手机获取验证码的倒计时效果

//创建一个全局并非队列
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//创建一个定时器
_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
//配置定时器，每秒执行
dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);
//创建事件处理器
dispatch_source_set_event_handler(_timer, ^{
timeout --;
if (timeout <= 0) {
//定时器结束，关闭
dispatch_source_cancel(_timer);
_timer = nil;
//回到主线程刷新UI
dispatch_async(dispatch_get_main_queue(), ^{
[_btn setTitle:@"获取验证码" forState:UIControlStateNormal];
});
}
else
{
//回到主线程刷新UI
dispatch_async(dispatch_get_main_queue(), ^{
[_btn setTitle:[NSString stringWithFormat:@"%ds后重新获取",timeout] forState:UIControlStateNormal];
});
}
});
//启动定时器
dispatch_resume(_timer);