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我想知道如何创建类似于 instagram 的 backstack 的东西。

我有一个应用程序，它有一个底部导航栏，它打开 3 个不同的片段（、、、MapFragment）HostFragment，ProfileFragment在里面，当用户单击一个按钮时HostFragment，我创建了一个名为 的新片段：Host2Fragment

    Tab1          Tab2           Tab3
[MapFragment][HostFragment][ProfileFragment]
                   .
                   .
                   .
             [Host2Fragment]

一些例子：

注意：Tab1/MapFragment 是主片段

  • 用户已开启Host2Fragment，按返回可返回HostFragment
    • 再次按下返回到Tab1
  • 用户在Host2Fragment，点击Tab2回到HostFragment

本质上，按下返回，应该只是剥离所有层回到主片段（Tab1），并在这样做的同时更新底部导航栏。我还想保留片段的状态，这样当我点击返回它时，它应该仍然处于相同的状态

到目前为止我所拥有的：

主要活动

class MainActivity : AppCompatActivity(){
    private val mapFragment: Fragment = MapFragment()
    private val hostFragment: Fragment = HostFragment()
    private val profileFragment: Fragment = ProfileFragment()
    private val fm = supportFragmentManager
    private var activeFragment = mapFragment

    lateinit var toolbar: ActionBar
    private val mOnNavigationItemSelectedListener =
        BottomNavigationView.OnNavigationItemSelectedListener { item ->

            when (item.itemId) {
            R.id.navigation_map -> {
                fm.beginTransaction().hide(activeFragment).show(mapFragment).commit()
                activeFragment = mapFragment
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_host -> {
                fm.beginTransaction().hide(activeFragment).show(hostFragment).commit()
                activeFragment = hostFragment
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_profile -> {
                fm.beginTransaction().hide(activeFragment).show(profileFragment).commit()
                activeFragment = profileFragment
                return@OnNavigationItemSelectedListener true
            }
        }
        false
    }

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

        fm.beginTransaction().add(R.id.container, profileFragment, "profileFragment").hide(profileFragment).commit()
        fm.beginTransaction().add(R.id.container, hostFragment, "hostFragment").hide(hostFragment).commit()
        fm.beginTransaction().add(R.id.container, mapFragment, "mapFragment").commit()

        toolbar = supportActionBar!!
        val bottomNavigation: BottomNavigationView = findViewById(R.id.navigationView)
        bottomNavigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)

        //the home Fragment
        bottomNavigation.selectedItemId = R.id.navigation_map
    }
}

主机片段

class HostFragment : Fragment(){
    companion object {
        private const val TAG = "HostFragment"
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        val view = inflater.inflate(R.layout.fragment_host, container, false)
        return view
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val button = view.findViewById(R.id.button) as Button
        button.setOnClickListener{
            Log.d(HostFragment.TAG, "Clicked Host button")
            fragmentManager?.beginTransaction()?.add(R.id.container, Host2Fragment(), "host2Fragment")?.commit()
        }
    }
}
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