How to use CoreData with multiple threads and ManagedObjectContexts
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I'm working on an application that is going to have a data store, a user
interface that reads from the data store and a worker thread that
updates the data store by making network calls via an IMAP library.
Right now, my application has an internal set of objects which bindings
are attached to. However, I'd like to move this to a CoreData store to
make persistence easier, and I'm not clear on how to work with CoreData
and multiple threads.
I read that it's suggested that each thread have it's own
NSManagedObjectContext. So if each of my threads has their own
NSManagedObjectContext, and I update one ManagedObjectContext, will the
ManagedObjectContext in the other thread be magically updated as well?
To make things more clear, if my worker thread updates it's
NSManagedObjectContext will the NSManagedObjectContext which the
bindings are attached to change as well? I'm guessing no, since that
seems too easy?
Or should I share the same NSManagedObjectContext and do a trylock on it
before either thread accesses it?
Thanks alot, and I apologize if this is in the docs someplace.
--
Matt Ronge
www.theronge.com -
On Nov 26, 2005, at 10:45 PM, Matt Ronge wrote:
> I read that it's suggested that each thread have it's own
> NSManagedObjectContext. So if each of my threads has their own
> NSManagedObjectContext, and I update one ManagedObjectContext, will
> the ManagedObjectContext in the other thread be magically updated
> as well? To make things more clear, if my worker thread updates
> it's NSManagedObjectContext will the NSManagedObjectContext which
> the bindings are attached to change as well? I'm guessing no, since
> that seems too easy?
It's not because it would be too easy; it's because that such change-
propagation is only sometimes what you want, and can have serious
performance and data-consistency implications. (Imagine changing an
object in one context and attempting to propagate the change to a
couple other contexts where you're getting ready to change the
object, or where you have just changed it and are in the process of
saving it.) Managed object contexts are relatively independent
scratchpads for your data
See /Developer/Examples/CoreData/BackgroundFetching for an example of
how to work with multiple contexts in multiple threads, and how to do
change propagation yourself. The short summary is to use an SQLite
persistent store, save the context where you make the changes, send
the object IDs for changed objects to your other thread, and have the
other thread refresh its objects with those IDs.
> Or should I share the same NSManagedObjectContext and do a trylock
> on it before either thread accesses it?
This won't do what you want or expect. More specifically, this
general pattern -- "I need to access a resource in multiple threads,
so I'll just put a lock around it" -- will only work if you control
*every* point of access to that resource and can make them all use
your lock. As you say in your post, you have interface elements
bound to these objects. This means that there is a point of access
to the object that is outside your control; Cocoa bindings aren't
thread-safe, and there are no hooks that you can use to make them
lock on your context.
Your best bet is to follow the BackgroundFetching example.
-- Chris -
> See /Developer/Examples/CoreData/BackgroundFetching for an example of
> how to work with multiple contexts in multiple threads, and how to do
> change propagation yourself. The short summary is to use an SQLite
> persistent store, save the context where you make the changes, send the
> object IDs for changed objects to your other thread, and have the other
> thread refresh its objects with those IDs.
I just looked in my developer/examples folder and I am unable to find
the BackgroundFetching sample code. I also did a google search and I
searched Apples dev site as well. Am I missing something? Does it have a
different name?
Thanks,
--
Matt Ronge
www.theronge.com -
Hi,
I was just looking at BackgroundFetching myself.
I think you need to download the new XCode 2.2
Cheers,
Peter
On Nov 26, 2005, at 9:19 PM, Matt Ronge wrote:
>> See /Developer/Examples/CoreData/BackgroundFetching for an example
>> of how to work with multiple contexts in multiple threads, and
>> how to do change propagation yourself. The short summary is to
>> use an SQLite persistent store, save the context where you make
>> the changes, send the object IDs for changed objects to your
>> other thread, and have the other thread refresh its objects with
>> those IDs.
>
> I just looked in my developer/examples folder and I am unable to
> find the BackgroundFetching sample code. I also did a google search
> and I searched Apples dev site as well. Am I missing something?
> Does it have a different name?
>
> Thanks,
> --
> Matt Ronge
> www.theronge.com
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-
On Nov 26, 2005, at 11:19 PM, Matt Ronge wrote:
>> See /Developer/Examples/CoreData/BackgroundFetching for an example
>> of how to work with multiple contexts in multiple threads, and
>> how to do change propagation yourself. The short summary is to
>> use an SQLite persistent store, save the context where you make
>> the changes, send the object IDs for changed objects to your
>> other thread, and have the other thread refresh its objects with
>> those IDs.
>
> I just looked in my developer/examples folder and I am unable to
> find the BackgroundFetching sample code. I also did a google search
> and I searched Apples dev site as well. Am I missing something?
> Does it have a different name?
I have Xcode 2.2 installed on my system and...
[G5:~] shawnce% ls /Developer/Examples/CoreData/BackgroundFetching/
BackgroundFetcher.h English.lproj
BackgroundFetcher.m
GenerateBackgroundFetchingData
BackgroundFetching.xcodeproj Info.plist
BackgroundFetchingAppDelegate.h SharedConstants.h
BackgroundFetchingAppDelegate.m SharedConstants.m
BackgroundFetching_DataModel.xcdatamodel main.m
BackgroundFetching_Prefix.pch version.plist
- Shawn
