CoreDataStack.swift

import CoreData

public final class CoreDataStack {
    public let persistentContainer: NSPersistentContainer
    
    public init(_ persistentContainer: NSPersistentContainer) {
        self.persistentContainer = persistentContainer
    }
    
    public private(set) lazy var mainContext: NSManagedObjectContext = {
        persistentContainer.viewContext.automaticallyMergesChangesFromParent = true
        return persistentContainer.viewContext
    }()
    
    public private(set) lazy var writingContext = BackgroundManagedObjectContext(persistentContainer) {
        $0.automaticallyMergesChangesFromParent = false
    }
    
    public private(set) lazy var readingContext = BackgroundManagedObjectContext(persistentContainer) {
        $0.automaticallyMergesChangesFromParent = true
    }
}


public final class BackgroundManagedObjectContext {
    var context: NSManagedObjectContext?
    
    public init(_ container: NSPersistentContainer, _ setup: (NSManagedObjectContext)->Void) {
        self.context = container.newBackgroundContext() ~ setup
    }
    
    deinit {
        context.map {
            context = nil
            $0.performAndWait {}
        }
    }
}

public extension BackgroundManagedObjectContext {
    func performTask(_ block: @escaping (NSManagedObjectContext) -> Void) {
        context?.perform { [weak self] in
            self?.context.map {
                block($0)
            }
        }
    }
}