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MVVM, SwiftUI, Combine and the question of CoreData

Forums > iOS

I am relatively new to iOS programming and I am having a hard time making peace between how CoreData works and what I know about software architecture.

So I am trying to setup a project around the MVVM pattern with Combine framework and SwfitUI.

My first problem is the @FetchRequest property wrapper. In every SwiftUI sample and tutorial I've seen, the view talks directly to the database. It is not just that I don't like this because I'm so religious about separation of concerns, it is making it hard for me to implement my use case. The local database is a cache for me, the actual source of truth is in the cloud, and the cache needs to keep up with the changes on the cloud. The observable pattern fits in nicely with this scenario; you serve what you have in the cache, and at the same time, you run your business logic to see if you need to fetch a fresh copy of your data, and if it is the case, you load your data and make another serve to the view layer. That will be hard to implement with @FetchRequest to say the least.

The other problem is when you need to load just some of the columns out of your database table, with CoreData that means that not only you'll be arranging a rather complex database fetch operation inside your view, you'll get a dictionary result at the end, which means a whole lot of string literals for keys and type castings for values in your view code.

I would normally just inject a repository into my view-model and, if required, I would return a light version of my entity from the repository with only the required columns on it.

And finally, I find it problematic that all entities are NSManagedObject instances. Besides similar violations of the MVVM pattern, this goes directly in the face of reactive functional programming (which Apple is trying to embrace with the Combine framework) where you're supposed to have a flow of immutable data.

Searching for a solution, I've found workarounds that basically work with NSFetchedResultsControllerDelegate to turn a simple FetchRequest into a Publisher, like this one and this one.

Using that approach and mapping CoreData entities to immutable entities in each repository, I was able to address all the above concerns. As it stands now, I have, for instance, a CDUser type which will be mapped to a User type before it leaves the UserRepository.

At this point, I am starting to doubt whether it is worth all the effort and the overhead that CoreData brings with it, just to keep using CoreData. Why not just ditch CoreData altogether and use something like Realm instead? That looks like a bold move though, considering how integral CoreData is to iOS programming. So I wanted to know the opinion of more experienced iOS programmers.

I know CoreData is a big frameworks and I am standing to lose some of its features here (like storage in iCloud which I don't need and batch loading which I could live without), but apart from those, would you call it a terribly bad idea to ditch CoreData for the reasons that I presented above.

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hi,

i've been going back and forth on several architecture ideas when Core Data is involved in a simple (well, it was intended to be simple) project called ShoppingList15. it has a simple Core Data graph: an Item (as in shopping item, like "Milk") and Location (as in a place in a grocery store where the Item is found, like "Dairy"), with a relationship of one Location to many Items. the app shares data across personal devices using NSPersistentCloudKitContainer, so it has a cloud component to it.

this project currently has 3 branches to it:

  • the master branch is based on @FetchRequests and a global (shared) instance of a PersistentStore that provides other access to Core Data objects.
  • an MVVM branch has a centralized DataManager (not a global) that vends Core Data objects to SwiftUI views, backed by NSFetchedResultsControllers that take the place of @FetchRequests. no SwiftUI views ever do anything directly to a Core Data object, since all useful properties of a Core Data object have been abstracted to read-only properties.
  • an MVVM-2 branch has a DataManager that does not vend Core Data objects to SwiftUI views, but instead, struct representations of those objects. SwiftUI views are now being given true, read-only data, and SwiftUI seems very happy.

the underlying problem in all of this: bridging the gap between Core Data (object-focused) and SwiftUI (struct-focused).

i am hopeful there will be some discussion on this thread. i am no expert ... i just try things out in public and hope for some useful feedback.

hope that helps,

DMG

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Core data work and software architechure have a great relation which I understand when I was developing an iOS app for Dear Lottery Result Today 1pm. Although you will also learn it with experience. But here I would like to give you an advice. Whenever you see any issue so try to search about that issue on youtube and you will get solution.

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