我是 GCD 新手，看似简单的使用方法对我来说并不适用。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“Top”，没有其他内容。中间的代码没有无限循环——它只是永远不会执行到底。我可以在代码中间放置断点，程序执行到某个点就会中断，之后就不会中断。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，synchronizationQueue 调度队列正在被释放，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？
为了响应人们询问中间的代码，程序执行在此方法调用中停止(由其中一行...代码...表示)，位于 if (fetchArray.count) 行处，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，如果我简单地注释掉对dispatch_async的调用，一切都会正常执行。它只在主线程上执行，我宁愿不这样做。
最佳答案
你的 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.
关于ios - dispatch_async永远不会完成整个 block 的执行，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19747960/