Add a note about process instantiation

[markjaquith]

Instantiation should happen unconditionally. If you only instantiate on-demand when you're pushing to the queue, it won't work.

I was confused about this. I had this code:

public function add_items( array $items = [] ) {
	$job = new MyJob();

	foreach ( $items as $item ) {
		$job->push_to_queue( $item );
	};

	$job->save()->dispatch();
}

This did not work, because on requests where I was not calling add_items(), MyJob() wasn't being instantiated.

[acafourek]

Took me awhile to discern what this meant, and while it's obvious in retrospect, I wonder if we could phrase this guidance more explicitly? My first crack:

---

The core WP_Background_Process class also processes the queue of tasks to be run and to do so, it needs to be instantiated on every page load. If you only load the class when you're adding things to the queue, that core class wont ever be loaded to actually do anything with the queue. So, make sure that you're doing something like this:

add_action('init', function(){
    new myQueueJob();
});

You'll still need to instantiate later when adding to the queue, like this:

     $background = new myQueueJob();
     $background->push_to_queue($item )->save()->dispatch();

This way, the core class can add items to the queue and work with that queue every time WP gets loaded.

[markjaquith]

Perhaps a bigger issue is that constructor side effects aren't a good pattern. This issue would be more obvious if you had to call $job->work(); or $job->listen();. It wasn't obvious to me that new-ing up an object would do anything except provide me that object.