多线程应用中NSManagedObjectContext的通用方法

我在这里读了很多关于NSManagedObjectContext和多线程应用程序的文章。我还介绍了CoreDataBooks示例,以了解独立线程如何需要自己的NSManagedObjectContext,以及save操作如何与主NSManagedObjectContext合并。我发现这个例子很好,但也太具体了。我想概括一下,不知道我的方法是否合理。

我的方法是使用一个泛型函数来获取当前线程的NSManagedObjectContext。该函数返回主线程的NSManagedObjectContext,但如果从其他线程中调用,则将创建一个新的(或从缓存中获取)。具体如下:

+(NSManagedObjectContext *)managedObjectContext {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *moc = delegate.managedObjectContext;

    NSThread *thread = [NSThread currentThread];

    if ([thread isMainThread]) {
        return moc;
    }

    // a key to cache the context for the given thread
    NSString *threadKey = [NSString stringWithFormat:@"%p", thread];

    // delegate.managedObjectContexts is a mutable dictionary in the app delegate
    NSMutableDictionary *managedObjectContexts = delegate.managedObjectContexts;

    if ( [managedObjectContexts objectForKey:threadKey] == nil ) {
        // create a context for this thread
        NSManagedObjectContext *threadContext = [[[NSManagedObjectContext alloc] init] autorelease];
        [threadContext setPersistentStoreCoordinator:[moc persistentStoreCoordinator]];
        // cache the context for this thread
        [managedObjectContexts setObject:threadContext forKey:threadKey];
    }

    return [managedObjectContexts objectForKey:threadKey];
}

commit 功能:

+(void)commit {
    // get the moc for this thread
    NSManagedObjectContext *moc = [self managedObjectContext];

    NSThread *thread = [NSThread currentThread];

    if ([thread isMainThread] == NO) {
        // only observe notifications other than the main thread
        [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(contextDidSave:)
                                                 name:NSManagedObjectContextDidSaveNotification
                                               object:moc];
    }

    NSError *error;
    if (![moc save:&error]) {
        // fail
    }

    if ([thread isMainThread] == NO) {
        [[NSNotificationCenter defaultCenter] removeObserver:self 
                                                    name:NSManagedObjectContextDidSaveNotification 
                                                  object:moc];
    }
}

contextDidSave: 犯罪 .

+(void)contextDidSave:(NSNotification*)saveNotification {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *moc = delegate.managedObjectContext;

    [moc performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                      withObject:saveNotification
                   waitUntilDone:YES];
}

最后,我们用以下方法清理NSManagedObjectContext的缓存:

+(void)initialize {
    [[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(threadExit) 
                                                 name:NSThreadWillExitNotification 
                                               object:nil]; 
}

+(void)threadExit {
    MyAppDelegate *delegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSString *threadKey = [NSString stringWithFormat:@"%p", [NSThread currentThread]];  
    NSMutableDictionary *managedObjectContexts = delegate.managedObjectContexts;    

    [managedObjectContexts removeObjectForKey:threadKey];
}

这是编译,似乎工作,但我知道线程问题可能是棘手的,由于种族条件。有人认为这种方法有问题吗?