A SilverStripe dropdown field that has it's options populated via ajax, based on the value of the field it depends on.
SilverStripe 4
// 1. Create a callable function that returns an array of options for the DependentDropdownField.
// When the value of the field it depends on changes, this function is called passing the
// updated value as the first parameter ($val)
$datesSource = function($val) {
if ($val == 'one') {
// return appropriate options array if the value is one.
}
if ($val == 'two') {
// return appropriate options array if the value is two.
}
};
$fields = FieldList::create(
// 2. Add your first field to your field list,
$fieldOne = DropdownField::create('FieldOne', 'Field One', ['one' => 'One', 'two' => 'Two']),
// 3. Add your DependentDropdownField, setting the source as the callable function
// you created and setting the field it depends on to the appropriate field
DependentDropdownField::create('FieldTwo', 'Field Two', $datesSource)->setDepends($fieldOne)
);