add method
Adds a field under name and registers listeners.
Implementation
void add(String name, ReactiveField<dynamic> field) {
if (_fields.containsKey(name)) {
throw ArgumentError('A field named "$name" already exists.');
}
_fields[name] = field;
_order.add(field);
field.value.addListener(_refresh);
field.error.addListener(_refresh);
_refresh();
}