How to use the TextWatcher class in kotlin?

This example demonstrates how to use the TextWatcher class in kotlin.
Step 1 − Create a new project in Android Studio, go to File ⇒New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:padding="12dp"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="50dp"
      android:text="Tutorials Point"
      android:textAlignment="center"
      android:textColor="@android:color/holo_green_dark"
      android:textSize="32sp"
      android:textStyle="bold" />
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_above="@id/etInput"
      android:layout_centerInParent="true"
      android:layout_marginBottom="30dp"
      android:text="Android Text Watcher"
      android:textColor="@android:color/holo_orange_dark"
      android:textSize="24sp"
      android:textStyle="bold" />
   <EditText
      android:id="@+id/etInput"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_centerInParent="true"
      android:hint="Input"
      android:maxLength="15" />
   <TextView
      android:id="@+id/textView"
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:layout_below="@id/etInput"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="12dp"
      android:textColor="@android:color/holo_red_dark"
      android:textSize="24sp"
      android:textStyle="bold|italic" />
</RelativeLayout>
Step 3 − Add the following code to MainActivity.kt
import android.os.Bundle
import android.text.Editable
import android.text.TextWatcher
import android.widget.EditText
import android.widget.TextView
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
   lateinit var input: EditText
   lateinit var output: TextView
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      title = "KotlinApp"
      input = findViewById(R.id.etInput)
      output = findViewById(R.id.textView)
      input.addTextChangedListener(textWatcher)
   }
   private val textWatcher = object : TextWatcher {
      override fun afterTextChanged(s: Editable?) {
      }
      override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) {
      }
      override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) {
         output.text = s
         if (start == 12) {
            Toast.makeText(applicationContext, "Maximum Limit Reached", Toast.LENGTH_SHORT)
            .show()
         }
      }
   }
}
Step 4 − Add the following code to androidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="app.com.kotlipapp">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click the Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen −
Click here to download the project code.