Using has_filter with a class based plugin to check whether the plugin is active or not

Usually I use this method to test whether a plugin is active (usually my own).

This would be in the plugin I’m checking for:

function my_plugin_setup() {
    // setup code here
}
add_action( 'plugins_loaded', 'my_plugin_setup' );

From another file or plugin I would use has_filter on the function which is fired on plugins_loaded:

if ( has_filter( 'plugins_loaded', 'my_plugin_setup' ) ) {
    // plugin is active, do something
}

How would I use this same method above but for a class based plugin? Ie the plugin code looks something like:

class My_Plugin {

    public function __construct() {
        add_action( 'plugins_loaded', array( $this, 'setup' ) );    
    }

    // other stuff below
}

Obviously this doesn’t work:

if ( has_filter( 'plugins_loaded', array( $this, 'My_Plugin' ) ) ) {
    // do something
}

Note: I’d like to use this method instead of using is_plugin_active because the plugin’s folder name might change and hence is_plugin_active would no longer work.

To do literally what you’re wanting here, the easiest way is to store the instance of the plugin class in a global variable at the end of your plugin:

class My_Plugin {
    static $instance;
    function __construct(){} /* or private, with singleton get_instance() method */
    function setup(){ ... }
}
$my_plugin_instance = new My_Plugin();
add_action( 'plugins_loaded', array( $my_plugin_instance, 'setup' ) );

Then you can do:

$has_plugin = (
    isset( $my_plugin_instance ) 
    &&
    has_filter( 'plugins_loaded', array( $my_plugin_instance, 'setup' )
);
if ( $has_plugin ) ...

However, this approach to see if a plugin has been loaded seems overly complicated. Why not just check to see if class_exists( 'My_Plugin' )? If the class does exist, then you know the plugin was loaded. No need to do any checks for if a filter was added.