NavigationManager.kt

class NavigationManager {

    lateinit var router: Router
    var childRouter: Router? = null
    
    //region Generic
    fun init(router: Router) {
        this.router = router
        if (!router.hasRootController()) {
            router.setRoot(RouterTransaction.with(SplashController()))
        }
    }

    fun back(): Boolean {
        return router.handleBack()
    }
    
    fun restartSplash() {
        router.setRoot(RouterTransaction.with(SplashController()))
        childRouter = null
    }
    //endregion
    
    //region BottomNavigationView bits
    
    @IdRes
    private var currentSelectedItemId: Int = -1
    private var routerStates = SparseArray<Bundle>()
    
    fun initHome(controller: Controller, container: ViewGroup, navbar: BottomNavigationView) {
        childRouter = controller.getChildRouter(container)

        navbar.setOnNavigationItemSelectedListener { onNavigationItemSelected(it) }

        if (routerStates.size() == 0) {
            // Select the first item
            currentSelectedItemId = R.id.bottombar_one
            home_dashboard()
        } else {
            // We have something in the back stack. Maybe an orientation change happen?
            // We can just rebind the current router
            childRouter?.rebindIfNeeded()
        }
    }
    
    private fun onNavigationItemSelected(item: MenuItem): Boolean {
        if (currentSelectedItemId == item.itemId) return true

        saveStateFromCurrentTab(currentSelectedItemId)
        currentSelectedItemId = item.itemId
        clearStateFromChildRouter()
        val bundleState = tryToRestoreStateFromNewTab(currentSelectedItemId)

        if (bundleState is Bundle) {
            childRouter?.restoreInstanceState(bundleState)
            childRouter?.rebindIfNeeded()
            return true
        }

        when (item.itemId) {
            R.id.bottombar_one -> bottombar_one()
            R.id.bottombar_two -> bottombar_two()
            R.id.bottombar_three -> bottombar_three()
            R.id.bottombar_four -> bottombar_four()
            else -> return false
        }

        return true
    }
    
    private fun clearStateFromChildRouter() {
        childRouter?.apply {
            setPopsLastView(true) /* Ensure the last view can be removed while we do this */
            popToRoot()
            popCurrentController()
            setPopsLastView(false)
        }
    }

    private fun saveStateFromCurrentTab(itemId: Int) {
        val routerBundle = Bundle()
        childRouter?.saveInstanceState(routerBundle)
        routerStates.put(itemId, routerBundle)
    }

    [ bottombar_one() ... bottombar_four() implementations, calling childRouter?.setRoot() with the appropriate subpage controller

    //endregion
}