Patterns for accessing remote data with Core Data?

Question

I am trying to write a Core Data application for the iPhone that uses an external data source. I'm not really using Core Data to persist my objects but rather for the object life-cycle management. I have a pretty good idea on how to use Core Data for local data, but have run into a few issues with remote data. I'll just use Flickr's API as an example.

The first thing is that if I need say, a list of the recent photos, I need to grab them from an external data source. After I've retrieved the list, it seems like I should iterate and create managed objects for each photo. At this point, I can continue in my code and use the standard Core Data API to set up a fetch request and retrieve a subset of photos about, say, dogs.

But what if I then want to continue and retrieve a list of the user's photos? Since there's a possibility that these two data sets might intersect, do I have to perform a fetch request on the existing data, update what's already there, and then insert the new objects?

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In the older pattern, I would simply have separate data structures for each of these data sets and access them appropriately. A recentPhotos set and and a usersPhotos set. But since the general pattern of Core Data seems to be to use one managed object context, it seems (I could be wrong) that I have to merge my data with the main pool of data. But that seems like a lot of overhead just to grab a list of photos. Should I create a separate managed object context for the different set? Should Core Data even be used here?

I think that what I find appealing about Core Data is that before (for a web service) I would make a request for certain data and either filter it in the request or filter it in code and produce a list I would use. With Core Data, I can just get list of objects, add them to my pool (updating old objects as necessary), and then query against it. One problem, I can see with this approach, however, is that if objects are externally deleted, I can't know, since I'm keeping my old data.

Am I way off base here? Are there any patterns people follow for dealing with remote data and Core Data? :) I've found a few posts of people saying they've done it, and that it works for them, but little in the way of examples. Thanks.

Answer 1

For a situation like this you could use Cocoa's archiving facilities to save the photo objects (and an index) to disk between sessions, and just overwrite it all every time the app calls home to Flickr.

But since you're already using Core Data, and like the features it provides, why not modify your data model to include a "source" or "callType" attribute? At the moment you're implicitly creating a bunch of objects with source "Flickr API", but you can just as easily treat the different API calls as unique sources and then store that explicitly.

To handle deletion, the simplest way would be to clear the data store each time it's refreshed. Otherwise you'd need to iterate over everything and only delete the photo objects with filenames that weren't included in the new results.

I'm planning to do something similar to this myself so I hope this helps.

PS: If you're not storing the photo objects between sessions at all, you could just use two different contexts and query them separately. As long as they're never saved, and the central store doesn't have anything in it already, it would work just like you describe.

Answer 2

It seems to me that your first instincts are right: you should use fetchrequests to update your existing store. The approach I used for an importer was the following: get a list of all the files that are eligible for importing and store it somewhere. I'm assuming here that getting that list is fast and lightweight (just a name and an url or unique id), but that really importing something will take a bit more time and effort and the user may quit the program or want to do something else before all the importing is done.

Then, on a separate background thread (this is not as hard as it sounds thanks to NSRunLoop and NSTimer, google on "Core Data: Efficiently Importing Data"), get the first item of that list, get the object from Flickr or wherever and search for it in the Core Data database (carefully read Apple's Predicate Programming Guide on setting up efficient, cached NSFetchRequests). If the remote object already lives in Core Data, update the information as necessary, if not insert. When that is done, remove the item from the to-be-imported list and move on to the next one.

As for the problem of objects that have been deleted in the remote store, there are two solutions: periodic syncing or lazy, on-demand syncing. Does importing a photo from Flickr mean importing the original thing and all its metadata (I don't know what the policy is regarding ownership etc) or do you just want to import a thumbnail and some info? If you store everything locally, you could just run a check every few days or weeks to see if everything in your local store is present remotely as well: if not, the user may decide to keep the photo anyway or delete it. If you only store thumbnails or previews, then you will need to connect to Flickr each time the user wants to see the full picture. If it has been deleted, you can then inform the user and delete it locally as well, or mark it as not being accessible any more.

Answer 3

You might try a combination of two things. This strategy will give you an interface where you get the results of a NSFetchRequest twice: Once synchronously, and once again when data has been loaded from the network.

1. Create your own subclass of NSFetchRequest that takes an additional block property to execute when the fetch is finished. This is for your asynchronous request to the network. Let's call it FLRFetchRequest

2. Create a class to which you pass this request. Let's call it FLRPhotoManager. FLRPhotoManager has a method executeFetchRequest: which takes an instance of the FLRFetchRequest and...

   1. Queues your network request based on the fetch request and passes along the retained fetch request to be processed again when the network request is finished.
   2. Executes the fetch request against your CoreData cache and immediately returns the results.
   3. Now when the network request finishes, update your core data cache with the network data, run the fetch request again against the cache, and this time, pull the block from the FLRFetchRequest and pass the results of this fetch request into the block, completing the second phase.

This is the best pattern I have come up with, but like you, I'm interested in other's opinions.