利用performSelectorInBackground和performSelectorOnMainThread实现多线程刷新UI

NSObject类的performSelectorOnMainThread和performSelectorInBackground可以实现简单的多线程编程技术
1、- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg
创建一个线程在子线程执行，aSelector代表了新创建的线程，arg是传入的参数
2、- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait;
该方法的作用是在主线程中，执行制定的方法（代码块）。
参数：
@selector就是，要定义我们要执行的方法。
withObject:arg定义了，我们执行方法时，传入的参数对象。类型是id。（我们可以传入任何参数）
waitUntilDone:YES指定，当前线程是否要被阻塞，直到主线程将我们制定的代码块执行完。
注意：
1.当前线程为主线程的时候，waitUntilDone:YES参数无效。
2.该方法，没有返回值

3.该方法主要用来用主线程来修改页面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];

}

执行结果：

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}