a counter viewmodel demonstrating how to use stateFlow in android

activity_main.mxl

<?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/mTxt"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="0"
        android:textSize="32sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.457" />

    <Button
        android:id="@+id/incrementBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="48dp"
        android:layout_marginEnd="160dp"
        android:text="+"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/mTxt" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.kt

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.activity.viewModels
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleCoroutineScope
import androidx.lifecycle.lifecycleScope
import androidx.lifecycle.repeatOnLifecycle
import kotlinx.coroutines.flow.collectLatest
import kotlinx.coroutines.launch
import me.astrocoder.blog.usingstateflow.databinding.ActivityMainBinding

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

        //button on click, call our viewModel
        binding.incrementBtn.setOnClickListener {
            //increment our counter value:
            myViewModel.incrementCounter()
        }

        //Launch myViewModel in a LifeCycleScope
        lifecycleScope.launch {
            //Repeat on LifeCycle ensures that the flow only emits data when needed
            // (I.e when the app is not in the background, or when the UI is visible),
            // this is helpful when a user presses the home button and our app goes into the background,
            // as we don’t want to collect data might put our app at a risk of a crash,
            // for example showing a modal when the app is in the background can cause it to crash
            repeatOnLifecycle(Lifecycle.State.STARTED){
                myViewModel.counter.collect{
                    binding.mTxt.text = it.toString()
                }
            }
        }
    }
}

MyViewModel.kt

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.activity.viewModels
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleCoroutineScope
import androidx.lifecycle.lifecycleScope
import androidx.lifecycle.repeatOnLifecycle
import kotlinx.coroutines.flow.collectLatest
import kotlinx.coroutines.launch
import me.astrocoder.blog.usingstateflow.databinding.ActivityMainBinding

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

        //button on click, call our viewModel
        binding.incrementBtn.setOnClickListener {
            //increment our counter value:
            myViewModel.incrementCounter()
        }

        //Launch myViewModel in a LifeCycleScope
        lifecycleScope.launch {
            //Repeat on LifeCycle ensures that the flow only emits data when needed
            // (I.e when the app is not in the background, or when the UI is visible),
            // this is helpful when a user presses the home button and our app goes into the background,
            // as we don’t want to collect data might put our app at a risk of a crash,
            // for example showing a modal when the app is in the background can cause it to crash
            repeatOnLifecycle(Lifecycle.State.STARTED){
                myViewModel.counter.collect{
                    binding.mTxt.text = it.toString()
                }
            }
        }
    }
}