The $post variable – Did I get the grasp of how the Backend actions get parsed?

Question

I was looking for a way to attach a custom meta box to the edit screen of a page at a specific template.

I found and tried this here and from my actual point of view this can't work that way. Let me explain why I think so: To determine, whether I'm on a screen using a specific template I need reference the global $post. But since this function to create the meta box is hooked into the admin_init the global $post will not yet exist.

So it has to put out an Error message, doesn't it? After those thoughts I tried to tie my meta_box_init function to edit_post, save_post and such. But the codex says, even they need $post to be set. So how could I achieve my page template specific meta box, really? Is there a clean solution?

Answer 1

As I've already wrote in question comments, at the very beginning of creating the new post or page first choose a template and save as draft. Meta box will appear after save if right template is used.

<?php
add_action( 'add_meta_boxes', 'my_add_meta_box' );
function my_add_meta_box() {
    // get template file name
    $template_basename = wp_basename( get_page_template() );
    // check if right template is used
    if('my-template.php' == $template_basename) {
        add_meta_box(
            'my_metabox',
            __('Metabox name'),
            'my_display_metabox',
            'page',
            'normal',
            'high'
        );
    }
}
function my_display_metabox($post) {
    wp_nonce_field( 'my_metabox', 'my_metabox_noncename' );
    // meta box HTML here
}