UIScrollView不响应setZoomScale:

我做错了什么?下面是我如何创建并填充我的scrollView

    -(void)viewDidAppear:(BOOL)animated {

    UIImageView *imgView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"bigimage.jpg"]];
    imgView.contentMode = UIViewContentModeScaleAspectFit;

    scrollView = [[UIScrollView alloc] initWithFrame:self.view.frame];
    scrollView.contentSize = imgView.frame.size;

    scrollView.minimumZoomScale =  scrollView.frame.size.width / scrollView.contentSize.width * 0.99;
    scrollView.maximumZoomScale =  2;

    [self.view addSubview:scrollView];


[scrollView addSubview:imgView];

    [scrollView setZoomScale:0.5 animated:YES];
    NSLog(@"current zoomScale: %f", scrollView.zoomScale);
    [imgView release];
}

NSLogged zoomScale为1.0,图像以全尺寸清晰显示。“收缩缩放”不起任何作用。 我已经尝试了几乎所有我能在网上找到的东西,但每个人似乎都在做这件事,而且对他们有效。

非常感谢您的帮助!