【ANDROID STUDIO】Live Data

 package com.example.viewmodel

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProvider
import com.example.viewmodel.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding
    private lateinit var viewModel: MainActivityViewModel
    private lateinit var viewModelFactory: MainActivityViewModelFactory


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
        viewModelFactory = MainActivityViewModelFactory(100)
        viewModel = ViewModelProvider(this, viewModelFactory).get(MainActivityViewModel::class.java)
        viewModel.totalData.observe(this, Observer {
            binding.resultTextView.text = it.toString()
        })

        binding.insertButton.setOnClickListener {
            viewModel.setTotal(binding.inputEditText.text.toString().toInt())
        }

    }
}

package com.example.viewmodel

import androidx.lifecycle.ViewModel
import androidx.lifecycle.ViewModelProvider

class MainActivityViewModelFactory(private val startingTotal : Int) : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MainActivityViewModel::class.java)) {
            return MainActivityViewModel(startingTotal) as T
        }
        throw IllegalArgumentException("Unknown View Model Class")
    }


}
package com.example.viewmodel

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

class MainActivityViewModel(startingTotal: Int) : ViewModel() {
    private var total = MutableLiveData<Int>()
    val totalData : LiveData<Int>
    get() = total


    init {
        total.value = startingTotal
    }

    fun setTotal(input: Int) {
        total.value = (total.value)?.plus(input)
    }
}
<?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">

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

        <EditText android:id="@+id/input_edit_text"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:ems="10"
            android:inputType="number"
            android:textSize="36sp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.323"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.064" />

        <Button android:id="@+id/insert_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="50dp"
            android:layout_marginTop="40dp"
            android:text="Add"
            android:textSize="36sp"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@+id/input_edit_text"
            android:layout_marginLeft="50dp" />

        <TextView
            android:id="@+id/result_text_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="45dp"
        android:layout_marginTop="32dp"
        android:layout_marginEnd="41dp"
        android:textSize="36sp"
        app:layout_constraintEnd_toEndOf="@+id/insert_button"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/insert_button"
            android:layout_marginRight="41dp"
            android:layout_marginLeft="45dp" />
        </androidx.constraintlayout.widget.ConstraintLayout>
    </layout>