[안드로이드] ViewModel을 이용한 Fragment간 데이터 공유

우선 Fragment를 변경할 수 있는 버튼과

Fragment를 교체할 수 있는 FrameLayout을 놔준다.

class MainActivity : AppCompatActivity() {

    private val binding by lazy { ActivityMainBinding.inflate(layoutInflater) }

    private val firstFragment by lazy { FirstFragment() }
    private val secondFragment by lazy { SecondFragment() }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
        switchFragment(firstFragment)
    }

    fun switchFirstFragment(view: View) { // 버튼 클릭 시 FirstFragment 표시
        switchFragment(firstFragment)
    }

    fun switchSecondFragment(view: View) { // 버튼 클릭 시 SecondFragment 표시
        switchFragment(secondFragment)
    }

    // Fragment를 교체하는 함수
    private fun switchFragment(fragment: Fragment) {
        supportFragmentManager.beginTransaction().replace(binding.frameLayout.id, fragment).commit()
    }
}

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/frameLayout"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@id/linearLayout"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent">
    </FrameLayout>

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="horizontal"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent">

        <Button
            android:id="@+id/firstFragmentButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:onClick="switchFirstFragment"
            android:text="First" />

        <Button
            android:id="@+id/secondFragmentButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:onClick="switchSecondFragment"
            android:text="Second" />
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

---

First/SecondFragment와 ViewModel를 만들어준다.

여기서 Fragment에 ViewModel을 선언할 때 viewModels()가 아닌 activityViewModels()를 사용한 이유는

ViewModel을 Activity에 종속시켜 같은 데이터를 공유하기 위함이다.

만약 viewModels()를 사용하게 된다면 각 Fragment에 종속되어 데이터가 공유되지 않는다.

class FirstFragment : Fragment() {

    private val viewModel: FragmentViewModel by activityViewModels()

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        val binding = FragmentFirstBinding.inflate(inflater, container, false)
        binding.editText.addTextChangedListener {
            viewModel.setInput(it.toString())
        }
        return binding.root
    }
}

<androidx.constraintlayout.widget.ConstraintLayout 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">

    <EditText
        android:id="@+id/editText"
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:hint="여기에 입력"
        android:textSize="30sp"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.95" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="428dp"
        android:gravity="center"
        android:text="First Fragment"
        android:textSize="40dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

class SecondFragment : Fragment() {

    private val viewModel: FragmentViewModel by activityViewModels()

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        val binding = FragmentSecondBinding.inflate(inflater, container, false)
        viewModel.input.observe(viewLifecycleOwner) {
            binding.inputText.text = it
        }
        return binding.root
    }
}

<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:orientation="vertical">

    <TextView
        android:id="@+id/inputText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="입력값"
        android:textSize="50sp"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.95" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="428dp"
        android:gravity="center"
        android:text="Second Fragment"
        android:textSize="40dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

class FragmentViewModel : ViewModel() {

    private val _input = MutableLiveData<String>()
    val input: LiveData<String> = _input

    fun setInput(input: String) {
        _input.value = input
    }
}

---

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