android - 为什么要支持FragmentManager.saveFragmentInstanceState(selectedFragment)! =空

我用A，B，C的三个按钮完成了BottomNavigationView。触摸B或C后，旋转屏幕将返回A。
我已经尝试过如下所示：

private fun bottomNavView() {
    bottom_nav_view.setOnNavigationItemSelectedListener {

        when (it.itemId) {
            R.id.navigation_home -> selectedFragment = HomeFragment.newInstance()

            R.id.navigation_add_photos -> selectedFragment = AddPhotosFragment.newInstance()


            R.id.navigation_profile -> selectedFragment = ProfileFragment.newInstance()
        }

        savedState =
            supportFragmentManager.saveFragmentInstanceState(selectedFragment)!!
        vm.setStateOfRotate(savedState)
        supportFragmentManager.beginTransaction()
            .replace(R.id.nav_host_fragment, selectedFragment)
            .commit()

        true
    }

上面会出现如下错误：

java.lang.IllegalStateException: Fragment AddPhotosFragment{e1f2f47 (8c74a7e8-3af7-4290-be4a-f16f8ea6229f)} is not currently in the FragmentManager

        at androidx.fragment.app.FragmentManagerImpl.saveFragmentInstanceState(FragmentManagerImpl.java:442)

        at com.squall.searchdesigner.MainActivity$bottomNavView$1.onNavigationItemSelected(MainActivity.kt:92)

92线表示↓

savedState = supportFragmentManager.saveFragmentInstanceState（selectedFragment）!!

================================================== =======

private fun bottomNavView() {
    bottom_nav_view.setOnNavigationItemSelectedListener {

        when (it.itemId) {
            R.id.navigation_home -> selectedFragment = HomeFragment.newInstance()

            R.id.navigation_add_photos -> selectedFragment = AddPhotosFragment.newInstance()


            R.id.navigation_profile -> selectedFragment = ProfileFragment.newInstance()
        }


        vm.setStateOfRotate(savedState)
        supportFragmentManager.beginTransaction()
            .replace(R.id.nav_host_fragment, selectedFragment)
            .commit()
        savedState =
            supportFragmentManager.saveFragmentInstanceState(selectedFragment)!!
        true
    }
}

但是，如果让：
savedState = supportFragmentManager.saveFragmentInstanceState（selectedFragment）!!

放在下面的下面
supportFragmentManager.beginTransaction（）。replace（R.id.nav_host_fragment，selectedFragment）.commit（）

savedState变为null。

savedState = null

问题是什么？

最佳答案

后来，我解决了使用伴侣对象存储所选片段的问题，如下所示：

companion object{
    var selectedFragment: Fragment? = null

}