方式一：手动开启线程
NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(test) object:nil];
[thread start];
方式二：自动开启线程
[NSThread detachNewThreadSelector:@selector(test) toTarget:self withObject:nil];
方式三:直接在后台创建一个子线程
[self performSelectorInBackground:@selector(test) withObject:nil];
退出线程
[NSThread exit];
