目标C-无法在主线程上更新UI

我在使用performSelectorOnMainThread更新我的UI时遇到了一些麻烦.这是我的情况.在我的viewDidLoad中,我设置了一个活动指示器和一个标签.然后我调用选择器从服务器检索一些数据.然后我调用选择器在延迟后更新UI.这是代码:

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.reloadSchools = [[UIAlertView alloc] init];
    self.reloadSchools.message = @"There was an error loading the schools. Please try again.";
    self.reloadSchools.title = @"We're Sorry";
    self.schoolPickerLabel = [[UILabel alloc]init];
    self.schoolPicker = [[UIPickerView alloc] init];
    self.schoolPicker.delegate = self;
    self.schoolPicker.dataSource = self;
    self.server = [[Server alloc]init];
    schoolList = NO;

    _activityIndicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
    [self.view addSubview:_activityIndicator];
    [self.view bringSubviewToFront:_activityIndicator];
    [_activityIndicator startAnimating];

    [NSThread detachNewThreadSelector: @selector(getSchoolList) toTarget: self withObject: nil];
    [self performSelector:@selector(updateUI) withObject:nil afterDelay:20.0];
}

选择器updateUI检查数据是否被检索,并调用主线程上的选择器以相应地更新UI.以下是这些部分的代码:

-(void)updateUI
{
    self.schools = [_server returnData];
    if(!(self.schools == nil)) {
        [self performSelectorOnMainThread:@selector(fillPickerView) withObject:nil waitUntilDone:YES];
    }
    else {
        [self performSelectorOnMainThread:@selector(showError) withObject:nil waitUntilDone:YES];
    }
}

-(void)showError {
    NSLog(@"show error");
    [_activityIndicator stopAnimating];
    [self.reloadSchools show];
    }
-(void)fillPickerView {
     NSLog(@"fill picker view");
     schoolList = YES;
     NSString *schoolString = [[NSString alloc] initWithData:self.schools encoding:NSUTF8StringEncoding];
     self.schoolPickerLabel.text  = @"Please select your school:";
     self.shoolArray = [[schoolString componentsSeparatedByString:@"#"] mutableCopy];
     [self.schoolPicker reloadAllComponents];
     [_activityIndicator stopAnimating];
}

当调用选择器fillPickerView时,活动指示器保持旋转,标签文本不会更改,并且选取器视图不会重新加载其内容.有人可以向我解释为什么我使用的方法不能在主线程上更新我的ui吗？