FetchPublisher

FetchPublisher.swift

//
//  FetchPublisher.swift
//  [REDACTED]
//
//  Created by Jeff Kelley on 6/20/19.
//  Copyright © 2019 Jeff Kelley. All rights reserved.
//

import CoreData
import Combine

@propertyDelegate
class FetchPublisher<PublisherType: Publisher, Entity>: NSObject, NSFetchedResultsControllerDelegate
where PublisherType.Output == Array<Entity>, Entity: NSManagedObject, PublisherType.Failure == Never {
    
    private var controller: NSFetchedResultsController<PublisherType.Output.Element>?
    private let publisher = PassthroughSubject<[Entity], PublisherType.Failure>()
    private let fetchRequest: NSFetchRequest<Entity>
    
    var value: PublisherType { return publisher.eraseToAnyPublisher() as! PublisherType }
    
    init(_ entityType: PublisherType.Output.Element.Type,
         sortDescriptors: [NSSortDescriptor]) {
        fetchRequest = entityType.fetchRequest() as! NSFetchRequest<Entity>
        fetchRequest.sortDescriptors = sortDescriptors
        
        super.init()
    }
    
    func performFetch(_ context: NSManagedObjectContext) {
        controller = NSFetchedResultsController(
            fetchRequest: fetchRequest,
            managedObjectContext: context,
            sectionNameKeyPath: nil,
            cacheName: Entity.entity().name)
        
        controller?.delegate = self
        
        do {
            try controller?.performFetch()
        }
        catch {
            fatalError(error.localizedDescription)
        }
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        publisher.send(controller.fetchedObjects as? PublisherType.Output ?? [])
    }
    
    var values: [Entity] { controller?.fetchedObjects ?? [] }
    
    private var context: NSManagedObjectContext? { controller?.managedObjectContext }
    
    public func delete(at indices: IndexSet) {
        let values = self.values
        indices.forEach { i in
            context?.delete(values[i])
        }
    }
    
}

typealias ManagedPublisher<T: NSManagedObject> = AnyPublisher<[T], Never>
typealias Fetched<T: NSManagedObject> = FetchPublisher<ManagedPublisher<T>, T>