SlapChat Fetched (Results Controller)

So far, we've been implementing viewDidAppear to handle refreshing our data, but the process (fetch, pass, reload) could be a little overkill. Also, what happens if we can't rely on viewDidAppear being called? Fortunately, there's a more sophisticated solution for refreshing our [core]data called NSFetchedResultsController.

To be vague, the controller intelligently feeds data to a tableView, only updating it when there are changes. To be more specific, it fires off delegate methods when the results of it's fetch change. Like most other Core Data things, there's a buncha boilerplate for implementing these delegate methods (which we provide). If you want to read more about how it works, check out the Apple docs.

Instructions

Let's integrate NSFetchedResultsController into good ol' slapChat.

Add Messages Button

Set it up so we can add new messages to our tableView without going to a different view controller, creating a situation where we can't rely on viewDidAppear for refreshing.

1. Add a plus button to your navigation bar.
2. When the plus button is tapped, have it create a new FISMessage.
   • For now, just set it's content to a string version of the current time.
   • Don't forget to save it!
3. Test it: run your app, hit the add button once, then run again to see if your message was added properly.
4. Trash your implementation of viewDidAppear; we won't be needing it anymore :D

Implementing Results Controller

We're going to set up the results controller within our tableView controller for a few reasons. First, you can only set the NSFRC's delegate (which you need) once. Second, Apple warns against changing NSFRC's fetch request or predicates post-initialization, so we want to set them up per view controller. Last but not least, we want our view controllers to be as self-contained as possible.

1. Add an NSFetchedResultsController property to your tableView controller.
2. In viewDidLoad, initialize it using initWithFetchRequest:managedObjectContext:sectionNameKeyPath:cacheName:
   • Be sure to add a predicate to your fetch request before you use it to initialize NSFRC; your app will crash otherwise!
   • you can enter nil for both sectionNameKeyPath: and cacheName:
3. Set your tableView controller as the NSFRC's delegate. Don't forget to conform to the appropriate protocol :)
4. Scroll down in the .m and check out the boilerplate NSFetchedResultsControllerDelegate implementation we included. Read through the methods to see how they will handle changes. Also uncomment them.
5. NSFRC has a fetchedResults property (a prime example of good naming conventions!). Remove your local messages array entirely and replace it in the data source with your NSFRC's array.
6. Finally, call performFetch: on your NSFRC at the end of viewDidLoad.
   • Enter nil for the argument, we'll get to how NSError's work later on.
7. Run it, and be amazed as you tap the add button and messages magically show up in your tableView!

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