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Making a Combine publisher from a FetchedResultsController
// 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>()
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 ?? [])

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kielgillard commented Jul 30, 2019


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josephlord commented Jul 30, 2019

Yes. This is probably completely redundant now


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jjatie commented Aug 26, 2019

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

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