为什么一定要在主线程刷新UI？
安全+效率：因为UIKit框架不是线程安全的框架，当在多个线程进行UI操作，有可能出现资源抢夺，导致问题。

其实：在子线程是不能更新UI的， 看到能更新的结果只是个假象。因为：在子线程代码完成之后，回到主线程，然后执行了子线程的更新UI的代码，由于这个时间很短，所以看起来是能够在子线程刷新UI的。想验证的话也很简单，看下面demo：点击按钮，会开启一个子线程，然后在子线程中刷新该按钮的标题，再新建一个按钮，最后一行代码是延迟5秒。

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view addSubview:self.button];   
}

-(UIButton *)button{
    if (!_button) {
        _button = [[UIButton alloc] initWithFrame:CGRectMake(0, 100, SCREEN_WIDTH, 100)];
        _button.backgroundColor = [UIColor yellowColor];
        [_button addTarget:self action:@selector(btnClicked) forControlEvents:UIControlEventTouchUpInside];
        [_button setTitle:@"UI" forState:UIControlStateNormal];
        [_button setTitleColor:[UIColor redColor] forState:UIControlStateNormal];
        _button.titleLabel.lineBreakMode = NSLineBreakByWordWrapping;
        _button.titleLabel.font = [UIFont systemFontOfSize:13];
    }
    return _button;
}

- (void)btnClicked {
    [NSThread detachNewThreadSelector:@selector(uiRefreashTest) toTarget:self withObject:nil];
}

- (void)uiRefreashTest {
    NSLog(@"当前线程:%@", [NSThread currentThread]);
    NSLog(@"主线程:%@", [NSThread mainThread]);
    
    //在子线程刷新该按钮的标题名字为子线程信息
    NSString *subStr = [NSString stringWithFormat:@"子线程:%@", [NSThread currentThread]];
    NSString *mainStr = [NSString stringWithFormat:@"主线程:%@", [NSThread mainThread]];
    [_button setTitle:subStr forState:UIControlStateNormal];
    CGRect labelTitleSize = [subStr boundingRectWithSize:CGSizeMake(SCREEN_WIDTH, SCREEN_HEIGHT) options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:13]} context:nil];
    _button.frame = CGRectMake(0, 100, labelTitleSize.size.width+10, labelTitleSize.size.height+10);
    
    //在子线程新建一个按钮,标题名字为主线程信息
    UIButton *newBtn = [[UIButton alloc] init];
    newBtn.backgroundColor = [UIColor greenColor];
    newBtn.titleLabel.font = [UIFont systemFontOfSize:13];
    newBtn.titleLabel.lineBreakMode = NSLineBreakByWordWrapping;
    [newBtn setTitle:mainStr forState:UIControlStateNormal];
    [newBtn setTitleColor:[UIColor blueColor] forState:UIControlStateNormal];
    CGRect labelTitleSize01 = [mainStr boundingRectWithSize:CGSizeMake(SCREEN_WIDTH, SCREEN_HEIGHT) options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:13]} context:nil];
    
    newBtn.frame = CGRectMake(0, 250, labelTitleSize01.size.width+10, labelTitleSize01.size.height+10);
    [self.view addSubview:newBtn];
    
    //在这个子线程延迟5秒钟
    sleep(5);
}

执行结果：

333.gif

结果分析：
从图中点击按钮后，虽然任务是在同一个子线程执行，但是并没有按顺序执行，而是首先立即执行NSLog日志输出，但是接下来并没有执行刷新按钮标题，和新建按钮的代码，而是优先执行sleep(5)延迟代码，然后才执行的刷新和新建控件的代码。