Check and load class of theme from a plugin

Question

I'm in the process of breaking down a theme to a bunch of plugins, I wanna do it one by one so hell won't break loose.

Question is, Suppose I have a plugin which depends on a class from the theme, where and how should I validate his existence and load it, and if not exist how do I properly show an error and avoid loading the plugin, is it on the activation hook? I'm pretty new to plugins, and would love to get best practices, or any good docs references regarding my issue, thanks!

I don't need to check another plugins existence, just another code that is currently part of the theme.

Answer 1

I'd recommend to not use methods from your theme in the plugins. Only the other way around. Let your plugins provide methods to be used in the theme. Then you could simply use is_plugin_active().

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Nonetheless you could use PHP's class_exists() to check for the existence of a class. Another common practice seems to be to let your plugin die; at the very top before anything else can be loaded.

<?php
/*
Plugin Name: My Plugin
Description: Lorem ipsum dolor sit amet.
Version: 1.0
Author: You
Author URI: https://example.com
*/

if ( ! class_exists('MyThemeClass') ) {
  die;
}

class MyPlugin() {
  // ...
}

new MyPlugin;