[LiveData] 안드로이드 LiveData 기본 사용법 - 기본 예제

class MainActivity : AppCompatActivity() {
    lateinit var binding : ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val liveText: MutableLiveData<String> = MutableLiveData()
        //button을 누르면 증가 될 숫자
        var count : Int = 0

        //- 첫 번째 매개변수인 this는 LifeCycleOwner인 MainActivity 입니다.
        //- 두 번째 매개변수인 Observer Callback은 LiveData(liveText)의 value의 변경을 감지하고 호출되는 부분입니다.
        liveText.observe(this, Observer {
            //it로 넘어오는 param은 LiveData의 value
            binding.textTest.text = it
        })

        // liveText의 value를 변경
        // liveText 자체를 변경시키면 안됨
        binding.btnChange.setOnClickListener {
            liveText.value = "HEllO Wolrd! ${++count}"
        }

    }
}

<?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">

    <TextView
        android:id="@+id/text_test"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toTopOf="@id/btn_change"
        app:layout_constraintVertical_chainStyle="packed"/>

    <Button
        android:id="@+id/btn_change"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="ADD 1"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@id/text_test"
        app:layout_constraintBottom_toBottomOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>