求解，ios7下 setNavigationBarHidden:YES 之后 页面会跟 Statusbar 叠在一起

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSString *verticalConstraint = @"V:|[v]";// for iOS6
    NSMutableDictionary *views = [NSMutableDictionary new];
    NSMutableArray *constraints = [NSMutableArray new];
    [views setObject:self.subview forKey:@"v"];
    if ([self respondsToSelector:@selector(topLayoutGuide)]) {
        // for iOS7
        [views setObject:self.topLayoutGuide forKey:@"topLayoutGuide"];
        verticalConstraint = @"V:[topLayoutGuide][v]";
    }
    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:verticalConstraint options:0 metrics:nil views:views]];
    // ... other constraints
    [self.view addConstraints:constraints];
    // Do any additional setup after loading the view from its nib.
}