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 //   related to making sliding from the side fragment transition
 //   https://www.reddit.com/r/androiddev/comments/3b6fe8/slide_in_a_new_fragment_over_the_existing_one/
  //  he can call remove() or hide() after transition animation completes. He will also need to call popBackStack() twice and override onBackPressed. But all of this are workarounds./**/

inline fun FragmentManager.inTransaction(func: FragmentTransaction.() -> FragmentTransaction) {
    beginTransaction().func().commit()
}
class MainActivity : AppCompatActivity(), NavigationView.OnNavigationItemSelectedListener {

    private val mFirebaseAnalytics: FirebaseAnalytics? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
  
        if (savedInstanceState == null) {
             supportFragmentManager.inTransaction {
                    replace(R.id.container, ListFragment())
            }
        }
        supportFragmentManager.addOnBackStackChangedListener(this)
    }
  

    override fun onBackStackChanged() {
        shouldDisplayHomeUp()
    }

    override fun onSupportNavigateUp(): Boolean {   //  This method is called when the up button is pressed. Just pop the back stack.
        if (supportFragmentManager.backStackEntryCount > 0) {
            supportFragmentManager.popBackStack()
            return false
        }
        finish()
        return true
    }

    fun popAllBackStackIfNeeded(manager: FragmentManager) {
        if (manager.backStackEntryCount > 0) {
           manager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE)
        }
    }   

    fun shouldDisplayHomeUp() { //  Enable Up button only if there are entries in the back stack
        val canBack = supportFragmentManager.backStackEntryCount > 0
        supportActionBar?.setDisplayHomeAsUpEnabled(canBack)
    }
}

// It's only abstract to log from subclasses with corresponding _tag
// Replace `abstract` with `open` if don't need this functionality
abstract class BaseFragment: Fragment() {

    abstract fun log(message: String)

    val compositeDisposable: CompositeDisposable by lazy { CompositeDisposable() }

    override fun onPause() {
        super.onPause()
        log("onPause")
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
            cleanUp()
        }
    }

    override fun onStop() {
        super.onStop()
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            cleanUp()
        }
    }

    private fun cleanUp() {
        compositeDisposable.clear()
    }

    fun showMessage(tv: TextView?, msg: String? = null) {
        tv?.visibility = View.VISIBLE
        msg?.let { tv?.text = it }
    }
}

abstract class RealmBaseFragment : BaseFragment() {

    lateinit var realm: Realm

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        log("onCreate")
        realm = Realm.getDefaultInstance()
    }

    override fun onDestroy() {
        super.onDestroy()
        log("onDestroy")
        realm.close()
    }
}

class BaseFragment: Fragment() {

    companion object {
        fun newInstance(): BaseFragment {
            val fragment = BaseFragment()
            val bundle = Bundle()
            fragment.arguments = bundle
            return fragment
        }
    }

    override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        super.onCreateView(inflater, container, savedInstanceState)
        val rootView = inflater?.inflate(R.layout.fragment_main, container, false)
        return rootView
    }
}

// https://medium.com/thoughts-overflow/how-to-add-a-fragment-in-kotlin-way-73203c5a450b
inline fun FragmentManager.inTransaction(func: FragmentTransaction.() -> FragmentTransaction) {
    beginTransaction().func().commit()
}

fun FragmentManager.add(id: Int, fragment: Fragment, name: String? = null) {
    beginTransaction()
            .add(id, fragment)
            .run { if (name != null) addToBackStack(name) else return@run this }
            .commit()
}

fun FragmentManager.replace(id: Int, fragment: Fragment, name: String? = null) {
    beginTransaction()
            .replace(id, fragment)
            .run { if (name != null) addToBackStack(name) else return@run this }
            .commit()
}