I created a framework to share my data objects between the App and the extension. This includes the data model and the sqlite file with my Core Data db.
I am concerned of what might happen if both App and Extension try to access this shared sqlite db.
What could go wrong if the App is making some changes to the db in the background while the extension is using it?
What's the best practice in this case?
As #CL notes, SQLite is fine with this. But you're not using SQLite directly, so you may need to do some Core Data-level work to maintain consistency. With an iOS app and an extension, you have two separate processes that can make changes to the data. Your code needs to account for this.
If your app extension only displays data (for example, a "today" extension that only displays data created in the app), you probably don't need to do anything special. If your app is running in the background and creating new data while the extension is visible, it's possible that the extension's data could be slightly out of date. If that's important, you can refresh it. But today extensions typically aren't visible for very long, so it's probably not worthwhile. In this case I'd use NSReadOnlyPersistentStoreOption when setting up the extension's Core Data stack just to be clear about the intention and prevent inadvertent changes.
If your app extension creates new data or modifies existing data, your app needs to be aware of this and respond appropriately. What changes you would make will depend on how exactly the extension handles the shared data. For modified data, your app probably needs to call refreshObject:mergeChanges: on any in-memory managed objects with NO as the second argument. It also probably needs to redo any fetches where the changes might affect a search predicate. That'll ensure you're getting the most recent updates. For new data you would need to re-fetch any data the app is working with to get new additions/deletions. A good time to check would be when the application comes to the foreground (i.e. when UIApplicationWillEnterForegroundNotification is posted).
Related
Background story
I am developing a big iOS app. This app works under specific assumptions. The main of them is that app should work offline with internal storage which is a snapshot of last synchronized state of data saved on server. I decided to use CoreData to handle this storage. Every time app launches I check if WiFi connection is enabled and then try to synchronize storage with server. The synchronization can take about 3 minutes because of size of data.
The synchronization process consists of several stages and in each of them I:
fetch some data from the server (XML)
deserialize it
save it in Core Data
Problem
Synchronization process can be interrupted for several reasons (internet connection, server down, user leaving application, etc). This may cause data to be out-of-sync.
Let's assume that synchronization process has 5 stages and it breaks after third. It results in 3/5 of data being updated in internal storage and the rest being out of sync. I can't allow it because data are strongly connected to each other (business logic).
Goal
I don't know if it is possible but I'm thinking about implementing one solution. On start of synchronization process I would like to create snapshot (some kind of copy) of current state of Core Date and during synchronization process work on it. When synchronization process completes with success then this snapshot could overwrite current CoreData state. When synchronization interrupts then snapshot can be simply aborted. My internal storage will be secured.
Questions
How to create CoreData snapshot?
How to work with CoreData snapshot?
How to overwrite CoreDate state with snapshot?
Thanks in advice for any help. Code examples, if it is possible, will be appreciated.
EDIT 1
The size of data is too big to handle it with multiple CoreData's contexts. During synchronization I am saving current context multiple times to cleanup memory. If I do not do it, the application will crash with memory error.
I think it should be resolved with multiple NSPersistentStoreCoordinators using for example this method: link. Unfortunately, I don't know how to implement this.
You should do exactly what you said. Just create class (lets call it SyncBuffer) with methods "load", "sync" and "save".
The "load" method should read all entities from CoreData and store it in class variables.
The "sync" method should make all the synchronisation using class variables.
Finally the "save" method should save all values from class variables to CoreData - here you can even remove all data from CoreData and save brand new values from SyncBuffer.
A CoreData stack is composed at its core by three components: A context (NSManagedObjectContext) a model (NSManagedObjectModel) and the store coordinator (NSPersistentStoreCoordinator/NSPersistentStore).
What you want is to have two different contexts, that shares the same model but use two different stores. The store itself will be of the same type (i.e. an SQLite db) but use a different source file.
At this page you can see some documentation about the stack:
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/CoreData/InitializingtheCoreDataStack.html#//apple_ref/doc/uid/TP40001075-CH4-SW1
The NSPersistentContainer is a convenience class to initialise the CoreData stack.
Take the example of the initialisation of a NSPersistentContainer from the link: you can have the exact same code to initialise it, twice, but with the only difference that the two NSPersistentContainer use a different .sqlite file: i.e. you can have two properties in your app delegate called managedObjectContextForUI and managedObjectContextForSyncing that loads different .sqlite files. Then in your program you can use the context from one store to get current data to show to the user and you can use the context that use the other store with a different .sqlite if you are doing sync operations. When the sync operations are finally done you can eventually swap the two files and after clearing and reloading the NSPersistentContainer (this might be tricky, because you will want to invalidate and reload all managed objects: you are switching to an entirely new context) you can then show the newly synced data to the user and start syncing again on a new .sqlite file.
The way I understand the problem is that you wish to be able download a large "object graph". It is however so large that it cannot be loaded at once in memory, so you would have to break it in chunks and then merge it locally into to Core data.
If that is the case, I think that's not trivial. I am not sure I can think of direct solution without understanding the object relations and even then it may be really overwhelming.
An overly simplistic solution may be to generate the sqlite file on the backend then download it in chunks; it seems ugly, but it serves to separate the business logic from the sync, i.e. the sqlite file becomes the transport layer. So, I think the essence to the solution would be to find a way to physically represent the data you are syncing in a format that allows for splitting it in chunks and that can afterwards be merged into a sqlite file (if you insist on using Core data).
Please also note that as far as I know Amazon (https://aws.amazon.com/appsync/) and Realm (https://realm.io/blog/introducing-realm-mobile-platform/) provide background sync of you local database, but those are paid services and you would have to be careful not be locked in (should not depend on their libs in your model layer, instead have a translation layer).
I’m trying to create an iPad app with information about architecture (buildings, cities, architects,…) but I don’t want to use internet connection. I would like to include all the data when building it so the user automatically gets everything when he downloads the app.
The user won’t be able to modify this data. The preferences will be stored separately. I have though that more data about, for example, other cities could be added with updates.
How should I do this?
Should I use Core Data?
I would like the data to be localizable and support at least two languages.
If your data has some structure to it (consistent fields, which seems likely) then CoreData can be a very good choice. You can put a .sqlite file in your application bundle as a resource, and have localized versions of it. Keep in mind though that any time your data changes you will have to push an app update, unless you write some code to extract the resource to the local storage, and to download new versions and update the data when necessary.
when dealing with CoreData, I've run into a few problems I'm trying to nip in the bud for future proofing the system out of the gate. The simple fact of the matter is that I've never done anything like this before (work with CoreData that is). While I've managed to figure out how to work with it in the app, I need to know a decent practice to signal an app between versions that default data needs to be refresh on first app launch.
So right now, in my AppDelegate, I setup my managed object context, and I perform a fetch request to see if there are any records at all in a particular table/entity. I only want this to happen on first launch so im not constantly rewriting the contents of the DB every app launch. Anyways, so it goes ahead and uses Object Models to handle inserting of data amongst the entities in question (theres a few)
Now, for this version of the app, it's going into the store without an API (thats a far future thing), but between versions released to the app store, we may have to update specific information within the entities (for example: prices), again I only want this refresh to happen on app launch. Also, the schema MIGHT change, Im not sure if or when, but I'd like to make sure this can accomodate that just in case.
I figured, versioning the coredata "Add Model Version" would do the trick, set the new db version as the active version, but when I launch the app in the simulator, nothing happens which tells me that the data inside is being retained.
Any help towards what it is that I should do to accomodate this would be appreciated. Thank you!
You should find the Core Data Model Versioning and Data Migration guide useful:
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/Introduction.html
You'll also probably find Method for import initial data with coredata useful.
I am creating an app where data needs to be displayed right away from the local datastore. I fire off a background thread when the app starts to determine if iCloud is available.
I looked everywhere and can't find the solution to this: when iCloud becomes available, can I change the "options" on the persistentStore to start using iCloud transactions?
I'm not sure what the proper approach is in this situation. Everything I try causes the application to crash.
Originally I had it so the iCloud checking wasn't in a background thread and the app worked fine, but occasionally timed out.
You have not to know when iCloud becomes available. You just work with data but you don't send them directly to iCloud. iOS does it instead you. So only it knows when and how it should send data.
No, you can't change the options on an NSPersistentStore object once it exists. You can only specify the options when adding the persistent store to the NSPersistentStoreCoordinator. The closest you could get to changing options would be to tear down the entire Core Data stack and start over with different options.
That wouldn't help, though, because:
Even if you have detected that iCloud is available (I'm guessing using NSFileManager, either via its ubiquityIdentityToken or by calling URLForUbiquityContainerIdentifier:), your call to addPersistentStoreWithType:configuration:URL:options:error: might still block for a while. If there's new data available in iCloud, it doesn't start downloading until you add the persistent store, and that method doesn't return until the download process is finished. And sometimes, iCloud just makes that method block for a while for no readily apparent reason.
If you let the user make any changes to the data while using non-iCloud options, those changes will not get automatically sent to the cloud later. Core Data only sends changes to iCloud when the data changes while iCloud is active-- which makes it generate a transaction. You'd have to load and re-save any changes the user made, or those changes would never make it to the cloud.
You have, unfortunately, hit on one of the major stumbling points when using Core Data with iCloud. You can't make the full data store available until Core Data finishes communicating with iCloud-- because your call to add the persistent store doesn't return until then. And you can't do anything to speed up that process. This is just one of the headaches you'll run into if you continue trying to use iCloud with Core Data.
Depending on the nature of your data you might be able to use two data stores, one purely local and one synced via iCloud. You could make the purely local data store available while the iCloud one tries to get its act together well enough to be useful. If you stick with one data store though, you're stuck with the delay.
I created a new App using UIManagedDocument. While on my devices everything is fine, I got a lot of bad ratings, because there are problems on other devices :(
After a lot of reading and testing, I decided to go back to the traditional Core Data stack.
But what is the best way to do this with an app, that is already in the app store?
How can I build this update? What should I take care of?
Thanks,
Stefan
I think you may be better off to determine your issues with UIManagedDocument and resolve them.
However, if you want to go to plain MOC, you only have a few things to worry about. The biggest is that the UIMD stores things in a file package, and depending on your options you may have to worry about change logs.
In the end, if you want a single sqlite file, and you want to reduce the possibility of confusion, you have a class that simply opens your UIManagedDocument, and fetches each object, then replicates it in the single sqlite file for your new MOC.
Now, you should not need a different object model, so you should not have any migration issues.
Then, just delete the file package that holds the UIManagedDocument, and only use your single file sqlite store.
Basically, on startup, you try to open the UIManagedDocument. If it opens, load every object and copy it into the new database. Then delete it.
From then, you should be good to go.
Note, however, that you may now experience some UI delays because all the database IO is happening on the main UI thread. To work around this, you may need to use a separate MOC, and coordinate changes via the normal COreData notification mechanisms. There are tons of documents, examples, and tutorials on that.
EDIT
Thanks for your answer. My problem with these issues is, that I'm not
able to reproduce them. All my Devices are working fine. But I got a
lot mails, about problems like this: - duplicate entries - no data
after stoping and restarting the app - some say, that the app works
fine for some days and stops working(no new data). These are all
strange things, that don't happen on my devices. So for me the best
way is to go back to plain MOC. My DB doesn't hold many user generated
data, all the data is loaded from a webservice, so it's no problem to
delete the data and start of using a new DB. – Urkman
Duplicate entries. That one sounds like the bug related to temporary/permanent IDs. There are lots of posts about that. Here is one: Core Data could not fullfil fault for object after obtainPermanantIDs
Not saving. Sounds like you are not using the right API for UIManagedDocument. You need to make sure to not save the MOC directly, and either use an undo manager or call updateChangeCount: to notify UIManagedDocument that it has dirty data that you want to be saved. Again, lots of posts about that as well. Search for updateChangeCount.
However, you know your app best, and it may just be better and easier to use plain Core Data.
Remember, if you are doing lots of imports from the web, to use a separate MOC, and have your main MOC watch for DidSave notifications to update itself with the newly imported data.
UIManagedDocument is a special kind of document, an UIDocument subclass, that stores its data using Core Data Framework. So it combines the power of document architecture and core data capabilities.
You can read more about document based architecture from Document Based App Programming Guide for iOS and I recommend WWDC2011 Storing Documents in iCloud using iOS5 session video. I also recommend Stanford CS193P: iPad and iPhone App Development (Fall 2011) Lecture 13.
What is created when you call saveToURL:forSaveOperation:completionHandler: is an implementation detail of UIManagedDocument and UIDocument and you should not really worry or depend on it. However in current implementation a folder containing an sqlite database file is being created.
No. All entities will be contained in a single database file also more generally called: a persistent store. It is possible to use more than one persistent store, but those are more advanced use cases and UIManagedDocument currently uses one.
UIManagedDocument's context refers to a NSManagedObjectContext from underlying Core Data Framework. UIManagedDocument actually operates two of those in parallel to spin off IO operations to a background thread. When it comes to the nature of a context itself here's a quote from Core Data Programming Guide:
You can think of a managed object context as an intelligent scratch pad. When you fetch objects from a persistent store, you bring temporary copies onto the scratch pad where they form an object graph (or a collection of object graphs). You can then modify those objects however you like. Unless you actually save those changes, however, the persistent store remains unaltered.
But it really is a good idea to take a look at the lectures and other material I posted above to get a general picture of the technologies used and their potential value to you as a developer in different situations.