Declaring an instance of class included in parent theme from child theme functions.php

Question

Please note this is all "working" - the question is about the best practice and to try and work out why I need to include the PHP file which contains the class in two places for it to work correctly.

I'm working on a simple parent theme to speed up my own development cycle - and have got to the point of testing a simple child theme.

I understand that the functions.php file for the child is called up before the parent functions.php - which includes all the PHP functions and a class for adding CPT's.

I would like to build the CPT from the child theme - I want the parent to contain all the reusable and update-able functionality - but not to pre-build added features into each WP install it's used on.

To the code question:

I tried to include the class (which is in a separate PHP file) in the child functions.php and then instantiate an instance - for example:

require_once TEMPLATEPATH."/library/cpt/cpt.php"; // CPT class ##
$cpt_tree = new Custom_Post_Type( "tree" ); // New CPT - "tree" ##

Reloading the page gives an error:

Fatal error: Class 'Custom_Post_Type' not found in C:\xampp\htdocs\site\wordpress\wp-content\themes\child\functions.php on line 14

So, then I included the same PHP file in the parent functions.php file - and it all works - as an experiment I removed the class from the child theme - and it gives the same error.

I know this question is lacking lots of code examples - I'm hoping it will ring a bell with someone - or someone who understands more about using Classes will be able to give me some pointers - seems silly to require this file in two places and I'll like to keep the functions.php file as clean as possible.

Answer 1

Handle all the class loading in your parent theme on a predictable action (point in time) and hook in later in your child theme.

Example:

add_action( 'wp_loaded', 'parent_prefix_load_classes', 10 );

function parent_prefix_load_classes()
{
    $classes = [ 'Extra_Comment_Walker', 'Extra_Nav_Menu_Walker' ];

    foreach ( $classes as $class )
    {
        locate_template( "php/class.$class.php", TRUE, TRUE );
    }
}

Create instances in your child theme make sure your code runs after the classes are loaded:

// Priority 11 to run after the parent theme's loader.
add_action( 'wp_loaded', 'child_prefix_create_objects', 11 );

function child_prefix_create_objects()
{
    $nav_walker = new Extra_Nav_Menu_Walker;
}

Rules of thumb:

• Never load anything just when the file (function.php) is called. Wait for wp_loaded.
• Use the priority argument to control the order of execution.

Some notes:

• Custom post types, taxonomies and shortcodes belong to plugins. They should never be part of a theme, because that would create a lock-in effect for the user. If a theme switch would break the content you did something wrong.
• Do not use require_once in a theme. locate_template() is more flexible. You can overwrite the whole class now in your in child theme if you use the same directory structure.