GCD时间轴

 __block int timeout=60; //倒计时时间
            dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
            dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
            dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
            dispatch_source_set_event_handler(_timer, ^{
                if(timeout<=0){ //倒计时结束，关闭
                    dispatch_source_cancel(_timer); //关闭时间轴
                    dispatch_async(dispatch_get_main_queue(), ^{
                        //设置界面的按钮显示 根据自己需求设置
                        [self.getSMSCodeBtn setTitle:OTKObtainSMSVerificationCodeForFree forState:UIControlStateNormal];
                        self.getSMSCodeBtn.userInteractionEnabled = YES;
                    });
                }else{
                    //            int minutes = timeout / 60;
                    int seconds = timeout % 61;
                    NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
                    dispatch_async(dispatch_get_main_queue(), ^{
                        //设置界面的按钮显示 根据自己需求设置
                        [self.getSMSCodeBtn setTitle:[NSString stringWithFormat:@"%@秒后重新获取",strTime] forState:UIControlStateNormal];
                        self.getSMSCodeBtn.userInteractionEnabled = NO;
                       
                    });
                    timeout--;
                   
                }
            });
            dispatch_resume(_timer);