When to refresh data in iOS

Question

I'm not sure this question is strictly limited to iOS, as it's more of a general application-design question on data integrity. But at the moment, I'm getting into iOS and have hit a "best practice" wall.

If we use the Twitter app as an example. We have one tab for our timeline and one tab for our profile. So (to keep things very simple) each one is a View Controller, backed by an object (model). I tweet on the timeline VC, then I head on over to my profile tab. Both viewDidLoad methods have already ran, so the data that was loaded to draw the UI is currently stale. The "count" of my tweets is now out of date. In the iOS world, what is the best methods/approaches to keeping the VC model data synced with the backend?

Is it on a time interval? Or network requests in the viewWillAppear method? Is it event driven, ie. when I tweet in one VC, and it's been saved in the backend web service, I notify any VCs that care that there's a new tweet

I'm not 100% sure this question will have an "answer" in the SO sense, but I'm just trying to understand what's the done thing in the iOS world (as someone who comes from the web development world).

Answer 1

Here's a loose sketch of some things our team has done on a networking application that might give you insight for your own architecture.

At the level of the VC (in viewWillAppear) we typically have multiple NSNotificationCenter observers that will call whatever update methods you need to run after a network update call.

Firing off these notifications is some network listener that lives on a background thread. It's job is to wait for responses from the backend server (typically JSON blobs) that contain updates to the data model. I believe ours has a time interval that will periodically phone home and check to see if we have new things to update.

Note you will necessarily have to devise an asynchronous solution for network calls as the passing/receiving of data packs can be unreliable, and take time, and thus should be computed in the background. You'll also need a way to handle data loss and other errors between server and device. And of course in order to see UI updates, you'll need to switch back to the main thread when updating the VC.

Assuming a some change of state (i.e. new message), then it makes the call to get the new data, which then fires off an NSNotification (with a new payload of info), that goes to the observer on the VC.