Override kirby user field to select current user by default

user.php

<?php

class UserField extends SelectField {

  public function input() {

    $select = new Brick('select');
    $select->addClass('selectbox');
    $select->attr(array(
      'name'         => $this->name(),
      'id'           => $this->id(),
      'required'     => $this->required(),
      'autocomplete' => $this->autocomplete(),
      'autofocus'    => $this->autofocus(),
      'readonly'     => $this->readonly(),
      'disabled'     => $this->disabled(),
    ));

    $select->append($this->option('null', '', $this->value() == 'null'));

    if($this->readonly()) {
      $select->attr('tabindex', '-1');
    }

    foreach(site()->users() as $user) {
      if ($this->value() == '') {
        $selected = $user->isCurrent();
      } else {
        $selected = $this->value() == $user->username();
      }

      $displayName = ($user->firstName() || $user->lastName())? $user->firstName().' '.$user->lastName() : $user->username();
      $select->append($this->option($user->username(), $displayName, $selected));
    }

    $inner = new Brick('div');
    $inner->addClass('selectbox-wrapper');
    $inner->append($select);

    $wrapper = new Brick('div');
    $wrapper->addClass('input input-with-selectbox');
    $wrapper->append($inner);

    if($this->readonly()) {
      $wrapper->addClass('input-is-readonly');
    } else {
      $wrapper->attr('data-focus', 'true');
    }

    return $wrapper;
  }


  public function validate() {
    return site()->users()->find($this->value()) || $this->value() == 'null';
  }

}