public function __construct(array $attributes = [])
 {
  parent::__construct($attributes);

  $this->setParentColumn('pid');
  $this->setOrderColumn('sort');
  $this->setTitleColumn('name');
 }

 protected function form()
 {
  return Admin::form(AreaModel::class, function (Form $form) {

   $form->text('name', '名称')->rules("required");
   $form->select('city_id','城市')->options(

    DealCityModel::selectOptionsNoRoot()

   )->load('pid', 'district')->loadOne('pid', 'district');//load方法是框架自带的，定义在vendor=>encore=>laravel-admin=>src=>Form=>Field=>Select文件中，loadone是自己写的，在后面会给出代码district为自己定义的方法，pid是根据城市的变化而变化的下拉列表框名称，也就是下面这个。

   $form->select('pid','地区')->options(array(0 =>'请选择地区'));

  });
 }

 public function district(Request $request)
 {
  $cityId = $request->get('q');

  $list = AreaModel::where(['city_id' => $cityId, 'pid' => 0])->get(['id', DB::raw('name as text')]);

  foreach ($list as $key => $value) {

   $arr[] = array("id" => $value->id, "text" => $value->text);

  }

  return $arr;//返回数组到地区的option

 }

 public function loadOne($field, $sourceUrl, $idField = 'id', $textField = 'text')
 {
  if (Str::contains($field, '.')) {
   $field = $this->formatName($field);
   $class = str_replace(['[', ']'], '_', $field);
  } else {
   $class = $field;
  }

  $script = <<<EOT
$(function(){
 var target = $(".$class");
 $.get("$sourceUrl"+$("{$this->getElementClassSelector()}").val(), function (data) {
  target.find("option").remove();
  $(target).select2({
   data: $.map(data, function (d) {
    d.id = d.$idField;
    d.text = d.$textField;
    return d;
   })
  });
 });
});
EOT;

  Admin::script($script);

  return $this;
 }