adds the meta box container.
	 */
	public function add_meta_box( $post_type ) {
		// Limit meta box to certain post types.
		$post_types = array( 'post', 'page' );

		if ( in_array( $post_type, $post_types ) ) {
			add_meta_box(
				'some_meta_box_name',
				__( 'Some Meta Box Headline', 'textdomain' ),
				array( $this, 'render_meta_box_content' ),
				$post_type,
				'advanced',
				'high'
			);
		}
	}

	/**
	 * Save the meta when the post is saved.
	 *
	 * @param int $post_id The ID of the post being saved.
	 */
	public function save( $post_id ) {

		/*
		 * We need to verify this came from the our screen and with proper authorization,
		 * because save_post can be triggered at other times.
		 */

		// Check if our nonce is set.
		if ( ! isset( $_POST['myplugin_inner_custom_box_nonce'] ) ) {
			return $post_id;
		}

		$nonce = $_POST['myplugin_inner_custom_box_nonce'];

		// Verify that the nonce is valid.
		if ( ! wp_verify_nonce( $nonce, 'myplugin_inner_custom_box' ) ) {
			return $post_id;
		}

		/*
		 * If this is an autosave, our form has not been submitted,
		 * so we don't want to do anything.
		 */
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
			return $post_id;
		}

		// Check the user's permissions.
		if ( 'page' == $_POST['post_type'] ) {
			if ( ! current_user_can( 'edit_page', $post_id ) ) {
				return $post_id;
			}
		} else {
			if ( ! current_user_can( 'edit_post', $post_id ) ) {
				return $post_id;
			}
		}

		/* OK, it's safe for us to save the data now. */

		// Sanitize the user input.
		$mydata = sanitize_text_field( $_POST['myplugin_new_field'] );

		// Update the meta field.
		update_post_meta( $post_id, '_my_meta_value_key', $mydata );
	}


	/**
	 * Render Meta Box content.
	 *
	 * @param WP_Post $post The post object.
	 */
	public function render_meta_box_content( $post ) {

		// Add an nonce field so we can check for it later.
		wp_nonce_field( 'myplugin_inner_custom_box', 'myplugin_inner_custom_box_nonce' );

		// Use get_post_meta to retrieve an existing value from the database.
		$value = get_post_meta( $post->ID, '_my_meta_value_key', true );

		// Display the form, using the current value.
		?>
		<label for="myplugin_new_field">
			<?php _e( 'Description for this field', 'textdomain' ); ?>
		</label>
		<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="<?php echo esc_attr( $value ); ?>" size="25" />
		<?php
	}
}