Making a Combine publisher from a FetchedResultsController

FetchedResultsControllerPublisher.swift

//
//  FetchedResultsControllerPublisher.swift
//  ListsModel
//
//  Created by Joseph Lord on 09/06/2019.
//  Copyright © 2019 Joseph Lord. All rights reserved.
//

import Foundation
import Combine
import CoreData

/// Create by passing in a FetchedResultsController
/// This will perform the fetch request on the correct queue and publish the resutls on the
/// publishers.
final public class FetchedResultsControllerPublisher<FetchType>
    where FetchType : NSFetchRequestResult {
    
    private let internalFRCP: FetchedResultsControllerPublisherInternal<FetchType>
    
    /// Pass in a configured fetchResults controller and this class will provide a choice of publishers
    /// for you depending on how you like your errors
    public init(fetchedResultsController: NSFetchedResultsController<FetchType>,
                performFetchNotRequired: Bool = false) {
        self.internalFRCP = FetchedResultsControllerPublisherInternal(
            fetchedResultsController: fetchedResultsController,
            performFetch: !performFetchNotRequired)
    }
    
    public lazy var publisherWithErrors: AnyPublisher<[FetchType], Error> = {
        return self.internalFRCP.publisher.eraseToAnyPublisher()
    }()
    public lazy var publisher: AnyPublisher<[FetchType], Never> = {
        return self.internalFRCP.publisher.replaceError(with: []).eraseToAnyPublisher()
    }()
}

final private class FetchedResultsControllerPublisherInternal<FetchType>
    : NSObject, NSFetchedResultsControllerDelegate
    where FetchType : NSFetchRequestResult {
    
    let publisher: PassthroughSubject<[FetchType], Error>
    let fetchedResultsController: NSFetchedResultsController<FetchType>
    init(fetchedResultsController: NSFetchedResultsController<FetchType>,
         performFetch: Bool) {
        self.fetchedResultsController = fetchedResultsController
        publisher = PassthroughSubject<[FetchType], Error>()
        super.init()
        fetchedResultsController.delegate = self
        fetchedResultsController.managedObjectContext.perform {
            do {
                if performFetch {
                    try fetchedResultsController.performFetch()
                }
                self.publisher.send(fetchedResultsController.fetchedObjects ?? [])
            } catch {
                self.publisher.send(completion: .failure(error))
            }
        }
    }
    
    @objc func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        publisher.send(fetchedResultsController.fetchedObjects ?? [])
    }
}

A @FetchRequest property wrapper is now available in iOS 13 beta 5.

Yes. This is probably completely redundant now

It's still nice to process results from an FRC using Combine, especially when not driving views.

Yes @FetchRequest works inside View where is environment managedObjectContex. But If I want to get obsarvable request inside ViewModel (ex. ObservableObject) such publisher is very nice and it also works with old UIKit