executeFetchRequest 上的“集合在枚举时发生了变异”

Question

我被一个问题困扰了好几个小时，并且在 stackoverflow 上阅读了所有有关此问题的内容（并应用了找到的所有建议），我现在正式需要帮助。 ;o)

这里是上下文：

在我的 iPhone 项目中，我需要在后台导入数据并将其插入到托管对象上下文中。按照此处找到的建议，这就是我正在做的事情：

• 保存主 moc
• 使用主 moc 使用的持久存储协调器实例化后台 moc
• 将我的控制器注册为后台 moc 的 NSManagedObjectContextDidSaveNotification 通知的观察者
• 在后台线程调用导入方法
• 每次收到数据，在后台moc插入
• 所有数据导入后，保存后台moc
• 在主线程上将更改合并到主 moc 中
• 取消将我的控制器注册为通知的观察者
• 重置并释放后台 moc

有时（并且是随机的）例外情况...

*** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x5e0b930> was mutated while being enumerated...

...当我在后台 moc 上调用 executeFetchRequest 时抛出，以检查导入的数据是否已存在于数据库中。我想知道是什么在改变集合，因为在导入方法之外没有任何东西运行。

我已经包含了我的控制器和我的测试实体的全部代码（我的项目由这两个类和未修改的应用程序委托组成）：

//
//  RootViewController.h
//  FK1
//
//  Created by Eric on 09/08/10.
//  Copyright (c) 2010 __MyCompanyName__. All rights reserved.
//


#import <CoreData/CoreData.h>

@interface RootViewController : UITableViewController <NSFetchedResultsControllerDelegate> {
    NSManagedObjectContext *managedObjectContext;
    NSManagedObjectContext *backgroundMOC;
}


@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain) NSManagedObjectContext *backgroundMOC;

@end


//
//  RootViewController.m
//  FK1
//
//  Created by Eric on 09/08/10.
//  Copyright (c) 2010 __MyCompanyName__. All rights reserved.
//


#import "RootViewController.h"
#import "FK1Message.h"

@implementation RootViewController

@synthesize managedObjectContext;
@synthesize backgroundMOC;

- (void)viewDidLoad {
    [super viewDidLoad];

    self.navigationController.toolbarHidden = NO;

    UIBarButtonItem *refreshButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh target:self action:@selector(refreshAction:)];

    self.toolbarItems = [NSArray arrayWithObject:refreshButton];
}

#pragma mark -
#pragma mark ACTIONS

- (void)refreshAction:(id)sender {
    // If there already is an import running, we do nothing

    if (self.backgroundMOC != nil) {
        return;
    }

    // We save the main moc

    NSError *error = nil;

    if (![self.managedObjectContext save:&error]) {
        NSLog(@"error = %@", error);

        abort();
    }

    // We instantiate the background moc

    self.backgroundMOC = [[[NSManagedObjectContext alloc] init] autorelease];

    [self.backgroundMOC setPersistentStoreCoordinator:[self.managedObjectContext persistentStoreCoordinator]];

    // We call the fetch method in the background thread

    [self performSelectorInBackground:@selector(_importData) withObject:nil];
}

- (void)_importData {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgroundMOCDidSave:) name:NSManagedObjectContextDidSaveNotification object:self.backgroundMOC];         

    FK1Message *message = nil;

    NSFetchRequest *fetchRequest = nil;
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"FK1Message" inManagedObjectContext:self.backgroundMOC];
    NSPredicate *predicate = nil;
    NSArray *results = nil;

    // fake import to keep this sample simple

    for (NSInteger index = 0; index < 20; index++) {
        predicate = [NSPredicate predicateWithFormat:@"msgId == %@", [NSString stringWithFormat:@"%d", index]];

        fetchRequest = [[[NSFetchRequest alloc] init] autorelease];

        [fetchRequest setEntity:entity];
        [fetchRequest setPredicate:predicate];

        // The following line sometimes randomly throw the exception :
        // *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x5b71a00> was mutated while being enumerated.

        results = [self.backgroundMOC executeFetchRequest:fetchRequest error:NULL];

        // If the message already exist, we retrieve it from the database
        // If it doesn't, we insert a new message in the database

        if ([results count] > 0) {
            message = [results objectAtIndex:0];
        }
        else {
            message = [NSEntityDescription insertNewObjectForEntityForName:@"FK1Message" inManagedObjectContext:self.backgroundMOC];
            message.msgId = [NSString stringWithFormat:@"%d", index];
        }

        // We update the message

        message.updateDate = [NSDate date];
    }

    // We save the background moc which trigger the backgroundMOCDidSave: method

    [self.backgroundMOC save:NULL];

    [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:self.backgroundMOC];

    [self.backgroundMOC reset]; self.backgroundMOC = nil;

    [pool drain];
}

- (void)backgroundMOCDidSave:(NSNotification*)notification {    
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:@selector(backgroundMOCDidSave:) withObject:notification waitUntilDone:YES];
        return;
    }

    // We merge the background moc changes in the main moc

    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
}

@end

//
//  FK1Message.h
//  FK1
//
//  Created by Eric on 09/08/10.
//  Copyright 2010 __MyCompanyName__. All rights reserved.
//

#import <CoreData/CoreData.h>

@interface FK1Message :  NSManagedObject  
{
}

@property (nonatomic, retain) NSString * msgId;
@property (nonatomic, retain) NSDate * updateDate;

@end

// 
//  FK1Message.m
//  FK1
//
//  Created by Eric on 09/08/10.
//  Copyright 2010 __MyCompanyName__. All rights reserved.
//

#import "FK1Message.h"

@implementation FK1Message 

#pragma mark -
#pragma mark PROPERTIES

@dynamic msgId;
@dynamic updateDate;

@end

这就是全部！整个项目都在这里。没有表格视图，没有 NSFetchedResultsController，只有在后台 moc 上导入数据的后台线程。

在这种情况下什么可以改变集合？

我很确定我遗漏了一些明显的东西，这让我发疯了。

编辑：

这是完整的堆栈跟踪：

    2010-08-10 10:29:11.258 FK1[51419:1b6b] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x5d075b0> was mutated while being enumerated.<CFBasicHash 0x5d075b0 [0x25c6380]>{type = mutable set, count = 0,
entries =>
}
'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x0255d919 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x026ab5de objc_exception_throw + 47
    2   CoreFoundation                      0x0255d3d9 __NSFastEnumerationMutationHandler + 377
    3   CoreData                            0x02287702 -[NSManagedObjectContext executeFetchRequest:error:] + 4706
    4   FK1                                 0x00002b1b -[RootViewController _fetchData] + 593
    5   Foundation                          0x01d662a8 -[NSThread main] + 81
    6   Foundation                          0x01d66234 __NSThread__main__ + 1387
    7   libSystem.B.dylib                   0x9587681d _pthread_start + 345
    8   libSystem.B.dylib                   0x958766a2 thread_start + 34
)
terminate called after throwing an instance of 'NSException'

Answer 1

我正在导入记录并在表格视图中显示记录。当我尝试在 backgroundThread 上保存记录时遇到同样的问题，如下所示

 [self performSelectorInBackground:@selector(saveObjectContextInDataBaseWithContext:) withObject:privateQueueContext];

虽然我已经创建了 PrivateQueueContext。只需将上面的代码替换为下面的代码

[self saveObjectContextInDataBaseWithContext:privateQueueContext];

当我已经创建了一个用于保存记录的 privateQueueConcurrencyType 时，在后台线程上保存真的是我的愚蠢工作。

Answer 2

好的，我想我已经解决了我的问题，我必须感谢 Fred McCann 的这篇博文：

http://www.duckrowing.com/2010/03/11/using-core-data-on-multiple-threads/

问题似乎来自我在主线程而不是后台线程上实例化我的后台 moc 的事实。当 Apple 告诉每个线程需要有自己的 moc 时，您必须认真对待：每个 moc 必须在将要使用它的线程中实例化！

移动以下几行...

// We instantiate the background moc

self.backgroundMOC = [[[NSManagedObjectContext alloc] init] autorelease];

[self.backgroundMOC setPersistentStoreCoordinator:[self.managedObjectContext persistentStoreCoordinator]];

...在 _importData 方法中（就在将控制器注册为通知的观察者之前）解决了这个问题。

感谢您的帮助，彼得。感谢 Fred McCann 的宝贵博文！