Using Core Data in the background
Use Core Data in both a single-threaded and multithreaded app.
Overview
Core Data works in a multithreaded environment. However, not every object under the Core Data framework is thread safe. To use Core Data in a multithreaded environment, ensure that:
Bind managed object contexts to the thread (queue) that they’re initialization on.
Bind managed objects that you retrieve from a context to the same queue as the context.
Comparing Main Queue and Private Queue Contexts
There are two types of managed object contexts: main queue and private queue. You define the type of context when you initialize it.
A main queue context (as defined by a NSManagedObjectContextConcurrencyType.mainQueueConcurrencyType) is specifically for use with your application interface. Only use it on the main queue of your app.
A private queue context (as defined by a NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType) creates its own queue upon initialization. Only use it on that queue. Because the queue is private and internal to the NSManagedObjectContext instance, you can only access it through the perform(_:) and the performAndWait(_:) methods.
Initializing and configuring a context
Use init(concurrencyType:) to create a new context. For example, to create a private queue context:
// Create a private queue context.
let context = NSManagedObjectContext(.privateQueue)The parameter you pass during initialization determines what type of NSManagedObjectContext you receive.
When you use the NSPersistentContainer, you configure the viewContext property as a main queue (NSManagedObjectContextConcurrencyType.mainQueueConcurrencyType) context, and configure the contexts associated with performBackgroundTask(_:) and newBackgroundContext() as a private queue (NSManagedObjectContextConcurrencyType.privateQueueConcurrencyType).
Avoiding problems
In general, avoid doing data processing on the main queue that’s not user-related. Data processing can be CPU-intensive, and if it’s performed on the main queue, it can result in unresponsiveness in the user interface. If your application processes data, such as importing data into Core Data from JSON, create a private queue context and perform the import on the private context.
Don’t pass managed object instances between queues. Doing so can result in corruption of the data and termination of the app. When it’s necessary to hand off a managed object reference from one queue to another, use NSManagedObjectID instances.
You retrieve the managed object ID of a managed object by calling the objectID accessor on the NSManagedObject instance.