监听NSOperationQueue 队列任务完成

// KVO，监听 operationCount，等于0说明完成

• (NSOperationQueue *)operationQueue {
  if (!_operationQueue) {
  _operationQueue = [[NSOperationQueue alloc] init];
  [_operationQueue addObserver:self forKeyPath:@"operationCount" options:0 context:nil];
  }
  return _operationQueue;
  }

• (void)observeValueForKeyPath:(NSString *)keyPath
  ofObject:(id)object
  change:(NSDictionary *)change
  context:(void *)context
  {
  if (object == self.operationQueue && [keyPath isEqualToString:@"operationCount"])
  {

    // 等于0表示完成
    if (0 == self.operationQueue.operations.count)
    {
  
    }
  

  }
  else
  {
  [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
  }
  }