Navigation controller top layout guide not honored with custom transition

Question

Short version:

I am having a problem with auto layout top layout guide when used in conjunction with custom transition and UINavigationController in iO

Answer 1

As @Rob mentioned, topLayoutGuide is not reliable when using custom transitions in UINavigationController. I worked around this by using my own layout guide. You can see the code in action in this demo project. Highlights:

A category for custom layout guides:

@implementation UIViewController (hp_layoutGuideFix)

- (BOOL)hp_usesTopLayoutGuideInConstraints
{
    return NO;
}

- (id)hp_topLayoutGuide
{
    id object = objc_getAssociatedObject(self, @selector(hp_topLayoutGuide));
    return object ? : self.topLayoutGuide;
}

- (void)setHp_topLayoutGuide:(id)hp_topLayoutGuide
{
    HPLayoutSupport *object = objc_getAssociatedObject(self, @selector(hp_topLayoutGuide));
    if (object != nil && self.hp_usesTopLayoutGuideInConstraints)
    {
        [object removeFromSuperview];
    }
    HPLayoutSupport *layoutGuide = [[HPLayoutSupport alloc] initWithLength:hp_topLayoutGuide.length];
    if (self.hp_usesTopLayoutGuideInConstraints)
    {
        [self.view addSubview:layoutGuide];
    }
    objc_setAssociatedObject(self, @selector(hp_topLayoutGuide), layoutGuide, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

HPLayoutSupport is the class that will act as a layout guide. It has to be a UIView subclass to avoid crashes (I wonder why this isn't part of the UILayoutSupport interface).

@implementation HPLayoutSupport {
    CGFloat _length;
}

- (id)initWithLength:(CGFloat)length
{
    self = [super init];
    if (self)
    {
        self.translatesAutoresizingMaskIntoConstraints = NO;
        self.userInteractionEnabled = NO;
        _length = length;
    }
    return self;
}

- (CGSize)intrinsicContentSize
{
    return CGSizeMake(1, _length);
}

- (CGFloat)length
{
    return _length;
}

@end

The UINavigationControllerDelegate is the one responsible for "fixing" the layout guide before the transition:

- (id )navigationController:(UINavigationController *)navigationController
                                   animationControllerForOperation:(UINavigationControllerOperation)operation
                                                fromViewController:(UIViewController *)fromVC
                                                  toViewController:(UIViewController *)toVC
{
    toVC.hp_topLayoutGuide = fromVC.hp_topLayoutGuide;
    id  animator;
    // Initialise animator
    return animator;
}

Finally, the UIViewController uses hp_topLayoutGuide instead of topLayoutGuide in the constraints, and indicates this by overriding hp_usesTopLayoutGuideInConstraints:

- (void)updateViewConstraints
{
    [super updateViewConstraints];
    id topLayoutGuide = self.hp_topLayoutGuide;
    // Example constraint
    NSDictionary *views = NSDictionaryOfVariableBindings(_imageView, _dateLabel, topLayoutGuide);
    NSArray *constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topLayoutGuide][_imageView(240)]-8-[_dateLabel]" options:NSLayoutFormatAlignAllCenterX metrics:nil views:views];
    [self.view addConstraints:constraints];
}

- (BOOL)hp_usesTopLayoutGuideInConstraints
{
    return YES;
}

Hope it helps.