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 ?? [])
    }
}