How to deal with concurrency on core data - swift

Question

I've been wandering around google, stackoverflow and internet trying to understand how to work with core data and deal with the concurrency.
Consider that we have 2 tables, Events and Rooms.
An Event can have 1+ Rooms.
FunctionA - AddEvent
FunctionB - AddRoom
FunctionC - SearchRoom -> returns RoomEntity or nil
My problem, I keep getting these errors
Error Domain=NSCocoaErrorDomain Code=133020 "Could not merge changes." UserInfo={conflictList=(
"NSMergeConflict (0x10a507160) for NSManagedObject (0x1092f00c0) with objectID '0xd000000000040000 <x-coredata://A34C65BD-F9F0-4CCC-A9FB-1B1F5E48C70E/Rooms/p1>' with oldVersion = 116 and newVersion = 124 and old object snapshot = {\n location = Lisboa;\n name = \"\\U00cdndico LX\";\n} and new cached row = {\n location = Lisboa;\n name = \"\\U00cdndico LX\";\n}"
Notice the information of the Rooms is equal
my approach is the following.
1- I call the webservice once ( it brings a json with data of 3 types of Events ) These 3 all have the same json structure and share the same managedObjectContext passed by parameter
2- I create a managedObject
var managedObjectContext: NSManagedObjectContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
managedObjectContext = persistentContainer.viewContext
managedObjectContext.parent?.mergePolicy = NSMergePolicyType.mergeByPropertyObjectTrumpMergePolicyType
3-
managedObjectContext.perform(
{
do
{
try self.deleteAllEventsFromDb()
FunctionA(eventList, managedObjectContext) -> save
FunctionA(eventList2, managedObjectContext) -> save
FunctionA(eventList3, managedObjectContext) -> save
self.DatabaseDispatchGroup.enter()
try managedObjectContext.save()
self.DatabaseDispatchGroup.leave()
completion(Result.Success(true))
}
catch let error as NSError
{
print("Could not save. \(error), \(error.userInfo)")
completion(Result.Success(false))
}
})
4- For each Event I execute the same FunctionA to create and save the data in database (managedObjectContext.insert(eventEntity)) . This will work over several tables but lets only consider Events and Rooms(FunctionB).
5- FunctionA contains functionB. Function B search for an existing Room(FunctionC->returns entity?) if it doesn't exists(nil), it creates the entity ( should I save here? )
6- If a Room exists, gets the entity and tries to update the data
Not sure if its making any difference but when I save I do these saves I do it between a dispatchGroup
DatabaseDispatchGroup.enter()
try managedObjectContext.save()
DatabaseDispatchGroup.leave()
I was using a static managedObjectContext which was used for all the database requests but now I decided to create a objectContext per function which accesses the database.
I do keep the same persistentContainer and the same DispatchGroup for all requests
private override init() {
persistentContainer = NSPersistentContainer(name: "DataModel")
persistentContainer.loadPersistentStores() { (description, error) in
if let error = error {
fatalError("Failed to load Core Data stack: \(error)")
}
}
}
It seems to me that my problem is that I am Storing the changes in memory, and always doing updates over the initial data meaning that when I execute save() the context the data is not updated for the next operation?
How/when am I suppose to execute the save function?
Thank you

Answer 1

Once context is being saved, global notification is being posted: ContextDidSave notification.
When using multiple contexts (and not using parent-child approach) you should use this notification and either:
Re-fetch/refresh data in case you need to update view or perform some operation on new data set (using either fetch request or refreshObjects: API).
Merge changes to other contexts (remember about thread confinement! do that only on proper context queues). (merge doc)
There are many articles about it, check for instance this tutorial
and documentation