一段倒计时的代码

__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);
dispatch_async(dispatch_get_main_queue(), ^{
//设置界面的按钮显示 根据自己需求设置
[_buttonResend setTitle:@"重发验证码" forState:UIControlStateNormal];
_buttonResend.userInteractionEnabled = YES;
_buttonResend.enabled = YES;
});
}else{
//            int minutes = timeout / 60;
int seconds = timeout % 60;
__block 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
DYYLog(@"__ForetPassWordViewController__%@",strTime);
if([strTime isEqualToString:@"00"])
{
strTime = @"60";
}
[_buttonResend setTitle:[NSString stringWithFormat:@"%@秒后重新发送",strTime] forState:UIControlStateDisabled];
_buttonResend.enabled = NO;
_buttonResend.userInteractionEnabled = NO;

});
timeout--;

}
});
dispatch_resume(_timer);