icloud core data sync

Question

I am starting out with building a iOS app which will utilize core data and will sync its data with other devices through iCloud. I was thinking of giving a "Sync" button so that the data is not modified at random moments by data from iCloud. The following situation would then occur :-

User A is working on device A and adds two records. User B is currently working on device B and does not press the Sync Button. At night, the user B decides to update the data in device B and presses the Sync Button (I have the following questions regarding this situation) :-

1. Can the method mergeChangesFromContextDidSaveNotification insert those two records after the Sync button is pressed ? (I think it can. However, just want to confirm before proceeding ahead with this model)
2. Is it possible to access the properties of those two objects before inserting them into the database of device B ?

Answer 1

I do not think that iCloud is designed for your use case. iCloud syncs data across devices for the same user ie one that is logged in with the same Apple ID in more than one device. The idea is that when the user opens the app on a different device, he can continue working with the data he created / modified on another device before.

There is quite a bit of latency in the updates, so even if you enable / disable syncing via a "Sync" button the actual delivery time is not guaranteed and can take pretty long in some cases depending on connection speeds, chance, the state of Apple servers, etc.

Instead, you could devise a customised solution based on your own server and data scheme.

Answer 2

Since you're using iCloud with Core Data-- no, you can't do anything like this. When iCloud syncs Core Data changes, it does so in the background. It downloads the changes, saves them to the data store, and then, after it's done it tells you about the change that has already happened. Calling mergeChangesFromContextDidSaveNotification does not save the changes-- they're already saved. What that call does is update your managed object context with new data from the persistent store. You get notified of new insertions after they have already happened. Think of the "did import" notification as if it were a "did save" notification from another thread, telling you about something that's already finished.

Now, if you have already loaded a managed object when you receive the import notification, you could compare its current properties to those from the most recent incoming change update. If you don't already have the object in memory, you can't get its old values, because they've already been overwritten.