Required result:

A forms::form component becomes invalid as part of front-end validation when some components hold a certain combination of values: in the example, a Text Field will be valid only if another Text Field contains a correct value and if the combination of the values of the fields is valid.

1. Create a form with two Text Fields.
   
2. Set field IDs, for example, to a and b.
3. Define a method on the form that adds error messages to components when they contain invalid values:

   //rules for validation of the fields:
   private void validateGroup(){
     //error message for field a:
     def String error1 := (a.getValue() == "1" and b.getValue()== "1") ? "Values must not equal 1." : null;
     //error message for field b:
     def String error2 := (b.getValue()== "3") ? "b value must not equal 3." : null;
     def String all := joinErrors(error1, error2);
     //setting the errors as custom error messages on a:
     if !all.isBlank() then
        a.setCustomErrorMessage(all);
     else
        a.setCustomErrorMessage(null)
     end
   }
   //concatenate errors from components:
   private String joinErrors(String... errors) {
      def String concatenated := join(errors, "<br>");
      concatenated.isEmpty() ? null : concatenated;
   }

4. Call the method whenever a value is changed on either of the fields.

   //Init on text fields:
   c.setOnChangeListener({  e ->
      validateGroup()
   })