View Model State in android studio: Kotlin

Question

As I was working on this code yesterday after I tried using SaveInstanceState for saving my textview output while changing my orientation from portrait to landscape mode but it didn't work. so i thought and realized that i could work it out using View Model state but. unlike SaveInstance State, I tried doing View Model but i find it little tough to do and fix the errors, here is the code for main activity and ViewModel

Main Activity kotlin

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var count = 10
        val textCount = findViewById<View>(R.id.textView) as TextView
         val buttonred = findViewById<View>(R.id.injury) as Button
         
         val buttoning = findViewById<View>(R.id.revive) as Button


            var viewModel = ViewModelProvider(this).get(MainActivityViewModel::class.java)

            buttonred.setOnClickListener {
                if (count >= 0) {

                   count--
                    textCount.text = count.toString()
                }

            }
           
            buttoning.setOnClickListener {
                if (count <= 10) {
                    count += 2
                    textCount.text = count.toString()
                }
            }
    }
}

Viewmodel kotlin code (note: i just started working on it, but i have no idea how to work it out)

class MainActivityViewModel: ViewModel() {
    var count = 10
    fun medivial()
    {
        count--
    }
}

here is the xml file

<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="60dp"
        android:layout_height="78dp"
        android:layout_marginStart="145dp"
        android:layout_marginTop="68dp"
        android:layout_marginEnd="145dp"
        android:textSize="50dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
<Button
        android:id="@+id/injury"
        android:layout_width="106dp"
        android:layout_height="wrap_content"
        android:layout_marginHorizontal="150dp"
        android:text="injury"
         />
<Button
        android:id="@+id/revive"
        android:layout_width="197dp"
        android:layout_height="wrap_content"
        android:layout_marginHorizontal="109dp"
        android:text="revive" />
</LinearLayout>

need help on this

Answer 1

You have to add LiveData to the ViewModel class and observe it. Your activity class cannot have any data like count . ViewModel has to carry all actions and just give data to View (MainActivity)

To Your Gradle (module.app) file add:

dependencies {
    def lifecycle_version = "2.2.0"

    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
}

MainActivityViewModel class:

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


internal class MainActivityViewModel : ViewModel() {

    private val _count: MutableLiveData<Int> = MutableLiveData()
    val count: LiveData<Int>
        get() = _count

    init {
        _count.value = START_VALUE
    }

    fun increment() {
        _count.value = _count.value!! + 1
    }

    fun decrement() {
        _count.value = _count.value!! - 1

    }

    companion object {
        private const val START_VALUE = 10
    }
}

MainActivity class:

import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.ViewModelProvider

class MainActivity : AppCompatActivity()
{
    private lateinit var viewModel: MainActivityViewModel

    override fun onCreate(savedInstanceState: Bundle?)
    {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val textCount = findViewById<View>(R.id.textView) as TextView
        val butIncrement = findViewById<View>(R.id.injury) as Button
        val butDecrement = findViewById<View>(R.id.vial) as Button

        viewModel = ViewModelProvider(this).get(MainActivityViewModel::class.java)

        viewModel.count.observe(this, {
            textCount.text = it.toString()
        })

        butIncrement.setOnClickListener {
            viewModel.increment()
        }

        butDecrement.setOnClickListener {
            viewModel.decrement()
        }
    }
}