主要重写onSaveInstanceState保存Fragment，切换后进行赋值

1.定义四个Fragment。在切换深色\浅色模式时会重新创建内存

    private var aFragment:Fragment? = null
    private var bFragment:Fragment? = null
    private var cFragment:Fragment? = null
    private var dFragment:Fragment? = null

2.在onCreate里面进行判断是第一次进入还是重新绘制

override fun onCreate(savedInstanceState: Bundle?) {
  。。。
       if(savedInstanceState == null){ // 判断是否是第一次绘制Activity
            // 设置默认页面
            replaceZhanTingPageFragment()
            // 更改默认页面的Icon为b页的对应icon
            binding.bottomNavigationView.selectedItemId = binding.bottomNavigationView.menu.getItem(1).itemId

        }else{
            // 如果切换了深色/浅色模式 将之前存放的fragment状态赋予新创建的Fragment
            aFragment= supportFragmentManager.getFragment(savedInstanceState,"aFragmentBundle")
            bFragment= supportFragmentManager.getFragment(savedInstanceState,"bFragmentBundle")
            cFragment= supportFragmentManager.getFragment(savedInstanceState,"cFragmentBundle")
            dFragment= supportFragmentManager.getFragment(savedInstanceState,"dFragmentBundle")
        }
        initView()
  。。。
}

3.每次绘制都要进行赋值按钮

    private fun initView(){
        binding.bottomNavigationView.setOnItemSelectedListener(NavigationBarView.OnItemSelectedListener { item ->
            val id = item.itemId
            when (id) {
                R.id.a-> replaceA()
                R.id.b-> replaceB()
                R.id.c-> replaceC()
                R.id.d-> replaceD()
            }
            true
        })
    }

4.四个初始化Fragment页面。防止重新创建布局重叠

private fun replaceA {
        val transaction = supportFragmentManager.beginTransaction()
        if(aFragment == null){
            aFragment = AFragment()
            transaction.add(R.id.framelayout_container,
                aFragment as AFragment, "aFragment")
        }
        hideFragment(transaction)
        transaction.show(aFragment!!)
        transaction.commit()
    }

private fun replaceB {
        val transaction = supportFragmentManager.beginTransaction()
        if(bFragment == null){
            bFragment = BFragment()
            transaction.add(R.id.framelayout_container,
                bFragment as BFragment, "bFragment")
        }
        hideFragment(transaction)
        transaction.show(bFragment!!)
        transaction.commit()
    }

private fun replaceC {
        val transaction = supportFragmentManager.beginTransaction()
        if(cFragment == null){
            cFragment = CFragment()
            transaction.add(R.id.framelayout_container,
                cFragment as CFragment, "cFragment")
        }
        hideFragment(transaction)
        transaction.show(cFragment!!)
        transaction.commit()
    }

private fun replaceD {
        val transaction = supportFragmentManager.beginTransaction()
        if(dFragment == null){
            dFragment = DFragment()
            transaction.add(R.id.framelayout_container,
                dFragment asDFragment, "dFragment")
        }
        hideFragment(transaction)
        transaction.show(dFragment!!)
        transaction.commit()
    }

5.隐藏所有的fragment

    private fun hideFragment(transaction: FragmentTransaction) {
        if (aFragment != null) {
            transaction.hide(aFragment!!)
        }
        if (bFragment != null) {
            transaction.hide(bFragment!!)
        }
        if (cFragment != null) {
            transaction.hide(cFragment!!)
        }
        if dFragment != null) {
            transaction.hide(dFragment!!)
        }
    }