No screen transition animation in case of backstack depth is not changed

[kvaster]

In some cases we need to replace current screen - i.e. to pop back current screen and push new one:

navigator.pop()
navigator.goTo(NewScreen)

And it looks like transition animation is not used in such case. But it's all OK in following:

navigator.pop()
navigator.pop()
navigator.goTo(NewScreen)

and NOT ok:

navigator.pop()
navigator.pop()
navigator.goTo(NewScreen)

I've create example app to show the problem: https://github.com/kvaster/circuit-test

[stagg]

I believe I follow whats happening, not sure whats suppose to be different between your last two examples? Thanks for the sample!
The navigation animations are state driven and inherently tied to the backstack depth as a signal for moving forward or backwards. With the previous (n-1) and current (n) states informing which screens are shown.

So a question I have would be around whats the expected animation when "replacing" the current screen?

• Does the pop screen animate away, and then animate in the goTo screen?
  • If this is the case you could reasonably handle this result in the previous screen presenter and then goTo
• Does the goTo screen animate in over the current screen, which would be removed from the back stack afterwards?
  • Leaving the new screen you could skip over the "replaced" screen using popUntil, with some edge cases for back handling
  • Custom navigator might help here too
  • We don't really have a good way to "mutate" the backstack in place for this case though
• Something else?

[kvaster]

Telling the truth, I was expected transition animation any time 'Screen' is changed. It may be a question which one enter or exit... Now it plays exit animation any time new depth < current depth and enter animation when new depth > current depth. And when new depth = current depth, screen changes instantly.

Also I'm not mutating backstack in place - i'm using pop and goTo.

My use case is to show 'Create new' screen (it's a one screen flow) and after that to go to 'Edit screen', but going back should go to screen before 'Create new'. And it would be really nice to keep animations...

[kvaster]

I found the code in AnimatedNavDecorator:

  val animatedNavEvent =
    when {
      !sameRoot -> AnimatedNavEvent.RootReset
      diff > 0 -> AnimatedNavEvent.GoTo
      diff < 0 -> AnimatedNavEvent.Pop
      // Somehow the back stack has not changed?
      else -> return@spec EnterTransition.None togetherWith ExitTransition.None
    }

It means it really detectes changes not only to backStackDepth, but to args aswell. And it will be really good to be able to animate diff=0 aswell (changing diff > 0 to diff >= 0 works good).

And it looks like I can create custom AnimatedNavDecoration, but it will be good to have this customisable using original implementation.

[stagg]

it will be really good to be able to animate diff=0

I suspect there was a reason for not animating here, but will need to dig around and verify. If there isn't anything will look at adding in a new AnimatedNavEvent (and transition) for the case.

My use case is to show 'Create new' screen (it's a one screen flow) and after that to go to 'Edit screen', but going back should go to screen before 'Create new'. And it would be really nice to keep animations...

This is what I was wondering about to determine the expected animations in this case, thank you!
Going to the Edit screen should still be an AnimatedNavEvent.GoTo and then we'd get the expected forward animation. It's that on the navigation back the 'Create new' should be skipped/dropped from the backstack.

I think this is a deeper backstack/navigation change, as there'd need to be an ability to indicate when to skip specific screens/records during any pop event. Potentially something a NavigationInterceptor should do 🤔

Example with a custom navigator:

 val skippingNavigator = remember {
  object : Navigator by navigator {
    override fun pop(result: PopResult?): Screen? {
      return Snapshot.withMutableSnapshot {
        // Skip over next screen if its Screen2
        if (peekBackStack.getOrNull(1) is TabScreen.Screen2) {
          navigator.pop()
        }
        navigator.pop(result)
      }
    }
  }
}

skip2.mp4

[kvaster]

I think that the only reason for not animating such case is already described in the code: "Somehow the back stack has not changed?"
But it can change.

But I see some problems:

• what if I will do pop+goto to the same screen. I.e. backstack change was triggered, but it's state now is really same
• what animation should be played if I will do pop+pop+goto ? In this case backward animation will be used, but, probably, forward should be used....