[self performSelectorInBackground:@selector(thread) withObject:nil];

- (void)thread
{
    for(int i=300;i>=0;i--)
    {
        _count = i;
        // 回调主线程
        [self performSelectorOnMainThread:@selector(mainThread) withObject:nil waitUntilDone:YES];
        sleep(1);
    }
}
// 此函数主线程执行
- (void)mainThread
{
    self.time.text=[NSString stringWithFormat:@"%d",_count];
    if (_count==0) {
        
    }
}