Question

In many of my UIViewControllers, I update certain controls based on the state of my data. For example, I might have an edit button on a UITableViewController that should only be enabled when there is one or more items. Or perhaps I want to limit the number of items that can be added, and disable the ‘add’ button otherwise.

Every time I add or delete an item (or take any other action that can add/remove items), I have to remember to update any controls that might need enabling/disabling. This is trivial for the most part, but doesn’t feel comfortable – there is a lot of repetition, and I have to remember to add the calls to updateControlEnabled (or whatever) whenever I add new functionality that might affect the data.

And then I noticed NSManagedObjectContextObjectsDidChangeNotification. Reading the docs, it looks like I can receive a notification whenever something changes in my managed object context. This seems ideal, but I have a few questions:

1. Is this an appropriate use of
   NSManagedObjectContextObjectsDidChangeNotification?

2. Should I anticipate any performance impact if a controller
   subscribes to these and parses each one to see if it needs to update
   the UI? I will be checking the userInfo for every change, instead of
   only those that I know I will care about.

3. Where should I subscribe to the notifications? My UIViewController has a
   reference to the context, which helps, but I don’t know where to
   subscribe (loadView? viewDidLoad? init?) such that the view
   controller will always have one and only one subscription.
4. The view controller will continue to receive and process notifications
   when it’s offscreen – enabling and disabling controls as the
   data model is affected from elsewhere. Is this ok?

I guess I’m mostly just wondering if anyone else uses this approach and if so, what their experience is.

Answer 1

Q) Is this an appropriate use of NSManagedObjectContextObjectsDidChangeNotification?

A) Yes – I used it on OSX for a similar purpose.

Q) Should I anticipate any performance impact if a controller subscribes to these and parses each one to see if it needs to update the UI? I will be checking the userInfo for every change, instead of only those that I know I will care about.

A) NO – it will normally be a very small set of objects – ones that were directly changed.

Q) Where should I subscribe to the notifications? My UIViewController has a reference to the context, which helps, but I don’t know where to subscribe (loadView? viewDidLoad? init?) such that the view controller will always have one and only one subscription.

A) Well, you cannot affect the UI til the view shows – so probably viewDidLoad or viewWillAppear. The problem with the later is you may get it a few times depending on push/pops, so maybe I’d do it in viewDidLoad.

Q) The view controller will continue to receive and process notifications when it’s offscreen – enabling and disabling controls as the data model is affected from elsewhere. Is this ok?

A) Sure – when the view reappears all the elements will be setup correctly.

What you want to do is a classical use of that notification. Just check the thread it comes in on – if its not the mainThread then you want to make all your changes in a block posted to the mainThread.