R – Producer Consumer Issue with Core Data

cocoacocoa-touchcore-dataiphone

I've a Core Data application. In the producer thread, I pull data from a web service and store it in my object and call save.
My consumer object is a table view controller that displays the same.
However, the app crashes and I get
NSFetchedResultsController Error: expected to find object (entity: FeedEntry; id: 0xf46f40 ; data: ) in section (null) for deletion

on the console. When I debug it, everything works fine. So I understood that it's like a race issue.

How is these kind of problem solved? What's the best way to design a producer-consumer app with core-data?

Best Answer

If you are targeting Leopard or later, Apple has made things a touch easier.

In your producer thread create a MOC with the same PSC as the MOC in your main thread. You can pull objects from your webservice in this thread, create the new objects, and save them as normal.

In your consumer thread, add your controller as an observer for the NSManagedObjectContextDidSaveNotification. Your callback should look something like:

- (void) managedObjectContextDidSave:(NSNotification *)notification
{
  NSManagedObjectContext *managedObjectContext = [notification object];
  if(managedObjectContext != self.managedObjectContext)
    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
}

In this way, objects saved in the producer thread will be automatically pulled in your consumer thread.

Compare method

Either you implement a compare-method for your object:

- (NSComparisonResult)compare:(Person *)otherObject {
    return [self.birthDate compare:otherObject.birthDate];
}

NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];

NSSortDescriptor (better)

or usually even better:

NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
                                           ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];

You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.

Blocks (shiny!)

There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:

NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
    return [a.birthDate compare:b.birthDate];
}];

Performance

The -compare: and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor as the latter relies on KVC. The primary advantage of the NSSortDescriptor method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView by clicking on the header row.

R – mach_msg_trap in core-data

mach_msg_trap is a function, not an error. Specifically, it's the function that the run loop calls to wait for something to arrive. That function is how your application can wait for something to happen (I/O completes, timer fires, something posts a notification, etc.) without using CPU time.

But I want the producer thead to check new data and if any, i want it to update my managedObjectContext.

I'm not a Core Data guy, so I don't know the specific solution, but I do know that you never send a message to another thread's MOC. You are right to have separate MOCs, and you have them for a reason.

I suggest you edit your question to clarify that you are asking how to signal one Managed Object Context that another Managed Object Context has changed the data store that both of them share. Your question really has nothing to do with mach_msg_trap.

Best Answer

Related Solutions

Ios – How to sort an NSMutableArray with custom objects in it

Compare method

NSSortDescriptor (better)

Blocks (shiny!)

Performance

R – mach_msg_trap in core-data

Related Topic