$this->value always return null in Field Plugin

The CMS Questions
1

I am trying to access the field value inside a field-plugin. I thought i could acces it like so:

my code:

<?php

Kirby::plugin('jb/myfield', [
  'fields' => [
    'myfield' => [
      'props' => [],

      'computed' => [
        'testing' => function(){
          return $this->value;
        }
      ],
    ]
  ],
  
]);

But that always returns null. What am i missing?

For context: my ultimate goal would be to disable the field depending on the field value.

(I checked out the native kirby fields and it looks as if this should work: kirby/config/fields/textarea.php at main · getkirby/kirby · GitHub)

I am on Kirby v5.0.0

thanks in advance! <3
joscha