Create fragments and implement fragment switching

import android.support.design.internal.BottomNavigationMenuView

import android.support.design.widget.BottomNavigationView

import android.annotation.SuppressLint

import android.support.v4.app.Fragment

import android.view.MenuItem

import io.eelo.appinstaller.categories.CategoriesFragment

import io.eelo.appinstaller.home.HomeFragment

import io.eelo.appinstaller.search.SearchFragment

import io.eelo.appinstaller.settings.SettingsFragment

import io.eelo.appinstaller.updates.UpdatesFragment

import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

class MainActivity : AppCompatActivity(), BottomNavigationView.OnNavigationItemSelectedListener {

    private var currentFragment: Fragment? = null

    private val homeFragment = HomeFragment()

    private val categoriesFragment = CategoriesFragment()

    private val searchFragment = SearchFragment()

    private val updatesFragment = UpdatesFragment()

    private val settingsFragment = SettingsFragment()

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)

        setContentView(R.layout.activity_main)

        // Show the home fragment by default

        showFragment(homeFragment)

        bottom_navigation_view.setOnNavigationItemSelectedListener(this)

        // Disable shifting of nav bar items

        removeShiftMode(bottom_navigation_view)

    }

    override fun onNavigationItemSelected(item: MenuItem): Boolean {

        when {

            item.itemId == R.id.menu_home -> {

                showFragment(homeFragment)

                return true

            }

            item.itemId == R.id.menu_categories -> {

                showFragment(categoriesFragment)

                return true

            }

            item.itemId == R.id.menu_search -> {

                showFragment(searchFragment)

                return true

            }

            item.itemId == R.id.menu_updates -> {

                showFragment(updatesFragment)

                return true

            }

            item.itemId == R.id.menu_settings -> {

                showFragment(settingsFragment)

                return true

            }

        }

        return false

    }

    private fun showFragment(fragment: Fragment) {

        supportFragmentManager

                .beginTransaction()

                .replace(R.id.frame_layout, fragment)

                .commit()

        currentFragment = fragment

    }

    @SuppressLint("RestrictedApi")

    private fun removeShiftMode(bottomNavigationView: BottomNavigationView) {

        val menuView = bottomNavigationView.getChildAt(0) as BottomNavigationMenuView

package io.eelo.appinstaller.categories

import android.os.Bundle

import android.support.v4.app.Fragment

import android.view.LayoutInflater

import android.view.View

import android.view.ViewGroup

import io.eelo.appinstaller.R

class CategoriesFragment : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        val view = inflater.inflate(R.layout.fragment_categories, container, false)

        return view

    }

}

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package io.eelo.appinstaller.home

import android.os.Bundle

import android.support.v4.app.Fragment

import android.view.LayoutInflater

import android.view.View

import android.view.ViewGroup

import io.eelo.appinstaller.R

class HomeFragment : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        val view = inflater.inflate(R.layout.fragment_home, container, false)

        return view

    }

}

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package io.eelo.appinstaller.search

import android.os.Bundle

import android.support.v4.app.Fragment

import android.view.LayoutInflater

import android.view.View

import android.view.ViewGroup

import io.eelo.appinstaller.R

class SearchFragment : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        val view = inflater.inflate(R.layout.fragment_search, container, false)

        return view

    }

}

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package io.eelo.appinstaller.settings

import android.os.Bundle

import android.support.v4.app.Fragment

import android.view.LayoutInflater

import android.view.View

import android.view.ViewGroup

import io.eelo.appinstaller.R

class SettingsFragment : Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {

        val view = inflater.inflate(R.layout.fragment_settings, container, false)

        return view

    }

}

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