Update FieldGroup after change in some of the fieldGroup fields

How can I update the fieldGroup after modifying some fields in the fieldGroup.
Changes is added after commit. But I want to see the change in the fields before committing.

I suggest that you use ItemPropertyChangeListener.
Try the following code out:

// a datasource that stores item with fields you want to track
entityDs.addItemPropertyChangeListener(e -> {
    // name of a property that was changed
    String property = e.getProperty();

    // do something else

Is it suitable for you?