<!--Permissions for internet-->
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="30dp"
    android:background="@color/purple_500">

    <!--Image view inside our dialog box-->
    <ImageView
        android:id="@+id/idIVNotification"
        android:layout_width="300dp"
        android:layout_height="300dp"
        android:layout_below="@id/idTVNotification"
        android:layout_centerHorizontal="true"
        android:layout_margin="20dp" />

    <!--Text view inside our dialog box-->
    <TextView
        android:id="@+id/idTVNotification"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="8dp"
        android:text="Notification Message"
        android:textAlignment="center"
        android:textColor="@color/white"
        android:textSize="18sp" />

</RelativeLayout>

import android.app.AlertDialog;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;
import com.squareup.picasso.Picasso;

public class MainActivity extends AppCompatActivity {

    // initializing th variable for firebase firestore.
    FirebaseFirestore db = FirebaseFirestore.getInstance();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // creating a variable for our alert dialog builder.
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);

        // creating a layout inflater variable.
        LayoutInflater inflater = getLayoutInflater();

        // below line is for inflating a custom pop up layout.
        View dialoglayout = inflater.inflate(R.layout.custom_pop_up_layout, null);

        // initializing the textview and imageview from our dialog box.
        TextView notificationTV = dialoglayout.findViewById(R.id.idTVNotification);
        ImageView notificationIV = dialoglayout.findViewById(R.id.idIVNotification);

        // creating a variable for document reference.
        DocumentReference documentReference = db.collection("MyData").document("Data");

        // adding snapshot listener to our document reference.
        documentReference.addSnapshotListener(new EventListener<DocumentSnapshot>() {
            @Override
            public void onEvent(@Nullable DocumentSnapshot value, @Nullable FirebaseFirestoreException error) {
                // inside the on event method.
                if (error != null) {
                    // this method is called when error is not null 
                    // and we gt any error
                    // in this cas we are displaying an error message.
                    Toast.makeText(MainActivity.this, "Error found is " + error, Toast.LENGTH_SHORT).show();
                    return;
                }
                if (value != null && value.exists()) {
                    // if the value from firestore is not null then we are
                    // getting our data and setting that data to our text view.
                    // after getting the value from firebase firestore
                    // we are setting it to our text view and image view.
                    notificationTV.setText(value.getData().get("NotificationMessage").toString());
                    Picasso.get().load(value.getData().get("NotificationImage").toString()).into(notificationIV);
                }
            }
        });

        // after setting the text to our text view.
        // we are displaying our alert dialog.
        builder.setView(dialoglayout);
        builder.show();
    }
}