Android: Single Activity with multiple Fragments, Navigation implements Fragment jumps, and Fragments share data through ViewModel

1、MainActivity

1、activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/fragmentContainerView"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph" />
</androidx.constraintlayout.widget.ConstraintLayout>

2、MainActivity

/**
 * kotlin_jetpack_navigation
 * 使用navigation  + dataBinding + viewModel
 * 三个fragment共享一个viewModel中的数据，在NavigationView中显示
 */
class MainActivity : AppCompatActivity() {
    
    
    override fun onCreate(savedInstanceState: Bundle?) {
    
    
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

2、HomeFragment, DetailFragment

• FragmentA: includes SeekBar and a button, click the button to jump to FragmentB
• FragmentB: A button for SeekBar plus one and minus one operation, a button to return to FragmentA, that is, after the addition and subtraction operation, the result of addition and subtraction is displayed on FragmentA.

nav_graph.xml

<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/nav_graph"
    app:startDestination="@id/homeFragment">

    <fragment
        android:id="@+id/detailFragment"
        android:name="cn.hk.navigation3.DetailFragment"
        android:label="fragment_detail"
        tools:layout="@layout/fragment_detail" >
        <action
            android:id="@+id/action_detailFragment_to_homeFragment"
            app:destination="@id/homeFragment" />
    </fragment>
    <fragment
        android:id="@+id/homeFragment"
        android:name="cn.hk.navigation3.HomeFragment"
        android:label="fragment_home"
        tools:layout="@layout/fragment_home" >
        <action
            android:id="@+id/action_homeFragment_to_detailFragment"
            app:destination="@id/detailFragment" />
    </fragment>
</navigation>

2.1、HomeFragment

1、fragment_home.xml

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <variable
            name="data"
            type="cn.hk.navigation3.MyViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".HomeFragment">


        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            app:layout_constraintGuide_begin="320dp"
            app:layout_constraintGuide_percent="0.5" />

        <TextView
            android:id="@+id/textView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(data.number)}"
            android:textSize="20sp"
            app:layout_constraintBottom_toTopOf="@+id/guideline"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Button"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="@+id/guideline" />

        <SeekBar
            android:id="@+id/seekBar"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toTopOf="@+id/textView"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

2、HomeFragment

class HomeFragment : Fragment() {
    
    


    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
    
    
        // Inflate the layout for this fragment
//        val myViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
        val myViewModel by activityViewModels<MyViewModel>()
        val binding = DataBindingUtil.inflate<FragmentHomeBinding>(
            inflater, R.layout.fragment_home, container, false
        )
        binding.also {
    
    
            it.data = myViewModel
            it.lifecycleOwner = activity
            it.button.setOnClickListener {
    
     v ->
                val controller = Navigation.findNavController(v)
                controller.navigate(R.id.action_homeFragment_to_detailFragment)
            }
            myViewModel.get().value?.also {
    
     numberValue ->
                it.seekBar.progress = numberValue
            }
            it.seekBar.setOnSeekBarChangeListener(object : SeekBar.OnSeekBarChangeListener {
    
    
                override fun onProgressChanged(p0: SeekBar?, p1: Int, p2: Boolean) {
    
    
                   myViewModel.set(p1)
                }

                override fun onStartTrackingTouch(p0: SeekBar?) {
    
    
                }

                override fun onStopTrackingTouch(p0: SeekBar?) {
    
    
                }

            })
        }

        return binding.root
    }

2.2、DetailFragment

1、fragment_detail.xml

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <variable
            name="data"
            type="cn.hk.navigation3.MyViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".DetailFragment">


        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            app:layout_constraintGuide_begin="320dp"
            app:layout_constraintGuide_percent="0.5" />

        <TextView
            android:id="@+id/textView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(data.number)}"
            android:textSize="20sp"
            app:layout_constraintBottom_toTopOf="@+id/guideline"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <Button
            android:id="@+id/add"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="add"
            android:onClick="@{()->data.count(1)}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toStartOf="@+id/jian"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="@+id/guideline" />

        <Button
            android:id="@+id/jian"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="cut"
            android:onClick="@{()->data.count(-1)}"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toEndOf="@+id/add"
            app:layout_constraintTop_toTopOf="@+id/guideline" />

        <Button
            android:id="@+id/button3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="返回"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/add" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

2、DetailFragment

class DetailFragment : Fragment() {
    
    


    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
    
    
        // Inflate the layout for this fragment
//      val myViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
        val myViewModel by activityViewModels<MyViewModel>()
        val binding = DataBindingUtil.inflate<FragmentDetailBinding>(
            inflater,
            R.layout.fragment_detail,
            container,
            false
        )
        binding.also {
    
    
            it.data = myViewModel
            it.lifecycleOwner = activity
            it.button3.setOnClickListener {
    
     v ->
                val controller = Navigation.findNavController(v)
                controller.navigate(R.id.action_detailFragment_to_homeFragment)
            }
        }
        return binding.root
    }

}

3、MyViewModel

class MyViewModel : ViewModel() {
    
    
    var number: MutableLiveData<Int> = MutableLiveData(0)

    fun get() = number

    fun set(result: Int) {
    
    
        number.value = result
    }


    fun count(x: Int) {
    
    
        number.value = number.value?.plus(x)
        if (number.value!! < 0)
            number.value = 0
    }
}

reference