MetaBox body is not showing (the callback function)

Question

We're creating a plugin where, for example, it shows a metabox on all post types. To kick things off, I'm using the Starter Kit.

<?php

namespace PLUGIN_NAME;

class Foo
{
  public function __construct() {
    add_action( 'admin_init', [ $this, 'wpd_add_meta_box' ] );
  }

function wpd_add_meta_box() {
        $screens = get_post_types();
        foreach ( $screens as $screen ) {
            add_meta_box(
                'global-notice',
                __( 'Global Box 💥', 'global-box' ),
                'global_box_meta_box_callback',
                $screen,
                'advanced',
                'high'
            );
        }
    }

  function global_box_meta_box_callback( $post ) {
    echo "We're alive!"; // Never shows inside the metabox that is rendered.
  }

}

On a post page, I see the metabox but without the callback function.

Moving the code inside the functions.php, using add_action( 'admin_init','wpd_add_meta_box'), everything works fine.

How to render a metabox's content from a plugin within a class? Thanks.

Answer 1

That global_box_meta_box_callback() is a class method (i.e. a function inside a class) and not a globally defined function, so you need to use [ $this, 'global_box_meta_box_callback' ] (or array( $this, 'global_box_meta_box_callback' )) when supplying the class method to add_meta_box():

class Foo
{
    ...

    function wpd_add_meta_box() {
        $screens = get_post_types();
        foreach ( $screens as $screen ) {
            add_meta_box(
                'global-notice',
                __( 'Global Box 💥', 'global-box' ),
                [ $this, 'global_box_meta_box_callback' ], // correct
//              'global_box_meta_box_callback',            // incorrect
                $screen,
                'advanced',
                'high'
            );
        }
    }

    function global_box_meta_box_callback( $post ) {
        // your code here
    }
}

See the PHP manual for more details about using/supplying a callable/callback — in your case, we're using the object method call (see/find // Type 3: Object method call on that manual).