Handler In Android

activity_main.xml

<?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">

    <Button
        android:id="@+id/btnStart"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="160dp"
        android:layout_marginLeft="160dp"
        android:layout_marginTop="139dp"
        android:layout_marginBottom="168dp"
        android:text="Start"
        app:layout_constraintBottom_toTopOf="@+id/progressBar"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="27dp"
        android:layout_marginLeft="27dp"
        android:layout_marginEnd="27dp"
        android:layout_marginRight="27dp"
        android:layout_marginBottom="344dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnStart" />

    <TextView
        android:id="@+id/txtCount"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textAlignment="center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/progressBar"
        tools:layout_editor_absoluteX="132dp" />
</androidx.constraintlayout.widget.ConstraintLayout>