Use of drupal_static() and drupal_static_reset() is deprecated

drupal_static() and drupal_static_reset() are deprecated and will be removed before Drupal 9.0.0. The reason is that it's not recommended to have any kind of in-memory caches based on the static keyword. It breaks encapsulation, reusability, makes testing hard etc, so don't use that in the first place. An ordinary property on an object can deal with almost the same kind of use-cases.

Before

$foo = &drupal_static::get(__FUNCTION__);
drupal_static_reset(__FUNCTION__);
drupal_static_reset();

After

class Foo {

  protected $bar;

  public function getBar() {
    if (!isset($this->bar) {
      $this->bar = // Do your logic here. 
    }
    return $this->bar;
  }

}