Android 导航组件 - 向上导航打开相同的片段

Question

我在执行时遇到问题

findNavController(R.id.main_nav_host).navigateUp()

或

findNavController(R.id.main_nav_host).popBackStack()

它reopens/navigates 返回到 same/current 片段，而不是返回到后台堆栈中的最后一个片段。

有人能为我指明正确的方向吗？

导航图：

<navigation xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:app="http://schemas.android.com/apk/res-auto"
            xmlns:tools="http://schemas.android.com/tools"
            android:id="@+id/main_navigation_root"
            app:startDestination="@+id/dest_main">

    <fragment
            android:id="@+id/dest_main"
            android:name="com.example.popularmovies.ui.main.views.MainMoviesFragment"
            android:label="@string/home"
            tools:layout="@layout/fragment_main_movies">

        <action
                android:id="@+id/action_dest_main_to_dest_movie_details"
                app:destination="@+id/dest_movie_details"
                app:enterAnim="@anim/slide_in_right"
                app:exitAnim="@anim/slide_out_left"
                app:popEnterAnim="@anim/slide_in_left"
                app:popExitAnim="@anim/slide_out_right" />

    </fragment>

    <fragment
            android:id="@+id/dest_movie_details"
            android:name="com.example.popularmovies.ui.details.movie.view.MovieDetailsFragment"
            android:label="@string/movie_details"
            tools:layout="@layout/fragment_movie_details"/>

</navigation>

MainActivity 布局：

<FrameLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <fragment
            android:id="@+id/main_nav_host"
            android:name="androidx.navigation.fragment.NavHostFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:defaultNavHost="true"
            app:navGraph="@navigation/main_navigation"/>

</FrameLayout>

主要活动：

class MainActivity : AppCompatActivity(), HasSupportFragmentInjector {

    @Inject
    lateinit var dispatchingAndroidInjector: DispatchingAndroidInjector<Fragment>

    private lateinit var appBarConfiguration: AppBarConfiguration

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

        initNavUi()
    }

    override fun onBackPressed() {

        findNavController(R.id.main_nav_host).popBackStack()
    }

    override fun onCreateOptionsMenu(menu: Menu): Boolean {
        // Inflate the menu; this adds items to the action bar if it is present.
        menuInflater.inflate(R.menu.menu_main, menu)
        return true
    }

    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        return when (item.itemId) {
            R.id.action_settings -> true
            else -> super.onOptionsItemSelected(item)
        }
    }

    override fun onSupportNavigateUp(): Boolean {

        return findNavController(R.id.main_nav_host).navigateUp()
    }

    override fun supportFragmentInjector(): AndroidInjector<Fragment> {

        return dispatchingAndroidInjector
    }

    private fun initNavUi() {

        val navController = Navigation.findNavController(this, R.id.main_nav_host)
        appBarConfiguration = AppBarConfiguration(
            setOf(R.id.dest_main)
        )

        NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration)
    }

}

目标主页片段：

class MainMoviesFragment : Fragment(), Injectable, MovieViewHolder.MovieClickListener {

    @Inject
    lateinit var viewModelFactory: ViewModelProvider.Factory

    private lateinit var fragmentViewModel: MainMoviesFragmentViewModel

    private lateinit var moviesRv: RecyclerView
    private lateinit var moviesAdapter: MainMoviesAdapter

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

        return inflater.inflate(R.layout.fragment_main_movies, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        initViews(view)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        fragmentViewModel = ViewModelProviders.of(this,viewModelFactory).get(MainMoviesFragmentViewModel::class.java)
        fragmentViewModel.start()

        observe()
    }

    override fun onMovieClicked(position: Int) {

        fragmentViewModel.onMovieClicked(position)
    }

    private fun initViews(view: View) {

        moviesRv = view.findViewById<RecyclerView>(R.id.fragment_main_movies_rv).apply{

            layoutManager = LinearLayoutManager(context)
            setHasFixedSize(true)

            moviesAdapter = MainMoviesAdapter(this@MainMoviesFragment)
            adapter = moviesAdapter
        }
    }

    private fun observe() {

        fragmentViewModel.moviesLiveData.observe(this, Observer { moviesAdapter.submitList(it) })
        fragmentViewModel.onMovieClickedLiveEvent.observe(this, Observer { handleMovieClickedEvent(it) })

    }

    private fun handleMovieClickedEvent(movieModel: MovieModel?){

        val action = MainMoviesFragmentDirections.actionDestMainToDestMovieDetails()
        findNavController().navigate(action)
    }

}

目标目标片段：

class MovieDetailsFragment : Fragment() {

    private lateinit var viewModel: MovieDetailsFragmentViewModel

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

        return inflater.inflate(R.layout.fragment_movie_details, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

        viewModel = ViewModelProviders.of(this).get(MovieDetailsFragmentViewModel::class.java)
    }

}

GitHub上的项目代码可以找到here

Answer 1

你的 onMovieClickedLiveEvent 用于你的 MainMoviesFragmentViewModel，每次你回到你的 MainMoviesFragment 时都会触发，因为 MutableLiveData 保存了当前值。这意味着 popBackStack() 工作正常，但随后您会立即返回到详细信息页面（注意：您仍然需要删除 onBackPressed() 中的代码，因为现在您无法退出通过点击后退按钮的应用程序）。

似乎，尤其是对于变量的名称，您应该使用 SingleLiveEvent class, instead of MutableLiveData directly, as per this blog post.

当然，在这种情况下，根本没有特别理由使用 LiveData 或通过 ViewModel。您的 MovieViewHolder 可以将 MovieModel 直接传递给 onMovieClicked，后者可以直接调用 handleMovieClickedEvent。这将避免使用 LiveData（旨在存储 state，而不是事件）并更好地模拟您实际想要实现的目标：事件侦听器。

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