multithreading - `-applicationDidBecomeActive` 怎么可能同时被调度到两个单独的后台线程上？

Question

我们遇到了一次奇怪的崩溃 - 现在我知道是什么原因造成的，但我注意到的一件奇怪的事情是所有崩溃日志在两个单独的背景上都有 -applicationDidBecomeActive线程

Thread 5:
0   libsystem_kernel.dylib               0x00007fffaebac456 semaphore_wait_trap + 10
1   MyApp                               0x0000000107389da1 -[OutputManager(TechSmithCloud) reloadCloudDestinations] (OutputManager+TechSmithCloud.m:59)
2   MyApp                               0x000000010728c4b4 __44-[AppController applicationDidBecomeActive:]_block_invoke (AppController.m:696)
3   libdispatch.dylib                    0x00007fffaea57f5f _dispatch_call_block_and_release + 12
4   libdispatch.dylib                    0x00007fffaea4f128 _dispatch_client_callout + 8
5   libdispatch.dylib                    0x00007fffaea51099 _dispatch_root_queue_drain + 917
6   libdispatch.dylib                    0x00007fffaea50cb7 _dispatch_worker_thread3 + 99
7   libsystem_pthread.dylib              0x00007fffaec9b746 _pthread_wqthread + 1299
8   libsystem_pthread.dylib              0x00007fffaec9b221 start_wqthread + 13


Thread 13:
0   libsystem_kernel.dylib               0x00007fffaebac456 semaphore_wait_trap + 10
1   MyApp                               0x0000000107389da1 -[OutputManager(TechSmithCloud) reloadCloudDestinations] (OutputManager+TechSmithCloud.m:59)
2   MyApp                               0x000000010728c4b4 __44-[AppController applicationDidBecomeActive:]_block_invoke (AppController.m:696)
3   libdispatch.dylib                    0x00007fffaea57f5f _dispatch_call_block_and_release + 12
4   libdispatch.dylib                    0x00007fffaea4f128 _dispatch_client_callout + 8
5   libdispatch.dylib                    0x00007fffaea51099 _dispatch_root_queue_drain + 917
6   libdispatch.dylib                    0x00007fffaea50cb7 _dispatch_worker_thread3 + 99
7   libsystem_pthread.dylib              0x00007fffaec9b746 _pthread_wqthread + 1299
8   libsystem_pthread.dylib              0x00007fffaec9b221 start_wqthread + 13

而且我无法重现它(我在 -applicationDidBecomeActive 中注销了一条语句，并且它一次只注销一次)

所以我不确定这怎么可能，或者这是否是一个实际问题

也许这是一个与信号量相关的东西？

代码如下:

- (void)applicationDidBecomeActive:(NSNotification *)aNotification
{
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^
                  {
                     [[OutputManager sharedOutputManager] reloadCloudDestinations];
                  });

}

编辑:-reloadCloudDestinations 的代码

-(void) reloadCloudDestinations
{
   [self setupCloudLibrary];
   TSCAccount* account = [TSCCloudServices activeAccount];
   if( account.status == TSCAccount_SignedIn )
   {
      dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
      __weak __typeof(self) weakSelf = self;

      @try
      {
         AFHTTPSessionManager *test = (AFHTTPSessionManager*)[(TSCAccountHTTPSession*)[self.libraryCore valueForKey:@"sessionManager"] valueForKey:@"httpClient"];
         NSLog(@"%@", test);
         [self.libraryCore destinationsWithActions:NEVER_TRANSLATE(@"publish,list") completionBlock:^(NSArray *destinations, NSError *error) {

            @try
            {
               if( error == nil )
               {
                  [weakSelf createButtonsForNewDestinationsNotAlreadyPresent:destinations];
               }
               else
               {
                  NSLog(@"destinationsWithBlock error: %@", error);
               }
            }
            @catch (NSException* exception)
            {
               NSLog(@"reloadCloudDestinations - destinationWithBlock completion - An exception was thrown %@", exception);
            }
            @finally
            {
              dispatch_semaphore_signal(semaphore);
            }
         }];

         dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
      }
      @catch (NSException* exception)
      {
         NSLog(@"reloadCloudDestinations - An exception was thrown %@", exception);
         dispatch_semaphore_signal(semaphore);
      }
   }
}

Answer 1

回溯具有符号__44-[AppController applicationDidBecomeActive:]_block_invoke。这不是 -applicationDidBecomeActive: 方法本身。它是编译器为 -applicationDidBecomeActive: 方法中出现的 block 生成的函数的名称。

阻塞函数出现在后台线程上，因为您将其分派(dispatch)到主队列以外的队列。这不是问题，只是一个解释。

它出现多次可能是因为您的应用程序在其生命周期中变得活跃，退出活跃，然后再次活跃。每次它变为事件状态时，都会在主线程上调用 -applicationDidBecomeActive:。这会将 block 提交到全局并发队列，并假设有可用的 CPU 核心和其他系统资源，然后它将在后台线程上执行。

一个悬而未决的问题是，为什么当崩溃发生时这些 block 仍在运行，而不是在短时间内完成。您没有显示 -[OutputManager reloadCloudDestinations] 的代码，因此很难知道。显然，它等待一个计数恰好为零或更少的信号量。