How to use Core Data synchronously? - ios

Question

I'm trying to make an iOS 8 App with Swift and i need to download data from JSON and save it , but i don't understand Core Data mechanism. (I'm coming from Android with ORM Lite and Windows Phone with sqlite-net).
I'm trying to make two tasks, "GetAllNewsTask" returning all News from database , and "UpdateAllNewsTask" downloading JSON and parsing it, save to database and return all News.
The function getEntitiesFromJson transform parsed JSON string to entity object
class func getEntitiesFromJson(json: JSONValue) -> [NewsEntity]?{
var rList : [NewsEntity] = []
var array = json.array
var countItr = array?.count ?? 0
if(array == nil){
return nil
}
if(countItr > 0){
for index in 0...countItr-1{
var news = NewsEntity()
var jsonVal = array?[index]
news.id = jsonVal?["id"].integer ?? 0
........
rList.append(news)
}
}
return rList
}
GetAllNewsTask (newsDao.findAll() currently return an harcoded empty array, i didn't found how to select all NewsEntity synchronously)
class GetAllNewsTask:NSOperation {
var result : Array<News>?
override func main() -> (){
result = executeSync()
}
func executeSync() -> Array<News>? {
let newsDao = NewsDAO()
let entities = newsDao.findAll()
return NewsModel.getVOsFromEntities(entities)
}
UpdateAllNewsTask
class UpdateAllNewsTask:NSOperation {
var result : Array<News>?
override func main() -> (){
result = executeSync()
}
func executeSync() -> Array<News>? {
let response = JsonServices.getAllNews()
var managedObjectContext = (UIApplication.sharedApplication().delegate as AppDelegate).managedObjectContext!
var entityDescription = NSEntityDescription.entityForName("NewsEntity", inManagedObjectContext: managedObjectContext)
var entities = NewsModel.getEntitiesFromJson(response)
//TODO insert new, update existing and remove old
return GetAllNewsTask().executeSync()
}
I'm trying to add or update all NewsEntity and delete old, in Java i used List.removeAll(Collection<T>) but i can't found how to do this in Swift.
I got an exception when i override equals and hashcode in NewsEntity class.
Before continuing, is it the correct way to do this ?
If yes there is any good tutorial which demonstrate how to do this?
If no what is the correct way ?

Answer 1

Typically Core Data transactions should always be performed on the object's Managed Object Context thread. For this reason you will see the performBlock and performBlockAndWait calls in NSManagedObjectContext.
Since you are using the main thread you are technically synchronous assuming you are making those update calls on the main thread. If you are not then I would suggest wrapping your synch call into a performBlockAndWait call.
That being said, you should leverage Apple's Documentation on the subject as they explain how you can implement multithreaded core data. You should always perform your server related updates on a background thread.
If you want to implement a removeAll feature you will need to manually fetch all the objects you want to remove and call context.deleteObject(managedObject). Alternatively if you want something more powerful that should enforce cascade deletion, you can set this in your model editor when you select the relationship. The following Delete Rules are available:
Nullify
Cascade
No Action
Deny
Finally, you might find this post useful in explaining some of the commonly used Core Data stack setups and the various performance of each.
Welcome to iOS and good luck:)
EDIT
As an aside you might find Ray Wenderlich provides some great Core Data Tutorials