Represents a listener for card input events. Note that events are not one-time events. For instance, a user can "complete" the CVC many times by deleting and re-entering the value.
Called when a potentially valid card number has been completed in the CardNumberEditText. May be called multiple times if the user edits the field.
Called when a potentially valid CVC has been entered. The only verification performed on the number is that it is the correct length. May be called multiple times, if the user edits the CVC.
Called when a expiration date (one that has not yet passed) has been entered. May be called multiple times, if the user edits the date.
Called whenever the field of focus within the widget changes.
Called when a valid postal code has been entered. May be called multiple times, if the user edits the field. If the CardWidget is not collecting US card, any non-empty postal is considered valid.