InfoPath Dev

I'm learning infopath and designing the form. I have couple of questions -

I would like to know how to display alerts in infopath.

I'm not using data validation and I have some required fields. I would like to check if required field is empty, then disable the submit button. If not, enable the submit button.

I have a textbox. is there a way to update the text in infopath form.

//Ensure validation is only done when inserting or changing a value

//Check the Length of the NewValue of the field

I would really apreciate if someone can help me to resolve these issue. I have been struggling for a week.

Thanks,

-Shraddha

to update textbox in infopath

XPathNavigator xnDoc = null;

xnDoc = this.MainDataSource.CreateNavigator();

xnDoctemp= xnDoc.SelectSingleNode("/my:myFields/my:fieldname", this.NamespaceManager);

xnDoctemp.SetValue("your value");

Thanks you for reply. how do I update original XPathNavigator?

i am not able to get your question (how to update original xpathnavigator).

Can u rephrase it