切换主题后已调用onDestroyView()的Fragment UI仍重叠显示

问题成因 #

切换系统深色主题时，Android会销毁并重建MainActivity（这是系统正常行为），你的代码存在两个核心问题：

1. Fragment实例不匹配：Activity重建时，savedInstanceState != null，代码里的全局appointmentsFragment等变量会重新创建新实例，但FragmentManager会自动恢复之前状态中存在的所有Fragment实例（包括之前被detach的），导致全局变量指向的实例和FragmentManager实际管理的实例并非同一个。
2. 无效的detach操作：后续切换标签时，你detach的是全局变量的新实例（该实例从未被添加到FragmentManager），而FragmentManager恢复的旧实例依然存在于容器中，多个Fragment的UI层叠显示，最终引发错乱。

此外，重建后FragmentManager已存在对应tag的实例，执行attach操作时旧的detached实例未被清理，进一步加重了UI重叠问题。

解决方案 #

方案1：修复实例匹配（基于detach/attach逻辑） #

初始化时优先从FragmentManager恢复已存在的Fragment实例，确保全局变量与FragmentManager管理的实例一致：

class MainActivity : AppCompatActivity() {
    private val appointmentsFragment by lazy {
        supportFragmentManager.findFragmentByTag(AppointmentsFragment::class.java.simpleName) ?: AppointmentsFragment()
    }
    private val chatFragment by lazy {
        supportFragmentManager.findFragmentByTag(ChatFragment::class.java.simpleName) ?: ChatFragment()
    }
    private val journalFragment by lazy {
        supportFragmentManager.findFragmentByTag(JournalFragment::class.java.simpleName) ?: JournalFragment()
    }
    private val profileFragment by lazy {
        supportFragmentManager.findFragmentByTag(ProfileFragment::class.java.simpleName) ?: ProfileFragment()
    }
    private lateinit var selectedFragment: Fragment

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        if (savedInstanceState == null) {
            supportFragmentManager.commit {
                setReorderingAllowed(true)
                add(R.id.fragmentContainer, appointmentsFragment, AppointmentsFragment::class.java.simpleName)
                selectedFragment = appointmentsFragment
            }
        } else {
            // 重建时从容器中获取当前显示的Fragment
            selectedFragment = supportFragmentManager.findFragmentById(R.id.fragmentContainer) ?: appointmentsFragment
        }

        bottomNavigation.setOnNavigationItemSelectedListener { item ->
            val selection = when(item.itemId) {
                R.id.appointments -> appointmentsFragment
                R.id.chat -> chatFragment
                R.id.journal -> journalFragment
                R.id.profile -> profileFragment
                else -> error("Only 4 tabs are available")
            }

            if (selectedFragment == selection) return@setOnNavigationItemSelectedListener true

            supportFragmentManager.commit {
                setReorderingAllowed(true)
                detach(selectedFragment)
                attach(selection)
                selectedFragment = selection
            }
            true
        }
    }
}

方案2：改用hide/show实现标签切换（更稳定） #

对于底部标签这种频繁切换的场景，hide/show仅隐藏UI、不会销毁Fragment视图，性能更优且能避免重建时的实例匹配问题：

class MainActivity : AppCompatActivity() {
    private val fragments = listOf(
        AppointmentsFragment::class.java.simpleName to AppointmentsFragment(),
        ChatFragment::class.java.simpleName to ChatFragment(),
        JournalFragment::class.java.simpleName to JournalFragment(),
        ProfileFragment::class.java.simpleName to ProfileFragment()
    ).associate { (tag, frag) ->
        tag to (supportFragmentManager.findFragmentByTag(tag) ?: frag)
    }
    private lateinit var selectedFragment: Fragment

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        if (savedInstanceState == null) {
            supportFragmentManager.commit {
                setReorderingAllowed(true)
                // 一次性添加所有Fragment，默认隐藏非初始页
                fragments.values.forEach { frag ->
                    add(R.id.fragmentContainer, frag, frag::class.java.simpleName)
                    if (frag !is AppointmentsFragment) hide(frag)
                }
                selectedFragment = fragments[AppointmentsFragment::class.java.simpleName]!!
            }
        } else {
            // 重建时找到当前可见的Fragment
            selectedFragment = fragments.values.firstOrNull { it.isVisible } 
                ?: fragments[AppointmentsFragment::class.java.simpleName]!!
            // 确保其他Fragment都处于隐藏状态
            fragments.values.filter { it != selectedFragment }.forEach { frag ->
                if (frag.isVisible) {
                    supportFragmentManager.commit { hide(frag) }
                }
            }
        }

        bottomNavigation.setOnNavigationItemSelectedListener { item ->
            val selection = when(item.itemId) {
                R.id.appointments -> fragments[AppointmentsFragment::class.java.simpleName]!!
                R.id.chat -> fragments[ChatFragment::class.java.simpleName]!!
                R.id.journal -> fragments[JournalFragment::class.java.simpleName]!!
                R.id.profile -> fragments[ProfileFragment::class.java.simpleName]!!
                else -> error("Only 4 tabs are available")
            }

            if (selectedFragment == selection) return@setOnNavigationItemSelectedListener true

            supportFragmentManager.commit {
                setReorderingAllowed(true)
                hide(selectedFragment)
                show(selection)
            }
            selectedFragment = selection
            true
        }
    }
}

内容的提问来源于stack exchange，提问作者Orbit