Combine, Publishers, and Core Data

Create a continuous stream of information from Core Data to your UI

In my previous piece, RxSwift, Observables, and Core Data, I presented a way to get an observable directly out of your Core Data cache. But some are already using Apple's Combine framework, so I prepared exactly the same solution for Combine.

The general idea is to get a stream of Core Data entities in the form of a publisher and send new values each time those Core Data entities for the given FetchRequest are updated, inserted, or deleted.

I will not get into many details on how you can use this stream. More information can be found in my previous piece.

We will need NSFetchRequest and a NSManagedObjectContext to initialize the following publisher:

CDPublisher.swift

Every time Core Data is updated, the CDPublisher sends new values to its subscriber.

I recommend not using the Core Data entity directly but converting it into a domain struct and using that.

Here’s an example for the Customer entity:

I would create the following struct:

struct DCustomer {
  let id: Int
  let name: String
  let phone: String
}

And map the result of the CDPublisher to that:

CDPublisher(request: Customer.fetchRequest(), context: database.viewContext)
.map { $0.map { DCustomer(id: $0.id, name: $0.name, phone $0.phone) } }

What to Do From Here

If you work with SwiftUI, you can update your ViewModel's Published property.

Keep the following in mind here:

• When assigning to a Published property, receive the values in the main thread.
• Do not use .assign(to: \.customers, on: self) because it causes a self-reference to self and your MyViewModel will never be deallocated.
• Use [weak self] in the .sink closure.
• Better solutions exist that could replace this ugly .map().recieve().sink().store(), but that would be a topic for another piece.

As always, happy coding.