Changing date not reflected in previous view - ios

Question

I am reading some book and I stumbled this thing.
In my ViewController when a user clicks change date button following
code is called:
- (IBAction)changeDate:(id)sender {
DateViewController *vc = [[DateViewController alloc] init];
[vc setItem: item];
[[self navigationController] pushViewController:vc animated:YES];
}
item is a pointer to a custom class object, which has ivar of type NSDate *;
Now, inside DateViewController when user already picked new date and wants
to navigate to previous view, I have following code:
- (void)viewWillDisappear:(BOOL)animated
{
NSLog(#"%#", [datePicker date]);
item.dateCreated = [datePicker date]; // get selected date
}
This code works and when user goes back from above code change is reflected
in item data structure and user can see new date. However, if I change above code, to following code, it doesn't work anymore, any clues why?
(This does NOT work):
- (IBAction)changeDate:(id)sender {
DateViewController *vc = [[DateViewController alloc] init];
vc.userDate = currentItem.dateCreated;
[[self navigationController] pushViewController:vc animated:YES];
}
DateViewController:
- (void)viewWillDisappear:(BOOL)animated
{
NSLog(#"%#", [datePicker date]);
self.userDate = [datePicker date];
}

Answer 1

In the first case, item is a mutable instance, because you can change the date that it contains. In the second case you are supplying the NSDate itself, which is immutable.
So, in the first case, you pass a reference to item which can be edited and these edits are available later.
But, the second case doesn't edit the original date, it just stores the chosen date into a property on the view controller which is in the middle of being dismissed.
Generally it is better to make the communication clear, so the view controller would pass the chosen date back to the caller by delegation or provide a property (like in your second case) that can be queried once the selection is made. Your first option is effectively hiding the data exchange by sharing the instance item while the second view controller is on display.
Immutable means that the object itself (its contents) can not be changed. It does not prevent any reference to the object from being changed. If we use arrays (where there are mutable an immutable versions) to demonstrate:
NSArray *a = [NSArray array];
NSArray *b = a;
[b editSomething]; // illegal (not a true method name but just an example of something you might want to try)
b = nil; // just nils b, no affect on a at all
And
NSMutableArray *a = [NSArray array];
NSMutableArray *b = a;
[b addObject:#"String"]; // edits a, because a and b are the same object
b = nil; // just nils b, no affect on a at all
The NSArray is like the situation where you just pass the NSDate. The NSMutableArray is like the situation where you pass item (because you can change the contents).