Automatic calculate when textField changes

Hello,
I am new to XCode 4.3.2 and IOS 5.1. (programming for the iPhone).
I am wondering if there is a way to execute a calculation when (or as a textField changes). I am finding it difficult to find an example like this.

As a basic example I would like to have a textField1 a textField2 and textField3.
When the numbers in textField1 or textField2 changes it would add the two numbers together and put the total in textField3.

Is it possible to automatically perform this calculation or do you have to wait until the 'Return' or 'Tab' key is pressed and you leave the field?
Thank you.