// 添加一个监听者
- (void)addObserver {
    
    // 1. 创建监听者
    /**
     *  创建监听者
     *
     *  @param allocator#>  分配存储空间
     *  @param activities#> 要监听的状态
     *  @param repeats#>    是否持续监听
     *  @param order#>      优先级, 默认为0
     *  @param observer     观察者
     *  @param activity     监听回调的当前状态
     */
    CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, kCFRunLoopAllActivities, YES, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {
        
        /*
         kCFRunLoopEntry = (1UL << 0),          进入工作
         kCFRunLoopBeforeTimers = (1UL << 1),   即将处理Timers事件
         kCFRunLoopBeforeSources = (1UL << 2),  即将处理Source事件
         kCFRunLoopBeforeWaiting = (1UL << 5),  即将休眠
         kCFRunLoopAfterWaiting = (1UL << 6),   被唤醒
         kCFRunLoopExit = (1UL << 7),           退出RunLoop
         kCFRunLoopAllActivities = 0x0FFFFFFFU  监听所有事件
         */
        switch (activity) {
            case kCFRunLoopEntry:
                NSLog(@"进入");
                break;
            case kCFRunLoopBeforeTimers:
                NSLog(@"即将处理Timer事件");
                break;
            case kCFRunLoopBeforeSources:
                NSLog(@"即将处理Source事件");
                break;
            case kCFRunLoopBeforeWaiting:
                NSLog(@"即将休眠");
                break;
            case kCFRunLoopAfterWaiting:
                NSLog(@"被唤醒");
                break;
            case kCFRunLoopExit:
                NSLog(@"退出RunLoop");
                break;
            default:
                break;
        }
    });
    
    // 2. 添加监听者
    /**
     *  给指定的RunLoop添加监听者
     *
     *  @param rl#>       要添加监听者的RunLoop
     *  @param observer#> 监听者对象
     *  @param mode#>     RunLoop的运行模式, 填写默认模式即可
     */
    CFRunLoopAddObserver(CFRunLoopGetCurrent(), observer, kCFRunLoopDefaultMode);
}

###ASDK实现方式

+ (void)registerTransactionGroupAsMainRunloopObserver:(_ASAsyncTransactionGroup *)transactionGroup {
  static CFRunLoopObserverRef observer;
  CFRunLoopRef runLoop = CFRunLoopGetCurrent();
  CFOptionFlags activities = (kCFRunLoopBeforeWaiting | kCFRunLoopExit);
  CFRunLoopObserverContext context = {0, (__bridge void *)transactionGroup, &CFRetain, &CFRelease, NULL};

  observer = CFRunLoopObserverCreate(NULL, activities, YES, INT_MAX, &_transactionGroupRunLoopObserverCallback, &context);
  CFRunLoopAddObserver(runLoop, observer, kCFRunLoopCommonModes);
  CFRelease(observer);
}

static void _transactionGroupRunLoopObserverCallback(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info) {
  ASDisplayNodeCAssertMainThread();
  _ASAsyncTransactionGroup *group = (__bridge _ASAsyncTransactionGroup *)info;
  [group commit];
}