viewDidLoad called before prepareForSegue finishes

Question

I was under the impression that viewDidLoad will be called AFTER prepareForSegue finishes. This is even how Hegarty teaches his Stanford class (as recently as Feb 2013).

However, for the first time today, I have noticed that viewDidLoad was called BEFORE prepareForSegue was finished. Therefore, the properties that I was setting in prepareForSegue were not available to the destinationViewController within the destinations viewDidLoad method.

This seems contrary to expected behavior.

UPDATE

I just figured out what was going on. In my destinationViewController I had a custom setter that would reload the tableView each time the "model" was updated:

DestinationViewController     - (void)setManagedObjectsArray:(NSArray *)managedObjectsArray     {         _managedObjectsArray = [managedObjectsArray copy];         [self.tableView reloadData];     } 

It turns out, since the destinationViewController is a subclass of UITableViewController...calling 'self.tableView' forces the view to load. According to Apple's documentation, calling the view property of a view controller can force the view to load. The view of a UITableViewController is the tableView.

http://developer.apple.com/library/ios/#documentation/uikit/reference/UIViewController_Class/Reference/Reference.html

Therefore, in prepareForSegue the following line was forcing the view of the destinationViewController to load:

vc.managedObjectsArray = <custom method that returns an array>; 

To fix the problem, I changed the custom setter of the destinationViewController's model to:

- (void)setManagedObjectsArray:(NSArray *)managedObjectsArray     {         _managedObjectsArray = [managedObjectsArray copy];         if ([self isViewLoaded]) {             [self.tableView reloadData];         }     } 

This will only reload the tableView if the tableView is on screen. Thus not forcing the view to load during prepareForSegue.

If anyone objects to this process, please share your thoughts. Otherwise, I hope this prevents a long sleepless night for someone.

Answer 1

I had run into similar confusions in the past. The general rules of thumb that I learned are:

1. prepareForSegue is called before destination VC's viewDidLoad
2. viewDidLoad is called only after ALL outlets are loaded
3. Do not attempt to reference any destination VC's outlets in source VC's prepareForSegue.

Another lesson learned with regards to prepareForSegue is to avoid redundant processing. For example, if you already segue a tableView cell to a VC via storyboard, you could run into similar race condition if you attempt to process BOTH tableView:didSelectRowAtIndexPath and prepareForSegue. One can avoid such by either utilizing manual segue or forgo any processing at didSelectRowAtIndexPath.

Answer 2

Thought I'd add a little explanation on what happened here:

prepareForSegue is called before the view is displayed

viewDidload is called the first time the view is accessed (view is lazy loaded).

What probably happened is that you accessed the view in prepareForSegue triggering the view loading manually.

Usually the flow is:

1. preformSegue
2. prepareForSegue
3. ViewController is added to the hierarchy
4. viewDidLoad.

But what probably happened in your case is:

1. preformSegue
2. prepareForSegue
3. |— In prepareForSegue you access the view
4. |— view is automatically loaded => viewDidLoad is invoked
5. Return from performSegue
6. ViewController is added to the hierarchy, no viewDidLoad (already called)

So yes usually the view property is not accessed before the ViewController is added to the hierarchy, but if it is, viewDidLoad can be triggered earlier.