kotlin android底部导航片段setRetainInstance（true）

Question

我创建了一个带有4个片段的底部导航项目，并在qponFragment的onCreate（）中放置了setHasOptionsMenu（true），以便在切换片段后保持qponFragment的相同内容。但是，它不起作用，qponFragment在从其他片段切换回来后仍然刷新。请帮助修复它，找出我的代码有什么问题。

这里有MainActivity.kt的代码

class MainActivity : AppCompatActivity() {

    private var mFirebaseAnalytics: FirebaseAnalytics? = null

    private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
        when (item.itemId) {
            R.id.navigation_qpon -> {
                //message.setText(R.string.title_qpon)
                actionBarIcon(R.drawable.ic_title_black)
                createQponFragment()
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_me-> {
                //message.setText(R.string.title_me)
                actionBarIcon(R.drawable.logged)
                createMeFragment()
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_tool -> {
                //message.setText(R.string.title_tool)
                actionBarIcon(R.drawable.logged)
                createToolFragment()
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_tutorial -> {
                //message.setText(R.string.title_tutorial)
                actionBarIcon(R.drawable.tutorial)
                createTutorialFragment()
                return@OnNavigationItemSelectedListener true
            }
        }
        false
    }

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

        // Obtain the FirebaseAnalytics instance.
        mFirebaseAnalytics = FirebaseAnalytics.getInstance(this)

        actionBarIcon(R.drawable.ic_title_black)


        createQponFragment()
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
    }

    fun actionBarIcon(imageName:Int) {

        setSupportActionBar(findViewById(R.id.my_toolbar))
        my_toolbar.setLogo(imageName)

        if (imageName == R.drawable.ic_title_black) {
            my_toolbar.setTitle("")
        }

        if (imageName == R.drawable.logged) {

            my_toolbar.setTitle("login name")

        }
        if (imageName == R.drawable.tutorial) {

            my_toolbar.setTitle("Tutorial")

        }


    }

    val manager = supportFragmentManager

    fun createQponFragment() {
        val transaction = manager.beginTransaction()
        val fragment = qponFragment()
        transaction.replace(R.id.fragmentholder,fragment)
        transaction.addToBackStack(null)
        transaction.commit()
    }

    fun createMeFragment() {
        val transaction = manager.beginTransaction()
        val fragment = meFragment()
        transaction.replace(R.id.fragmentholder,fragment)
        transaction.addToBackStack(null)
        transaction.commit()
    }

    fun createToolFragment() {
        val transaction = manager.beginTransaction()
        val fragment = toolFragment()
        transaction.replace(R.id.fragmentholder,fragment)
        transaction.addToBackStack(null)
        transaction.commit()
    }

    fun createTutorialFragment() {
        val transaction = manager.beginTransaction()
        val fragment = tutorialFragment()
        transaction.replace(R.id.fragmentholder,fragment)
        transaction.addToBackStack(null)
        transaction.commit()
    }
}

这里有qponFragment.kt的代码

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setRetainInstance(true)
}

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                          savedInstanceState: Bundle?): View? {

    setHasOptionsMenu(true)

    return inflater.inflate(R.layout.fragment_qpon, container, false)

}

Answer 1

如果您尝试保留片段实例而不是每次都创建新的实例，它应该可以工作。请找到代码bel

class MainActivity : AppCompatActivity() {

private var mFirebaseAnalytics: FirebaseAnalytics? = null

private var meFragment:Fragment? = null
var toolFragment :Fragment? =null
var qponFragment:Fragment? =null
var tutorialFragment:Fragment? = null
private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
    when (item.itemId) {
        R.id.navigation_qpon -> {
            //message.setText(R.string.title_qpon)
            actionBarIcon(R.drawable.ic_title_black)
            createQponFragment()
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_me-> {
            //message.setText(R.string.title_me)
            actionBarIcon(R.drawable.logged)
            createMeFragment()
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_tool -> {
            //message.setText(R.string.title_tool)
            actionBarIcon(R.drawable.logged)
            createToolFragment()
            return@OnNavigationItemSelectedListener true
        }
        R.id.navigation_tutorial -> {
            //message.setText(R.string.title_tutorial)
            actionBarIcon(R.drawable.tutorial)
            createTutorialFragment()
            return@OnNavigationItemSelectedListener true
        }
    }
    false
}

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

    // Obtain the FirebaseAnalytics instance.
    mFirebaseAnalytics = FirebaseAnalytics.getInstance(this)

    actionBarIcon(R.drawable.ic_title_black)


    createQponFragment()
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
}

fun actionBarIcon(imageName:Int) {

    setSupportActionBar(findViewById(R.id.my_toolbar))
    my_toolbar.setLogo(imageName)

    if (imageName == R.drawable.ic_title_black) {
        my_toolbar.setTitle("")
    }

    if (imageName == R.drawable.logged) {

        my_toolbar.setTitle("login name")

    }
    if (imageName == R.drawable.tutorial) {

        my_toolbar.setTitle("Tutorial")

    }


}

val manager = supportFragmentManager

fun createQponFragment() {
    val transaction = manager.beginTransaction()
    if(qponFragment == null) qponFragment = qponFragment()   // *****code changed here***********
    transaction.replace(R.id.fragmentholder,qponFragment)
    transaction.addToBackStack(null)
    transaction.commit()
}

fun createMeFragment() {
    val transaction = manager.beginTransaction()
    if(meFragment == null) meFragment = meFragment()      
    transaction.replace(R.id.fragmentholder,meFragment)
    transaction.addToBackStack(null)
    transaction.commit()
}

fun createToolFragment() {
    val transaction = manager.beginTransaction()
    if(toolFragment == null) toolFragment = toolFragment() 
    transaction.replace(R.id.fragmentholder,toolFragment)
    transaction.addToBackStack(null)
    transaction.commit()
}

fun createTutorialFragment() {
    val transaction = manager.beginTransaction()
    val fragment = tutorialFragment()
    if(tutorialFragment == null) toolFragment = tutorialFragment()   // *****code changed here***********
    transaction.replace(R.id.fragmentholder,tutorialFragment)
    transaction.replace(R.id.fragmentholder,fragment)
    transaction.addToBackStack(null)
    transaction.commit()
   }
}

显示错误：