DynamicCounter: Use FetchedResultsController to keep a dynamic count, the KVO

DynamicCounter.swift

//
//  DynamicCounter.swift
//
//  Created by deeje cooley on 11/11/18.
//  Copyright © 2018 deeje LLC.  All rights reserved.
//

import CoreData

public class DynamicCounter: NSObject {
    
    @objc dynamic public var count: Int = 0
    
    fileprivate let moc: NSManagedObjectContext
    fileprivate let frc: NSFetchedResultsController<NSFetchRequestResult>
    
    init(container: NSPersistentContainer, entityName: String, predicate: NSPredicate? = nil) {
        moc = container.newBackgroundContext()
        moc.automaticallyMergesChangesFromParent = true
        
        let request = NSFetchRequest<NSFetchRequestResult>(entityName: entityName)
        request.predicate = predicate ?? NSPredicate(value: true)
        frc = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc, sectionNameKeyPath: nil, cacheName: nil)
        
        super.init()
        
        frc.delegate = self
        try? frc.performFetch()
        count = frc.fetchedObjects?.count ?? 0
    }
    
}

extension DynamicCounter: NSFetchedResultsControllerDelegate {
    
    public func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        count = frc.fetchedObjects?.count ?? 0
    }
    
}