NSOutlineView not refreshing when objects added to managed object context from NSOperations

Question

Background

• Cocoa app using core data Two processes - daemon and a main UI
• Daemon constantly writing to a data store
• UI process reads from same data store
• Columns in NSOutlineView in UI bound to an NSTreeController
• NSTreeControllers managedObjectContext is bound to Application with key path of delegate.interpretedMOC
• NSTreeControllers entity is set to TrainingGroup (NSManagedObject subclass is called JGTrainingGroup)

What I want

When the UI is activated, the outline view should update with the latest data inserted by the daemon.

The Problem

Main Thread Approach

I fetch all the entities I'm interested in, then iterate over them, doing refreshObject:mergeChanges:YES. This works OK - the items get refreshed correctly. However, this is all running on the main thread, so the UI locks up for 10-20 seconds whilst it refreshes. Fine, so let's move these refreshes to NSOperations that run in the background instead.

NSOperation Multithreaded Approach

As soon as I move the refreshObject:mergeChanges: call into an NSOperation, the refresh no longer works. When I add logging messages, it's clear that the new objects are loaded in by the NSOperation subclass and refreshed. It seems that no matter what I do, the NSOutlineView won't refresh.

What I've tried

I've messed around with this for 2 days solid and tried everything I can think of.

• Passing objectIDs to the NSOperation to refresh instead of an entity name.
• Resetting the interpretedMOC at various points - after the data refresh and before the outline view reload.
• I'd subclassed NSOutlineView. I discarded my subclass and set the view back to being an instance of NSOutlineView, just in case there was any funny goings on here.
• Added a rearrangeObjects call to the NSTreeController before reloading the NSOutlineView data.
• Made sure I had set the staleness interval to 0 on all managed object contexts I was using.

I've got a feeling this problem is somehow related to caching core data objects in memory. But I've totally exhausted all my ideas on how I get this to work.

I'd be eternally grateful to anyone who can shed any light as to why this might not be working.

Code

Main Thread Approach

// In App Delegate
-(void)applicationDidBecomeActive:(NSNotification *)notification {
    // Delay to allow time for the daemon to save
    [self performSelector:@selector(refreshTrainingEntriesAndGroups) withObject:nil afterDelay:3];
}

-(void)refreshTrainingEntriesAndGroups {
    NSSet *allTrainingGroups    = [[[NSApp delegate] interpretedMOC] fetchAllObjectsForEntityName:kTrainingGroup];
    for(JGTrainingGroup *thisTrainingGroup in allTrainingGroups)
        [interpretedMOC refreshObject:thisTrainingGroup mergeChanges:YES];

    NSError *saveError = nil;
    [interpretedMOC save:&saveError];
    [windowController performSelectorOnMainThread:@selector(refreshTrainingView) withObject:nil waitUntilDone:YES];
}

// In window controller class
-(void)refreshTrainingView {
    [trainingViewTreeController rearrangeObjects]; // Didn't really expect this to have any effect. And it didn't.
    [trainingView reloadData];
}

NSOperation Multithreaded Approach

// In App Delegate (just the changed method)
-(void)refreshTrainingEntriesAndGroups {
    JGRefreshEntityOperation  *trainingGroupRefresh = [[JGRefreshEntityOperation alloc] initWithEntityName:kTrainingGroup];
    NSOperationQueue          *refreshQueue = [[NSOperationQueue alloc] init];
    [refreshQueue setMaxConcurrentOperationCount:1];
    [refreshQueue addOperation:trainingGroupRefresh];

    while ([[refreshQueue operations] count] > 0) {
        [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.05]];

    // At this point if we do a fetch of all training groups, it's got the new objects included. But they don't show up in the outline view.
    [windowController performSelectorOnMainThread:@selector(refreshTrainingView) withObject:nil waitUntilDone:YES];
}

// JGRefreshEntityOperation.m
@implementation JGRefreshEntityOperation

@synthesize started;
@synthesize executing;
@synthesize paused;
@synthesize finished;

-(void)main {
    [self startOperation];

    NSSet *allEntities    = [imoc fetchAllObjectsForEntityName:entityName];
    for(id thisEntity in allEntities)
        [imoc refreshObject:thisEntity mergeChanges:YES];

    [self finishOperation];
}

-(void)startOperation {
    [self willChangeValueForKey:@"isExecuting"];
    [self willChangeValueForKey:@"isStarted"];
    [self setStarted:YES];
    [self setExecuting:YES];
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isStarted"];

    imoc = [[NSManagedObjectContext alloc] init];
    [imoc setStalenessInterval:0];
    [imoc setUndoManager:nil];
    [imoc setPersistentStoreCoordinator:[[NSApp delegate] interpretedPSC]];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(mergeChanges:) 
                                                 name:NSManagedObjectContextDidSaveNotification 
                                               object:imoc];
}

-(void)finishOperation {
    saveError = nil;

    [imoc save:&saveError];
    if (saveError) {
        NSLog(@"Error saving. %@", saveError);
    }

    imoc = nil;

    [self willChangeValueForKey:@"isExecuting"];
    [self willChangeValueForKey:@"isFinished"];
    [self setExecuting:NO];
    [self setFinished:YES];
    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

-(void)mergeChanges:(NSNotification *)notification {
    NSManagedObjectContext *mainContext = [[NSApp delegate] interpretedMOC];
    [mainContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                                  withObject:notification
                               waitUntilDone:YES];  

}

-(id)initWithEntityName:(NSString *)entityName_ {
    [super init];
    [self setStarted:false];
    [self setExecuting:false];
    [self setPaused:false];
    [self setFinished:false];
    [NSThread setThreadPriority:0.0];
    entityName = entityName_;
    return self;
}

@end

// JGRefreshEntityOperation.h
@interface JGRefreshEntityOperation : NSOperation {
    NSString *entityName;
    NSManagedObjectContext  *imoc;
    NSError *saveError;
    BOOL started;
    BOOL executing;
    BOOL paused;
    BOOL finished;
}

@property(readwrite, getter=isStarted) BOOL started;
@property(readwrite, getter=isPaused) BOOL paused;
@property(readwrite, getter=isExecuting) BOOL executing;
@property(readwrite, getter=isFinished) BOOL finished;

-(void)startOperation;

-(void)finishOperation;

-(id)initWithEntityName:(NSString *)entityName_;

-(void)mergeChanges:(NSNotification *)notification;

@end

UPDATE 1

I just found this question. I can't understand how I missed it before I posted mine, but the summary is: Core Data wasn't designed to do what I'm doing. Only one process should be using a data store.

http://stackoverflow.com/questions/2054180/nsmanagedobjectcontext-and-nsarraycontroller-reset-refresh-problem

However, in a different area of my application I have two processes sharing a data store with one having read only access and this seemed to work fine. Plus none of the answers to my last question on this topic mentioned that this wasn't supported in Core Data.

I'm going to re-architect my app so that only one process writes to the data store at any one time. I'm still skeptical that this will solve my problem though. It looks to me more like an NSOutlineView refreshing problem - the objects are created in the context, it's just the outline view doesn't pick them up.

Answer 1

I ended up re-architecting my app. I'm only importing items from one process or the other at once. And it works perfectly. Hurrah!