iOS 安全地在主线程执行一个Block

//主线程同步队列
#define dispatch_main_sync_safe(block)
    if ([NSThread isMainThread]) {
        block();
    } else {
        dispatch_sync(dispatch_get_main_queue(), block);
    }
//主线程异步队列
#define dispatch_main_async_safe(block)
    if ([NSThread isMainThread]) {
        block();
    } else {
        dispatch_async(dispatch_get_main_queue(), block);
    }

//用法
  dispatch_main_async_safe(^{
                    //需要执行的代码片段;
                });

或者

#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block)
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {
        block();
    } else {
        dispatch_async(dispatch_get_main_queue(), block);
    }
#endif