//主執行緒同步佇列
#define dispatch_main_sync_safe(block)\
    if ([NSThread isMainThread]) {\
        block();\
    } else {\
        dispatch_sync(dispatch_get_main_queue(), block);\
    }
//主執行緒非同步佇列
#define dispatch_main_async_safe(block)\
    if ([NSThread isMainThread]) {\
        block();\
    } else {\
        dispatch_async
 
(dispatch_get_main_queue(), block);\
    }
//用法
  dispatch_main_async_safe(^{
                    //需要執行的程式碼片段;
                });