调用performSelector：onThread：withObject：waitUntilDone：在同一个线程上不止一次导致延迟

Question

代码如下：

// Code in some object that will do work for an application:  

- (BOOL)shouldBeRunning  
{  
  [lockRunning lock];  
  BOOL shouldBeRunning= shouldRun;  
  [lockRunning unlock];  

  return shouldBeRunning;  
}  

- (void)stopRunning  
{  
  [lockRunning lock];  
  shouldRun= FALSE;  
  [lockRunning unlock];  
}  

- (void)threadEntryPoint:(id)object  
{  
  NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];  

  NSRunLoop *runLoop = [NSRunLoop currentRunLoop];  

  // From an example I saw awhile back:  
  // A runloop with no sources returns immediately from runMode:beforeDate:  
  // That will wake up the loop and chew CPU. Add a dummy source to prevent it.  
  NSMachPort *dummyPort = [[NSMachPort alloc] init];  
  [runLoop addPort:dummyPort forMode:NSDefaultRunLoopMode];  
  [dummyPort release];  
  [pool release];  

  while ([self shouldBeRunning])   
  {  
    NSAutoreleasePool *loopPool = [[NSAutoreleasePool alloc] init];  
    [runLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];  
    [loopPool drain];  
  }   
}  

- (BOOL)startRunning:(NSError **)errorPtr  
{  
  [self stopRunning];  // Stop if we are already running.  

  [runWorker release];  
  runWorker= [[NSThread alloc] initWithTarget:self selector:@selector(threadEntryPoint:) object:nil];  

  if(!runWorker)  
    return (FALSE);  

  // Start up the thread.  
  shouldRun= TRUE;  

  [runWorker start];  

  return TRUE;  
}  

- (void)doLotsOfStuff  
{  
  // Some operation that is long and intensive  
  // that should be done in the background.  
  // This function will call the app delegate, which will display the  
  // results. It will also notify the app on completion.  
}    

- (void)doStuff
{  
  // Commented out for illustrative purposes.  
  //[self startRunning];  // Fire thread up.  

  [self performSelector:@selector(doLotsOfStuff) onThread:runWorker withObject:nil waitUntilDone:NO];  
}    

// Out in the delegate:  
- (void)applicationDidFinishLaunching:(NSNotification*)aNotification  
{  
  // Do setup....  
  [workObject startRunning];  // Start the worker thread in the worker object.  
}  

- (void)buttonHandler:(id)sender  
{  
  [workObject doStuff];  
}  

因此，在应用程序中有一个按钮。用户将按下它，任务将在工作线程上运行。该任务将为应用程序提供反馈。在这种情况下，按钮被禁用，直到任务完成。我只是不想显示所有代码。

如果编写了代码，如果我按下按钮一次，任务就会立即运行。通常，按下第二个按钮会产生相同的结果。但是，有时第二次按，但几乎总是第三次或之后，将导致执行任务的显着延迟。我把调试语句放进去，可以观察代码在线程上执行performSelector，然后有一个延迟，最后运行任务。

如果我取消注释doStuff中重新创建线程的行（使applicationDidFinishLaunching中的行冗余），当然它每次都能完美运行。

据我所知，线程正处于无响应的状态。

有关可能发生的事情的任何想法？设置和处理代码有什么明显错误吗？任何意见都赞赏。