ios - dispatch_async 永远不会完成整个 block 的执行

Question

我是 GCD 的新手，对它的简单使用似乎对我不起作用。我有以下代码:

+ (void)synchronizationTimerFired:(NSTimer *)theTimer
{
    if ((synchronizationUpNeededFlag) || (synchronizationDownNeededFlag))
    {
        if ((!synchronizationUpInProgressDepthQuantity) && (!synchronizationDownInProgressDepthQuantity))
        {
            dispatch_queue_t synchronizationQueue = dispatch_queue_create("synchronizationQueue",NULL);
            dispatch_async(synchronizationQueue, ^(void) {

                NSLog(@"Top");
                ...code...
                ...code...
                ...code...
                NSLog(@"Bottom");

            });
        }
    }

    // Check if there is no timer, or if it is not currently valid,
    // and yet if synchronization is turned on,
    // then establish a repeating timer to attend to synchronization related matters.
    if ((!synchronizationTimer) || (!synchronizationTimer.isValid))
    {
        if (synchronizationOnFlag)
        {
            synchronizationTimer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(synchronizationTimerFired:) userInfo:nil repeats:YES];
        }
    }
}

日志显示为“Top”，仅此而已。中间的代码没有无限循环——它只是永远不会执行完。我可以在中间的代码中放置断点，有一个点程序执行会中断，之后就不会了。中间有一个点，有时执行会在断点处停止，有时则不会。

在我看来，好像正在释放 synchronizationQueue 调度队列，但我无法调用 dispatch_retain，因为编译器提示 dispatch_retain 不能在 ARC 中使用。我错过了什么？

为了回应人们询问中间代码的问题，程序执行在该方法调用(由...代码行之一表示)中的 if (fetchArray.count) 行停止, 在下面评论。

+ (NSDate *)latestParseReceivedDownUpdatedAtDateForCoreDataEntityNameString:(NSString *)coreDataEntityNameString
{
    NSDate *functionReturnValue = nil;

    // Create fetchRequest
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:coreDataEntityNameString];

    // Set sort descriptor
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"parseReceivedDownUpdatedAtDate" ascending:NO]]];

    // We are only interested in one result
    [fetchRequest setFetchLimit:1];

    // Execute fetchRequest
    NSError *fetchError = nil;
    NSArray *fetchArray = [JBSAPPDELEGATE.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];

    if (fetchArray == nil)
    {
        NSLog(@"Unresolved error %@, %@", fetchError, [fetchError userInfo]);
        abort();
    }

    // If there are any records at all in our persistent store, we'll have exactly one.
    // But that doesn't mean it won't be nil, as if that record has never come down from
    // parse it will be a nil date on the managed object.
    if (fetchArray.count) // PROGRAM EXECUTION STOPS EITHER HERE, OR JUST BEFORE HERE
    {
        NSManagedObject *managedObject = [fetchArray objectAtIndex:0];
        functionReturnValue = [managedObject valueForKey:@"parseReceivedDownUpdatedAtDate"];
    }

    return functionReturnValue;
}

我要补充一点，如果我只是简单地注释掉对 dispatch_async 的调用，那么一切都会正常执行。它只在主线程上执行，我不希望它这样做。

Answer 1

您的 managedObjectContext 是 NSManagedObjectContext 吗？如果是这样，您是否在用于调度队列的特定线程上创建了托管对象上下文？如果您阅读 NSManagedObjectContext 上的文档，它会说:

...a context assumes the default owner is the thread or queue that allocated it—this is determined by the thread that calls its init method. You should not, therefore, initialize a context on one thread then pass it to a different thread. Instead, you should pass a reference to a persistent store coordinator and have the receiving thread/queue create a new context derived from that.