android - Kotlin与导航-如何将数据保留在先前的 fragment 中

Question

我正在使用导航图，并且我的片段A具有RecylerView。回到片段B时，我不想丢失片段A中的数据。
我该怎么做？

Answer 1

分步解决方案。使用ViewModel更改片段后，创建导航图并将ListView内容保存在片段中:

添加到build.gradle (Module: app)中:

dependencies { 
    ...

    def nav_version = "2.3.0"
    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
    implementation "androidx.navigation:navigation-ui-ktx:$nav_version"

    def lifecycle_version = "2.2.0"
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
}

和

android {
    ...

    kotlinOptions {
        jvmTarget = "1.8"
    }
}

创建两个片段(FragemntSecond可以为空):

FragemntStart:

<androidx.appcompat.widget.LinearLayoutCompat xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    >

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Start Fragment"
        />

    <Button
        android:id="@+id/butGoTo2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Go To Second"
        />

    <Button
        android:id="@+id/butAddRandomValue"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add random value"
        />

    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        />

</androidx.appcompat.widget.LinearLayoutCompat>

创建导航图，添加两个片段并将它们连接到图上。在MainActivity中添加FragmentContainerView:

<androidx.fragment.app.FragmentContainerView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/nav_host_fragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:defaultNavHost="true"
    app:navGraph="@navigation/navigation"
    />

创建一个ViewModel类:

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class CommonViewModel : ViewModel()
{
    // This list will keep all values for Your ListView
    private val _list: MutableLiveData<ArrayList<String>> = MutableLiveData()
    val list: LiveData<ArrayList<String>>
        get() = _list

    init
    {
        _list.value = ArrayList()
    }

    // function which will add new values to Your list
    fun addNewValue()
    {
        _list.value!!.add((0..100).random().toString())
        _list.value = _list.value //this will notify observers
    }
}

将ViewModel添加到StartFragment中:

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ArrayAdapter
import androidx.fragment.app.Fragment
import androidx.lifecycle.ViewModelProvider
import androidx.navigation.Navigation
import kotlinx.android.synthetic.main.fragment_start.*

class StartFragment : Fragment()
{
    private lateinit var viewModel: CommonViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View?
    {
        viewModel = ViewModelProvider(requireActivity()).get(CommonViewModel::class.java) // init view model
        return inflater.inflate(R.layout.fragment_start, container, false)
    }

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

        butGoTo2.setOnClickListener(Navigation.createNavigateOnClickListener(R.id.action_startFragment_to_secondFragment)) // set clickListener to navigate to second fragment
        butAddRandomValue.setOnClickListener { viewModel.addNewValue() } // listener for second button to add new value to ListView

        // set up observer. Every time You add sth to list, ListView will be updated
        viewModel.list.observe(viewLifecycleOwner, {
            listView.adapter = ArrayAdapter(
                requireContext(),
                android.R.layout.simple_list_item_1,
                it
            )
        })
    }
}

现在，您可以转到第二个片段，当您返回到“开始”时，您将不会丢失任何数据。这是一个简单的示例，说明如何使用ViewModel在ListView中保存数据。使用RecyclerView时，可以创建扩展ListAdapter的适配器，而在观察器中，只需编写 adapter.submitList(it)即可。

如果您想从ViewModel访问第二个片段中的数据(例如this list)，则可以这样操作:

class SecondFragment : Fragment()
{
    private val viewModel: CommonViewModel by activityViewModels() // retrieve the same view model
    
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View?
    {
        Log.d("MyTag", "${viewModel.list.value}") // here You can access list which is used in StartFragment 
        return inflater.inflate(R.layout.fragment_second, container, false)
    }
}