iOS 7视图使用topLayoutGuide在状态栏下重新定位

Question

我已经看到许多令人困惑的解决方案来处理iOS 7中的状态栏但是我喜欢Apple提到的解决方案 - https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIViewController_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40006926

....但它不起作用！此方法按预期调用，但childView不向下移动20像素。

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];

    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7) {
        UIView *childView = self.childController.view;
        id topGuide = self.topLayoutGuide;
        CGFloat topBarOffset = self.topLayoutGuide.length; //I checked this valued is 20
        NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings (childView, topGuide);
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat: @"V:[topGuide]-20-[childView]"
                                                                          options: 0
                                                                          metrics: nil
                                                                            views: viewsDictionary
                                   ]
        ];
    }
}

以下为我工作，但我真的想使用topLayoutGuide使用AutoLayout来解决这个问题。

- (void)layoutSubviews
{
    [super layoutSubviews];

    self.backgroundColor = [UIColor blackColor];
    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7) {
        CGRect frame = self.bounds;
        frame.origin.y = 20;
        frame.size.height -= 20;
        self.childView.frame = frame;
    } else {
        self.childView.frame = self.bounds;
    }
}

为什么使用topLayoutGuide无法正常工作？感谢。