2 Answers

• Accepted Answer

  You can replace the left nav button with a custom button and then add Event Listener to it. This works in simulator:

  var win = Titanium.UI.currentWindow;
  var b = Titanium.UI.createButton({title:'Back'});
  win.leftNavButton = b;
  b.addEventListener('click', function()
  {
     alert('I was clicked'); // to confirm its being called
     // do the stuff here
     win.close();
  });
  

• Thank you, great. I've made now a much more complex solution but it works too and might be handy in some other case:

  on the main window:

  Titanium.App.addEventListener('saveForm', function(e){
    if (e.fid == "domagic" && e.value == "true") {
      doMagic();
    }else{
      form[e.fid] = e.value;
      nomagic();
    }
  }
  
  childWin.addEventListener("close", function(e) {
     if (form.foobar != "") {
       Titanium.App.fireEvent("saveForm",{form: "domagic", value: "true"});
    } else {
       Titanium.App.fireEvent("saveForm",{form: "domagic", value: "false"});
    }
  }
  

  and on the child window

  
  someField.addEventListener('change', function(e){ 
      Titanium.App.fireEvent("saveForm",{form: e.source.fid, value: e.source.value});
  });