Ancillary Classes in CI4

There is no CI 4 get_instance like CI 3 it's all OOP now.

Helpers are load by an array anywhere or by the array in the BaseController.

How to call the controller function or model function inside a  particular helper function?

function allcustomers($x)
{
$ci =& get_instance();
$customer_datas=$ci->invoice_model->getallcustomers($x);
echo "<select id='customer' name='customer' class='form-control required' required>";
echo '<option value="">Please Select</option>';
foreach($customer_datas as $listcustomer)
{
echo "<option value='$listcustomer[customer_id]'>$listcustomer[customer_name]</option>";
}
echo "</select>";
}