An extendable protocol for NSManagedObject subclasses for simple data fetching

!Fetchable.swift

//
//  Fetchable.swift
//  CoreDataGenerics
//
//  Created by Rob Nadin on 29/07/2015.
//  Copyright © 2015 Rob Nadin. All rights reserved.
//

import CoreData

protocol Fetchable {
    typealias FetchableType: NSManagedObject = Self
    
    static var entityName: String { get }
    static func objectsInContext(context: NSManagedObjectContext, predicate: NSPredicate?, sortedBy: String?, ascending: Bool) throws -> [FetchableType]
    static func singleObjectInContext(context: NSManagedObjectContext, predicate: NSPredicate?, sortedBy: String?, ascending: Bool) throws -> FetchableType?
    static func objectCountInContext(context: NSManagedObjectContext, predicate: NSPredicate?) -> Int
    static func fetchRequest(context: NSManagedObjectContext, predicate: NSPredicate?, sortedBy: String?, ascending: Bool) -> NSFetchRequest
}

extension Fetchable where Self: NSManagedObject, FetchableType == Self {
    static var entityName: String {
        let components = NSStringFromClass(FetchableType).componentsSeparatedByString(".")

        return components.last ?? components.first!
    }
    
    static func singleObjectInContext(context: NSManagedObjectContext, predicate: NSPredicate? = nil, sortedBy: String? = nil, ascending: Bool = false) throws -> FetchableType? {
        let results: [FetchableType] = try objectsInContext(context, predicate: predicate, sortedBy: sortedBy, ascending: ascending)
        guard results.count > 0 else {
            return nil
        }
        
        return results.first
    }
    
    static func objectCountInContext(context: NSManagedObjectContext, predicate: NSPredicate? = nil) -> Int {
        let request = fetchRequest(context, predicate: predicate)
        let error: NSErrorPointer = nil
        let count = context.countForFetchRequest(request, error: error)
        guard error != nil else {
            return 0
        }
        
        return count
    }
    
    static func objectsInContext(context: NSManagedObjectContext, predicate: NSPredicate? = nil, sortedBy: String? = nil, ascending: Bool = false) throws -> [FetchableType] {
        let request = fetchRequest(context, predicate: predicate, sortedBy: sortedBy, ascending: ascending)
        
        return try context.executeFetchRequest(request) as! [FetchableType]
    }
    
    static func fetchRequest(context: NSManagedObjectContext, predicate: NSPredicate? = nil, sortedBy: String? = nil, ascending: Bool = false) -> NSFetchRequest {
        let request = NSFetchRequest(entityName: entityName)
        request.predicate = predicate
        if let key = sortedBy {
            request.sortDescriptors = [NSSortDescriptor(key: key, ascending: ascending)]
        }
        
        return request
    }
}

Event.swift

//
//  Event.swift
//  CoreDataGenerics
//
//  Created by Rob Nadin on 29/07/2015.
//  Copyright © 2015 Rob Nadin. All rights reserved.
//

import Foundation
import CoreData

@objc(Event)
class Event: NSManagedObject {

// Insert code here to add functionality to your managed object subclass

}

extension Event: Fetchable {}

GetEvents.swift

do {
    let events = try Event.objectsInContext(managedObjectContext)
    for event in events {
        print(event)
    }
} catch {
    print("Couldn't get Event objects")
}