NSManagedObject+Additions.swift

import Foundation
import CoreData

public protocol Managed: class, NSFetchRequestResult {
    static var entityName: String { get }
    static var defaultSortDescriptors: [NSSortDescriptor] { get }
}

public extension Managed {

    public static var defaultSortDescriptors: [NSSortDescriptor] {
        return []
    }

    static var sortedFetchRequest: NSFetchRequest<Self> {
        let fetchRequest = NSFetchRequest<Self>(entityName: entityName)
        fetchRequest.sortDescriptors = defaultSortDescriptors
        return fetchRequest
    }
}

extension NSManagedObjectContext {

    func insert<T: Managed>() -> T {
        let model = NSEntityDescription.insertNewObject(forEntityName: T.entityName, into: self) as! T
        return model
    }
    
    func insertObject<A: NSManagedObject>() -> A where A:Managed {
        guard let obj = NSEntityDescription.insertNewObject(forEntityName: A.entityName, into: self) as? A else {
            fatalError("wrong object type")
        }
        return obj
    }
    
    public func saveOrRollback() -> Bool {
        do {
            try save()
            return true
        }
        catch {
            rollback()
            return false
        }
    }
    
    public func performChanges(block: @escaping () -> Void) {
        perform {
            block()
            _ = self.saveOrRollback()
        }
    }
}