Return the values of each field on the form.
The returned values depends on the
fieldsome fields the values is a string whilst others its a list of string.
Consult the specific
fieldto get the returned value.
a map, where keys a field identifier used when adding field to form and values are the fields respective values.
This is the right place to perform form wide validations. That is validating fields against each other, also validate against parent form fields.
At this point you have access to the getValues() of both parent form and current form you can use this values to compare against.
The recommend approach is to a create check that implements FormAwareInterface and add it to the validator.
This method is invoked before the field specific validations have been run.