iCloud with Core Data and File sync

Question

I'm working on an Application that uses Core Data with iCloud (with the great improvement given by iOS7). This application stores data to describe a task with this information:

• name a NSString
• date a NSDate
• image a NSString which describes a path to the picture

The pictures could be stored in Documents or Library Directory (I have to decide which is the more convenient folder), by the way, in the same folder with a unique name.

Now I'd like to activate iCloud sync for the images too otherwise the experience of the user will be incomplete (I just sync DB data.. no images, a strange/wrong behaviour for an app).

I'm really confused by Apple Documentation. I can't find a way to understand exactly how iCloud data works for this kind of needs. I just want to sync every file of a folder as soon as they will be created. So my questions are:

• Could you share some good resources to learn how to use iCloud for file sync
• Have I to use UIDocument and other iCloud API? or is there something "automatic". Quite a new bye/stupid question, I know :(
• Are there any problem using Core Data and Document based iCloud synch in the same app?

Note: I know that I can sync data just by adding file in the document folder and hoping that users activate document sync... but this is not what I want obviously.

Answer 1

It is pretty straight forward to use both Core Data transaction log synchronisation and file based synchronisation in the same app to achieve what you want to achieve.

So you would set up your Core Data stack to use iCloud options and synchronise data changes via iCloud. At the same time you would store your images in the Apps iCloud container so they get synchronised as well. Just remember you need to use a relative reference to the images in your Core Data fileURL because the full pathname will vary depending on the device the app is running on. So for example you would just store the image filename in Core Data and use a standard directory such as 'iCloudContainer/Documents/Images/' to store them. 'iCloudContainer' being the URL you get by calling the [[NSFileManager defaultManager] URLForUbiquityContainerIdentifier:] API.

I have uploaded sample Core Data apps that use iCloud for transaction log synchronisation (ie synchronising data in Core Data) and that use file synchronisation for storing Core Data backup files in iCloud which can then be accessed by any device. You should be able to use the same code for moving backup files to and from iCloud for your images. Just remember you have to trigger download of files from iCloud before you can use them either by doing a coordinated read or by initialising the download using NSFileManager.

http://ossh.com.au/design-and-technology/software-development/sample-library-style-ios-core-data-app-with-icloud-integration/

http://ossh.com.au/design-and-technology/software-development/sample-library-style-ios-core-data-app-with-icloud-integration/sample-apps-explanations/

Download and run the sample apps and use the built in Backup File Manager to make backup files and to copy them to and from iCloud using different devices. Then just use the same code when storing your image files.

Your App does have to handle things like the user changing iCloud account, logging into or out of iCloud etc. and them move the core data file and image files accordingly.

Answer 2

The only way to have this happen automatically is to create a binary data attribute in your model for the images. If you do this, you will probably want to check the external binary storage allowed option, so the photos end up stored as files and not in the database.

If you would rather store the photos external to your store, you will have to do more work. You will need to migrate the photos into the iCloud container yourself, using the NSFileManager methods, for example. You could also use a class like iCloud Access if you find that easier.

The downside to handling the photos yourself is that you can never be sure that they have all arrived on your device when the Core Data store syncs, so it could be one or more photos are missing, even though there are entries for them in the store. You would have to make sure your app could handle this scenario, perhaps showing a placeholder image until the real photo was accessible.

There are no issues using Core Data and Document syncing in one app. In fact, they are exactly the same under the covers. From iClouds point of view, they are all just files to be transferred.