FragmentStateChanger.kt

package com.example.mvvm.ui

import androidx.fragment.app.FragmentManager
import androidx.fragment.app.FragmentTransaction
import com.example.mvvm.R
import com.example.mvvm.ui.base.BaseFragment
import com.example.mvvm.ui.base.BaseKey
import com.example.mvvm.ui.main.MainActivity
import com.zhuinden.simplestack.StateChange

/**
 * Created by Owner on 2017. 06. 29..
 */
class FragmentStateChanger(
    private val activity: MainActivity,
    private val fragmentManager: FragmentManager,
    private val containerId: Int
) {
    fun handleStateChange(stateChange: StateChange) {
        val fragmentTransaction: FragmentTransaction = fragmentManager.beginTransaction().disallowAddToBackStack()
        if (stateChange.direction == StateChange.FORWARD) {
            fragmentTransaction.setCustomAnimations(
                R.anim.slide_in_from_right,
                R.anim.slide_out_to_left,
                R.anim.slide_in_from_right,
                R.anim.slide_out_to_left
            )
        } else if (stateChange.direction == StateChange.BACKWARD) {
            fragmentTransaction.setCustomAnimations(
                R.anim.slide_in_from_left,
                R.anim.slide_out_to_right,
                R.anim.slide_in_from_left,
                R.anim.slide_out_to_right
            )
        }
        val previousState: List<BaseKey<*>> = stateChange.getPreviousKeys<BaseKey<*>>()
        val newState: List<BaseKey<*>> = stateChange.getNewKeys<BaseKey<*>>()
        for (oldKey in previousState) {
            val fragment = fragmentManager.findFragmentByTag(oldKey.fragmentTag)
            if (fragment != null) {
                if (!newState.contains(oldKey)) {
                    fragmentTransaction.remove(fragment)
                } else if (!fragment.isDetached) {
                    fragmentTransaction.detach(fragment)
                }
            }
        }
        for (newKey in newState) {
            var fragment: BaseFragment<*> = fragmentManager.findFragmentByTag(newKey.fragmentTag) as BaseFragment<*>
            if (newKey == stateChange.topNewKey()) {
                if (fragment.isRemoving) { // Fragments are quirky, they die asynchronously. Ignore if they're still there.
                    fragment = newKey.newFragment()
                    fragmentTransaction.replace(containerId, fragment, newKey.fragmentTag)
                } else if (fragment.isDetached) {
                    fragmentTransaction.attach(fragment)
                }
            } else {
                if (!fragment.isDetached) {
                    fragmentTransaction.detach(fragment)
                }
            }
            // bindViewModel here being captioned as Unreachable code
            fragment.bindViewModel(activity.backstackDelegate.getService(newKey, newKey.viewModelTag))
        }
        fragmentTransaction.commitNow()
    }

}