Runloop介绍
http://www.cnblogs.com/tangbinblog/archive/2012/12/07/2807088.html

巧妙使用block

+ (void)spinUntilCompletion:(void (^)(void(^completion)()))block
{
  __block volatile atomic_bool didFinish = false;
  block(^{
    atomic_fetch_or(&didFinish, true);//先返回didFinish,再修改didFinish
  });
  while (!atomic_fetch_and(&didFinish, false)) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:FBWaitInterval]];
  }
}

• atomic_fetch_or原子操作bool值，或
• atomic_fetch_and原子操作bool值，与，先返回本身值做判断

执行完block操作，在block中调用completion，didFinish值为true，则runloop停止，否则while循环一直监听didFinish值。