iOS --短信验证码倒计时

不足之处请指出

//开启倒计时
-(void)vertifierClick{

__block NSInteger time = 29; //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(time <= 0){ //关闭倒计时

dispatch_source_cancel(_timer);
dispatch_async(dispatch_get_main_queue(), ^{
//可以点击按钮
_verifyBtn.userInteractionEnabled = YES;
//设置按钮上的字
[_verifyBtn setTitle:@"重新发送" forState:UIControlStateNormal];

});

}else{

int seconds = time % 60;
dispatch_async(dispatch_get_main_queue(), ^{
//禁止点击按钮
_verifyBtn.userInteractionEnabled = NO;
//设置按钮显示读秒效果
[_verifyBtn setTitle:[NSString stringWithFormat:@"重新发送(%.2d)", seconds] forState:UIControlStateNormal]；
});
time--;
}
});
dispatch_resume(_timer);
}