利用performSelectorInBackground和performSelectorOnMainThread实现多线程

类型是id。

（我们能够传入不论什么參数）
waitUntilDone:YES指定，当前线程是否要被堵塞。直到主线程将我们制定的代码块运行完。

注意：
1.当前线程为主线程的时候，waitUntilDone:YES參数无效。
2.该方法。没有返回值

3.该方法主要用来用主线程来改动页面UI的状态。

sample code：

- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
    
    _label=[[UILabel alloc] initWithFrame:CGRectMake(40, 40, 60, 40)];
    _label.textColor=[UIColor redColor];
    _label.text=@"123";
    [self.view addSubview:_label];
    
    [self performSelectorInBackground:@selector(backWork) withObject:nil];
}
-(void)backWork
{
    NSLog(@"the thread is %@",[NSThread currentThread]);
    sleep(2);
    [self performSelectorOnMainThread:@selector(mainWork) withObject:nil waitUntilDone:NO];
}

-(void)mainWork
{
    NSLog(@"the main thread is %@",[NSThread currentThread]);

    _label.text=@"456";
    _label.textColor=[UIColor greenColor];

}

2014-08-19 11:03:59.101 testApp[1848:3107] the thread is <NSThread: 0x8c504a0>{name = (null), num = 2}

2014-08-19 11:04:01.103 testApp[1848:60b] the main thread is <NSThread: 0x8c444c0>{name = (null), num = 1}