//創建一個全局并非隊列  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);   //創建一個定時器    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);   //配置定時器，每秒執行   dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);   //創建事件處理器   dispatch_source_set_event_handler(_timer, ^{    timeout --;     if (timeout <= 0) {       //定時器結束，關閉        dispatch_source_cancel(_timer);       _timer = nil;       //回到主線程刷新UI       dispatch_async(dispatch_get_main_queue(), ^{       [_btn setTitle:@"獲取驗證碼" forState:UIControlStateNormal];       });     }    else {  //回到主線程刷新UI   dispatch_async(dispatch_get_main_queue(), ^{     [_btn setTitle:[NSString stringWithFormat:@"%ds后重新獲取",timeout] forState:UIControlStateNormal];     });     }    });  //啟動定時器dispatch_resume(_timer);