I would be opening a URL from objective-c in a MAC OS X Application ( safari would suffice).
But, I want to know if the user closes the tab or navigates to other page or submits it. So that I can take a corrective action.
One solution that I could offer involves AppleScripting and/or Apple Events.
Open up Safari in Script Editor and take a look at the dictionary of possibilities. Safari doesn't have a way of sending other applications notifications when things change in relation to its windows, but you can poll* Safari periodically from your app to see what the current URL associated with each window is.
Check out the answers to this question to see how people are grabbing the URL from the frontmost window in various web browsers. And these answers can likely be extended to cover all windows.
* yes, I do realize that polling is a "bad thing" in terms of programming these days. Until Safari can be told to do notifications to other apps, polling is the only way I can think of.