var textfield = Ti.UI.createTextfield();
textfield.addEventListener('android:back', function() {
  // this method is never called, so this event does not run on textfield
});

textfield.addEventListener('focus', function() {
  // this method is called at step 1
});

textfield.addEventListener('blur', function() {
  // this method is not called at step 2 because
  // the back button only hide the keyboard but the focus is not lost
});

// what code should I use to catch event when the keyboard is hidden
// when pressing the back button ?

textField.addEventListener('blur', function(e) {
    var val = e.value; // Get current value when the keyboard closed (the right way) as opposed to textField.value (the wrong way)
    alert("The text is : "+value);
});

var win = Ti.UI.currentWindow;
win.addEventListener('android:back', function(e) {
    // do crazy things
});