How to draw a smooth line following my finger using Kotlin?

This example demonstrates how to draw a smooth line following my finger using Android 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"
   tools:context=".MainActivity">
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_centerInParent="true"
      android:text="Draw lines on the Activity"
      android:textAlignment="center"
      android:textColor="@android:color/background_dark"
      android:textSize="32sp"
      android:textStyle="bold" />
   <ImageView
      android:id="@+id/imageView"
      android:layout_width="wrap_content"
      android:layout_height="match_parent"
      android:layout_alignParentStart="true"
      android:layout_alignParentTop="true"
      android:layout_alignParentEnd="true"
      android:contentDescription="TODO" />
</RelativeLayout>

Step 3 − Add the following code to src/MainActivity.kt

import android.graphics.Bitmap
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.os.Bundle
import android.view.MotionEvent
import android.view.View
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity(), View.OnTouchListener {
   private lateinit var imageView: ImageView
   private var bitmap: Bitmap? = null
   var canvas: Canvas? = null
   var paint: Paint? = null
   var downX = 0f
   var downY: kotlin.Float = 0f
   var upX: kotlin.Float = 0f
   var upY: kotlin.Float = 0f
   override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      title = "KotlinApp"
      imageView = findViewById(R.id.imageView)
      val currentDisplay = windowManager.defaultDisplay
      val dw = currentDisplay.width.toFloat()
      val dh = currentDisplay.height.toFloat()
      bitmap = Bitmap.createBitmap(dw.toInt(), dh.toInt(), Bitmap.Config.ARGB_8888)
      canvas = Canvas(bitmap)
      paint = Paint()
      paint!!.color = Color.RED
      paint!!.strokeWidth = 10F
      imageView.setImageBitmap(bitmap)
      imageView.setOnTouchListener(this)
   }
   override fun onTouch(v: View?, event: MotionEvent?): Boolean {
      when (event!!.action) {
         MotionEvent.ACTION_DOWN -> {
            downX = event.x
            downY = event.y
         }
         MotionEvent.ACTION_MOVE -> {
         }
         MotionEvent.ACTION_UP -> {
            upX = event.x
            upY = event.y
            canvas!!.drawLine(downX, downY, upX, upY, paint)
            imageView.invalidate()
         }
         MotionEvent.ACTION_CANCEL -> {
         }
         else -> {
         }
      }
      return true
   }
}

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.