It’s been a while since my last post. I am back and hoping to dedicate more time to writing tutorials and open sourcing interesting things. What this post is about? In this post I want to go through the recently published library and explain the motivation behind that, as well as the implementation. HidesNavigationBarWhenPushed hidesNavigationBarWhenPushed in action (With interactive pop gesture recognizer) Motivation While on various projects, time to time I get designs where view controller A has visible, and view controller B has it hidden. It was always a pain to make it work. hacking navigation bar The simplest solution would be: And it would work like this: That did not quite work, since the key requirement was to navigation bar title and button transitions the same, as if the navigation bar was visible. keep So I thought… What if at the time of push or pop I could hide the existing navigation bar, and show the fake one instead? Investigation Before jumping straight to the coding, I decided to better understand how works behind the scene. That process involved two steps: UINavigationBar Reading through the UINavigationBar runtime header Checking what is inside navigation bar via Debug View Hierarchy That made me understand two things: Navigation bar itself is always positioned with equal to status bar height, but it’s childs are laid out with negative , to simulate as if the navigation bar had equal to 0. origin.y origin.y origin.y Navigation bar contains a child of class , which is responsible for drawing the actual background. In runtime headers I found that there is a getter . _UIBarBackground _backgroundView The position of navigation bar is offset vertically by the height of the status bar. The vertical position of the background view is negative, to simulate that navigation bar is position with origin.y equal to 0. Implementation After the investigation I had enough information to start hacking on that. I developed a mini-plan on how it should work. There are two cases I had to worry about: Transition from view controller, which has navigation bar visible, to the one which has navigation bar hidden, and back Transition from view controller, which has navigation bar hidden, to the one which has navigation bar visible, and back Let’s go through these cases in a little bit more detail. From visible to hidden and back When I push from the current view controller, which has the navigation bar visible, to the target view controller, which has the navigation bar hidden, I copy the navigation bar and add it to the current view controller’s view and I hide the background view on the original navigation bar, which is the subview of the navigation controller’s view. In this case I can guarantee, that the navigation bar background remains only in the current view controller, but not in the target view controller. See implementation. When I pop from the current view controller, which has the navigation bar hidden, to the target view controller, which has the navigation bar visible, I wait for the navigation transition to finish. At the end of the transition I remove the fake navigation bar from the target view controller’s view and unhide the background view on the original navigation bar. See implementation. From hidden to visible and back When I push from the current view controller, which has the navigation bar hidden, to the target view controller, which has the navigation bar visible, I copy navigation bar, but I add it to the target view controller only when is called. At the end of this push transition I have to remove the fake navigation bar from the target view controller and unhide the background view on the original navigation bar. viewWillAppear See implementation. When I pop from the current view controller, which has the navigation bar visible, to the target view controller, which has the navigation bar hidden, I copy the navigation bar and add it to the current view controller, and hide the background view on the original navigation bar. In case if transition is failed and we returned back to the current view controller, I add logic to remove fake navigation bar and unhide the background view on the original navigation bar on of the current view controller. viewWillAppear See implementation. Bugs and bug-fixes Surprisingly, there was only a single bug I’ve encountered and had to fight against. Since is the private API and is managed by Apple, there were cases when this view was hidden or unhidden by Apple. To prevent that and keep it in sync with my state, I’ve added logic to observe flag on background view and override it with my value, if it has unexpectedly changed. _backgroundView hidden See implementation. Conclusion To be honest, it is risky to include like this into the production application, since it uses Apple’s internal API which can change at any point of time. On the other hand, I had this logic in production for more than a year and did not encounter any bugs. hack If you have any questions, please drop me a line! Thank you for reading! If you liked this post, please hit ‘Clap’ (the 👏 button), so other people can read it too! 😉 , if you want to chat. Hit me on Twitter
