实现原理：开启子线程监控主线程的RunLoop的状态，并设定超时时间。如果发生了超时，而且此时RunLoop的状态为kCFRunLoopBeforeSources 或  kCFRunLoopAfterWaiting，认为主线程卡顿；

CFRunLoopObserverRef _runloopObserver;dispatch_semaphore_t _semaphore;NSInteger _timeOutCount;CFRunLoopActivity _activity;void runLoopObserverCallBack(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info)
{ViewController *vc = (__bridge ViewController *)info;vc->_activity = activity;switch (activity) {case kCFRunLoopAfterWaiting:NSLog(@"kCFRunLoopAfterWaiting");break;case kCFRunLoopBeforeSources:NSLog(@"kCFRunLoopBeforeSources");break;case kCFRunLoopBeforeWaiting:NSLog(@"kCFRunLoopBeforeWaiting");break;case kCFRunLoopBeforeTimers:NSLog(@"kCFRunLoopBeforeTimers");break;default:break;}dispatch_semaphore_t semaphore = vc->_semaphore;dispatch_semaphore_signal(semaphore);
}//开始监控
-(void)startMonitor
{if(_runloopObserver){return;}_semaphore = dispatch_semaphore_create(0);CFRunLoopObserverContext context = {0, (__bridge void *)(self), NULL, NULL};_runloopObserver = CFRunLoopObserverCreate(kCFAllocatorDefault, kCFRunLoopAllActivities, YES, 0, runLoopObserverCallBack, &context);CFRunLoopAddObserver(CFRunLoopGetMain(), _runloopObserver, kCFRunLoopCommonModes);//开启子线程循环监控dispatch_async(dispatch_get_global_queue(0, 0), ^{while (YES) {dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 3 * NSEC_PER_SEC);long semaphoreWait = dispatch_semaphore_wait(_semaphore, time);if(semaphoreWait != 0){ //超时if(!_runloopObserver){_timeOutCount = 0;_semaphore  = 0;_activity = 0;return;}if(_activity == kCFRunLoopAfterWaiting || _activity == kCFRunLoopBeforeSources){NSLog(@"发现一次延时");if(++_timeOutCount < 3){continue;}}}else{_timeOutCount = 0;}}});
}

完整的demo路径：https://github.com/xiaobai1315/MonitorMainQueue