Android : Validate android form via Text Change Listener vs. Upon Submit form button

To validate information in a form in an android application inputted in EditText I am doing it using the setError method of EditText which is better than showing a dialog after the user hits submit button.

However, I am not sure if it is better to call the validate() method as the user is changing text of each input field or at the submission of the form? What is considered more user-friendly and better usability?

Thanks