UIScrollview自动布局似乎无法在iOS8 / Xcode 6预览中使用？

编辑这是在种子5中修复的,所以应该忽略这个注释！

该规则应该是(参见https://developer.apple.com/library/prerelease/ios/technotes/tn2154/_index.html),如果滚动视图(其子视图或子视图)的内容固定到滚动视图的所有四个边界,则它设置内容大小.

然而,在iOS 8中,这失败了 – 但是以一种奇怪的方式.只有当确定子视图的高度和宽度的约束都是绝对的而不是内在的时,它才会失败.

因此,例如,考虑该技术说明底部的代码,其中滚动视图和一个非常大的图像视图在代码中创建(这里是;我已经纠正了一个丢弃了at符号的小错误)：

- (void)viewDidLoad {
    UIScrollView *scrollView;
    UIImageView *imageView;
    NSDictionary *viewsDictionary;
    // Create the scroll view and the image view.
    scrollView  = [[UIScrollView alloc] init];
    imageView = [[UIImageView alloc] init];
    // Add an image to the image view.
    [imageView setImage:[UIImage imagenamed:@"MyReallyBigImage"]];
    // Add the scroll view to our view.
    [self.view addSubview:scrollView];
    // Add the image view to the scroll view.
    [scrollView addSubview:imageView];
    // Set the translatesAutoresizingMaskIntoConsTraints to NO so that the views
    // autoresizing mask is not translated into auto layout consTraints.
    scrollView.translatesAutoresizingMaskIntoConsTraints  = NO;
    imageView.translatesAutoresizingMaskIntoConsTraints = NO;
    // Set the consTraints for the scroll view and the image view.
    viewsDictionary = NSDictionaryOfVariableBindings(scrollView,imageView);
    [self.view addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"H:|[scrollView]|" 
        options:0 metrics: 0 views:viewsDictionary]];
    [self.view addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"V:|[scrollView]|" 
        options:0 metrics: 0 views:viewsDictionary]];
    [scrollView addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"H:|[imageView]|" 
        options:0 metrics: 0 views:viewsDictionary]];
    [scrollView addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"V:|[imageView]|" 
        options:0 metrics: 0 views:viewsDictionary]];
}

该代码有效(假设您的图像非常大),因为图像视图的大小受内在约束的影响.但现在更改最后两行,如下所示：

[scrollView addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"H:|[imageView(1000)]|" 
        options:0 metrics: 0 views:viewsDictionary]];
    [scrollView addConsTraints:[NSLayoutConsTraint 
        consTraintsWithVisualFormat:@"V:|[imageView(1000)]|" 
        options:0 metrics: 0 views:viewsDictionary]];

现在你所拥有的是一个可在iOS 7上滚动但在iOS 8上不可滚动的滚动视图.进一步的调查显示这是因为内容大小保持在(0,0);它不尊重内容视图的绝对宽度和高度约束.