On Apr 24, 2009, at 6:24 PM, Daniel Child wrote:
I have a Core Data app that imports data via a separate managed
object contexts. Without multithreading, the operation works fine.
But since the import takes over a minute, I want to do the import in
a different thread. The data consists of an archived table of data
with records.
This is my first time attempting to use threads, and the basic
problem I'm having is figuring out the best way to pass the complex
object (the table of records) from one thread to another. Since
there are something like 50,000 records, deep-copying would be ugly.
Indeed. Furthermore, since you're working with Core Data, you may not
actually want those 50K objects passed from one thread to another;
Core Data strongly prefers threads to work in entirely separate
managed object contexts. If you use one NSManagedObjectContext per
thread, but these contexts use the same NSPersistentStoreCoordinator,
Core Data has just the solution for you.
This must be a common issue with multithreading. Is there an elegant
way to get access to deeper layers of a complex object from within
another thread? i.e. a way to make 2) or 3) above work?
In fact, you can probably get away with not passing the objects
"across" to the main thread at all. Since your user interface will be
"fed" from the managed object context you've created for the main
thread — whether you're using bindings or doing your user interface
manually — and you can set that context to use the same coordinator as
your background thread, you can just do appropriate fetches and
relationship traversals against the main thread's context to build up
the object graph you're actually presenting to the user.
If you do want to still pass information from the background thread to
the main thread, all you need to pass are the managed object IDs of
the (saved) objects the main thread will be interested in. You can
just use -[NSObject
performSelectorOnMainThread:withObject:waitUntilDone:] for this and
pass either individual object IDs or collections of them. Then on the
main thread, you can ask the main thread's managed object context for
the managed object with that ID, and it'll hand you back an
appropriate one that you can use for your user interface.
-- Chris
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