如题，点击底部导航栏的item时，需要点击两次才会进行fragment的切换，代码如下：

class MainActivity : AppCompatActivity() {

    private val TAG = " MainActivity "

    private lateinit var _binding : ActivityMainBinding

    private val binding get() = _binding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        _binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
        // 打开默认第一页
        switchFragment(FragmentCreator.getFragment(FragmentCreator.INDEX_TOP1))
        // 监听事件
        initListener()
        logD(TAG," onCreate() ")
    }

    // 监听事件
    private fun initListener() {
        // 监听底部导航栏的点击事件
        binding.bottomNavigationBar.setOnNavigationItemReselectedListener {
            when(it.itemId){
                R.id.bottom_one -> {
                    switchFragment(FragmentCreator.getFragment(FragmentCreator.INDEX_TOP1))
                    logD(TAG,"one")
                }
                R.id.bottom_two -> {
                    switchFragment(FragmentCreator.getFragment(FragmentCreator.INDEX_TOP2))
                    logD(TAG,"two")
                }
                R.id.bottom_three -> {
                    switchFragment(FragmentCreator.getFragment(FragmentCreator.INDEX_TOP3))
                    logD(TAG,"three")
                }
                R.id.bottom_four -> {
                    switchFragment(FragmentCreator.getFragment(FragmentCreator.INDEX_TOP4))
                    logD(TAG,"four")
                }
            }
        }
    }

    // 切换Fragment, 传入Fragment实例参数
    private fun switchFragment(fragment: Fragment?) {
        val supportFragmentManager = supportFragmentManager
        val transaction = supportFragmentManager.beginTransaction()
        transaction.replace(R.id.main_container, fragment!!)
        transaction.commit()
    }
}

class FragmentCreator {

    companion object{
        const val INDEX_TOP1 = 1
        const val INDEX_TOP2 = 2
        const val INDEX_TOP3 = 3
        const val INDEX_TOP4 = 4

        private val top1: HomeFragment by lazy {
            HomeFragment.instance
        }

        private val top2: Top2Fragment by lazy {
            Top2Fragment.instance
        }

        private val top3: Top3Fragment by lazy {
            Top3Fragment.instance
        }

        private val top4: Top4Fragment by lazy {
            Top4Fragment.instance
        }

        fun getFragment(index: Int): Fragment?{
            when(index){
                INDEX_TOP1 -> {
                    return top1
                }
                INDEX_TOP2 -> {
                    return top2
                }
                INDEX_TOP3 -> {
                    return top3
                }
                INDEX_TOP4 -> {
                    return top4
                }

            }
            return null
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorBackground">

    <FrameLayout
        android:id="@+id/main_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />


    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_navigation_bar"
        android:layout_width="match_parent"
        android:layout_height="56dp"
        app:labelVisibilityMode="labeled"
        app:itemTextColor="@drawable/selected_color"
        android:layout_alignParentBottom="true"
        app:menu="@menu/my_navigation_items"/>

</RelativeLayout>

其余Fragment类似

class HomeFragment: Fragment() {

    private lateinit var _binding:FragmentHomeBinding
    
    private val binding get() = _binding

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        _binding = FragmentHomeBinding.inflate(layoutInflater,container,false)
        return binding.root
    }

    companion object{
        val instance = HomeFragment()
    }
}