What is it and from where it is loaded?

Question

I am new at wordpress. I found a hook in theme back-combat.php file which is prevents to load previous version of wordpress for current theme customization. such as

add_action('load-customize.php', 'my_function_name');

My question is what is this hook and from where it is loaded because load-customize.php

and which way I can learn quick about wordpress core and learning plugin development?

Answer 1

This is from a dynamic hook that's generated in the wp-admin/admin.php file and fires right after the admin_init action.

Here's the inline documentation for the core screens part:

/**
 * Fires before a particular screen is loaded.
 *
 * The load-* hook fires in a number of contexts. This hook is for core screens.
 *
 * The dynamic portion of the hook name, `$pagenow`, is a global variable
 * referring to the filename of the current page, such as 'admin.php',
 * 'post-new.php' etc. A complete hook for the latter would be
 * 'load-post-new.php'.
 *
 * @since 2.1.0
 */
 do_action( "load-{$pagenow}" );

When we visit /wp-admin/customize.php the generated action is load-customize.php.

We can then e.g. check if it has already been fired with:

did_action( 'load-customize.php' );

that returns the number of times it has fired so far, during the page load.

Check the code reference for more info on did_action().

Another example is the wp-admin/edit.php and the corresponding load-edit.php action.