jmfieldman / cadmium Goto Github PK

Hi there. Been seeing how the library feels in a project, and after getting used to the way you've guided things so far (my last bump was on how to use an object created in a transaction in a dispatch to main - ended up having to get the objectID and re-querying for it in the main thread), I've run up against something I can't figure out.

Cd.transact {
    let txObj = Cd.useInCurrentContext(self.fetchedResultsController.objectAtIndexPath(indexPath) as! CdManagedObject)!
    Cd.delete(txObj)
}

I get the exception:
*** Terminating app due to uncaught exception 'Cadmium Exception', reason: 'You cannot modify a managed object on the main thread. Only from inside a transaction.'

It's happening starting in NSManagedObjectContextDidSaveNotification:

dispatch_async(dispatch_get_main_queue()) {
    _mainThreadContext?.mergeChangesFromContextDidSaveNotification(notification) // <-- HERE
}

And then raising in CdManagedObject's willChangeValueForKey(), on the only relationship property I have in this class. e.g.

extension MyObj {
    @NSManaged var text: String
    @NSManaged var relationship: MyOtherObj?
}

#0  0x00000001001a01bc in CdManagedObject.willChangeValueForKey(String) -> () at Cadmium/CdManagedObject.swift:123
#1  0x00000001001a05e8 in @objc CdManagedObject.willChangeValueForKey(String) -> () ()
#2  0x00000001833406c8 in _sharedIMPL_setvfk_core ()
#3  0x000000018336978c in -[NSManagedObject(_NSInternalMethods) _propagateDelete:] ()
#4  0x00000001833b4744 in -[NSManagedObjectContext _mergeChangesFromDidSaveDictionary:usingObjectIDs:] ()
#5  0x00000001833b4c20 in -[NSManagedObjectContext mergeChangesFromContextDidSaveNotification:] ()
#6  0x00000001001a41c0 in static CdManagedObjectContext.(initializeMasterContexts(coordinator : NSPersistentStoreCoordinator?) -> ()).(closure #1).(closure #1) at Cadmium/CdManagedObjectContext.swift:76

Any thoughts? Thanks!

Either need to make an easier way to do this and/or a related example in the README or the example project. And my first solution of re-querying the object by ID is both inefficient and getting away from the design goals of the framework, I think.

The code from your comment in #1 was the first thing I had tried when I ran into it:

Cd.transact {
    let obj = try! Cd.create(MyObj.self)
    obj.prop = 3
    try! Cd.commit()
    dispatch_async(dispatch_get_main_queue()) {
        let myMainThreadObj = Cd.useInCurrentContext(obj)!
        // Do something with object
    }
}

But I run into this inside of useInCurrentContext():

        guard let originalContext = object.managedObjectContext else {
            Cd.raise("You cannot transfer an object without a existing context.  This object may be transient, or it's original context has been destroyed.")
        }

I assume because the temporary context for the transaction is gone and so the object has no MOC.

I got it to work using a synchronous dispatch to main - what do you think about potentially keeping that temporary context for the transaction around in the case of heavy-ish work done in the synchronous code on main? If that doesn't seem bad, then I think that's the best route to go. My first concern was with stuff like a race condition with the restoring of existingContext and existingNoCommit, but I guess those are protected because that thread is blocked until the newWriteContext.preformBlockAndWait is done. This is what I'm using now:

Cd.transact {
    let obj = try! Cd.create(MyObj.self)
    obj.prop = 3
    try! Cd.commit()
    dispatch_sync(dispatch_get_main_queue()) { // <-- SYNCHRONOUS
        let myMainThreadObj = Cd.useInCurrentContext(obj)!
        // Do something with object
    }
}