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

Question

我用 A、B、C 三个按钮做了一个 BottomNavigationView。触摸 B 或 C 后，旋转屏幕将返回到 A。我试过如下:

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行表示↓

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)!!

放在下一行的下面supportFragmentManager.beginTransaction().replace(R.id.nav_host_fragment, selectedFragment) .commit()

savedState 变为空。

保存状态 = null

问题是什么？

Answer 1

后来，我解决了使用伴生对象存储选定 fragment 的问题，如下所示:

companion object{
    var selectedFragment: Fragment? = null

}