Core Data & GCD: Passing the correct managed object context to custom NSManagedObjects

Question

I get runtime errors which seem to result from my incorrect implementation of GCD in combination with my custom NSManagedObjects.

Nested in a GCD

Answer 1

First,

"how to pass the correct managed object context down to my custom NSManagedObjects."

We create NSManagedObject with NSManagedObjectContext. not the other way around. So, when you have a NSManagedObject, you can access the NSManagedObjectContext by asking its property: – managedObjectContext as listed in Apple Document

Second,

When working with CoreData, multi-threading could be a little tricky. especially for the beginner. The are all kind of details that you need to take care of.

I highly recommend you checkout the Parent-Child NSManagedContext. then, using MagicRecord.

By using MagicRecord, you can simply Grand Central Dispatch with a Block like this:

[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){

    // here use the `localContext` as your NSManagedContext and do things in the background.
    // it will take care of all the rest.

}];

If you need to pass NSManagedObject into this block, remember to only pass the NSManagedObjectID instead of the proprety.

this is a example.

NSManagedObjectID *coaMapID = CoaMap.objectID;

[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext){
    coaMap *aCoaMap = (coaMap *)[localContext existingObjectWithID:coaMapID error:&error];
    // then use aCoaMap normally.
}];

Hope this helped.