swift - ViewController slide animation

Question

I want to create an animation like the iOS app facebook at tabswitch^[1]. I have already tried to develop some kind of animation, the problem that occurs is that the old view controller becomes invisible directly on the switch, instead of fading out slowly while the new controller is sliding in fast.

I've found this SO question How to animate Tab bar tab switch with a CrossDissolve slide transition? but the as correct marked solution does not really work for me (it is not a slide it is a fade transition). What I'd also like to get is the function to make slide left or right to switch the tabs. Like it was on a older version of facebook.

What I've got so far is this:

extension TabBarController: UITabBarControllerDelegate  {
    func tabBarController(_ tabBarController: UITabBarController, shouldSelect viewController: UIViewController) -> Bool {
        guard let fromView = selectedViewController?.view,
              let toView = viewController.view else { return false }
        if fromView != toView {
            toView.transform = CGAffineTransform(translationX: -90, y: 0)
            UIView.animate(withDuration: 0.25, delay: 0.0, options: .curveEaseInOut, animations: {
                toView.transform = CGAffineTransform(translationX: 0, y: 0)
            })
        }; return true
    }
}

---

class TabBarController: UITabBarController {
    override func viewDidLoad() {
        super.viewDidLoad()
        delegate = self
    }
}

How to fix this?

---

^[1] I would very much like to add a gif from the Facebook app. The problem is that I don't want to censor the video and just reveal too much of my data. (Even if fb already has them). Also on youtube I didn't find a suitable recording. Please try it yourself in the fb app in iOS.

Answer 1

I've never seen Facebook so I don't know what the animation is. But you can have any animation you like when a tab bar controller changes its tab (child view controller), coherently and without any hacks, using the built-in mechanism that Apple provides for adding custom animation to a transition between view controllers. It's called custom transition animation.

Apple first introduced this mechanism in 2013. Here's a link to their video about it: https://developer.apple.com/videos/play/wwdc2013/218/

I immediately adopted this in my apps, and I think it makes them look a lot spiffier. Here's a demo of a tab bar controller custom transition that I like:

The really cool thing is that once you've decided what animation you want, making the transition interactive (i.e. drive it with a gesture instead of a button click) is easy:

Now, you might be saying: Okay, but that's not quite the animation I had in mind. No problem! Once you've got the hang of the custom transition architecture, changing the animation to anything you like is easy. In this variant, I just commented out one line so that the "old" view controller doesn't slide away:

So let your imagination run wild! Adopt custom transition animations, the way that iOS intends.