使用GCD创建倒计时

__block int timeout=300; //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);

//dispatch_release(_timer);

dispatch_async(dispatch_get_main_queue(), ^{

//设置界面的按钮显示 根据自己需求设置

});

}else{

int minutes = timeout / 60;

int seconds = timeout % 60;

NSString *strTime = [NSString stringWithFormat:@"%d分%.2d秒后重新获取验证码",minutes, seconds];

dispatch_async(dispatch_get_main_queue(), ^{

//设置界面的按钮显示 根据自己需求设置

NSLog(@"----- %@-----",strTime);

});

timeout--;

}

});

dispatch_resume(_timer);