Check whether the validity of the entire form has changed since it was last checked, and
if so fire the {@link #validitychange validitychange} event. This is automatically invoked
when an individual field's validity changes.
function() {
        var me = this,
            valid = !me.hasInvalidField();
        if (valid !== me.wasValid) {
            me.onValidityChange(valid);
            me.fireEvent('validitychange', me, valid);
            me.wasValid = valid;
        }
    }