created GameView, GameActivity, Background classes. implemented infinite background illusion

        android:supportsRtl="true"

        android:theme="@style/Theme.8bitInvader"

        tools:targetApi="31">

        <activity

            android:name=".GameActivity"

            android:theme="@style/Theme.Design.NoActionBar" />

        <activity

            android:name=".MainActivity"

            android:exported="true">

package com.example.a8_bitinvader;

import android.content.res.Resources;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

public class Background {

    int xPos = 0, yPos = 0;

    Bitmap background;

    /*

     * Creates background from 'background1.jpeg' in res/drawable directory

     */

    Background(int screenWidth, int screenHeight, Resources res) {

        background = BitmapFactory.decodeResource(res, R.drawable.background);

        background = Bitmap.createScaledBitmap(background, screenWidth, screenHeight, false);

    }

}

package com.example.a8_bitinvader;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

import android.graphics.Point;              // necessary to get ScreenWidth and ScreenHeight

import android.view.WindowManager;

public class GameActivity extends AppCompatActivity {

    private GameView gameView;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        // puts app into fullscreen when GameActivity is running

        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);

        Point point = new Point();

        getWindowManager().getDefaultDisplay().getSize(point);

        gameView = new GameView(this, point.x, point.y);

        setContentView(gameView);

    }

    @Override

    protected void onPause() {

        super.onPause();

        gameView.pause();

    }

    @Override

    protected void onResume() {

        super.onResume();

        gameView.resume();

    }

}

package com.example.a8_bitinvader;

import android.content.Context;

import android.view.SurfaceView;

import android.graphics.Canvas;

import android.graphics.Paint;             // necessary when drawing Bitmaps with Canvas objects

public class GameView extends SurfaceView implements Runnable{

    private Thread thread;

    private boolean isRunning;

    private int screenWidth, screenHeight;

    private float screenRatioWidth, screenRatioHeight;

    private Background background1, background2;

    private Paint paint;

    public GameView(Context context, int screenWidth, int screenHeight) {

        super(context);

        this.screenWidth = screenWidth;

        this.screenHeight = screenHeight;

        screenRatioWidth = 1080f / screenWidth;

        screenRatioHeight = 1920f / screenHeight;

        background1 = new Background(screenWidth, screenHeight, getResources());

        background2 = new Background(screenWidth, screenHeight, getResources());

        background2.yPos = -screenHeight;

        paint = new Paint();

    }

    /*

     * Continuously loops through update(), draw(), and sleep() if the app is running

     * Overrides from Runnable class, which Thread automatically calls once resume() initializes said Thread

     */

    @Override

    public void run() {

        while(isRunning) {

            update();

            draw();

            // creates 30 millisecond delay in between draw() and update()

            try {

                Thread.sleep(30);

            } catch (InterruptedException e) {

                throw new RuntimeException(e);

            }

        }

    }

    /*

     * updates position of backgrounds.

     * checks if either background is offscreen. If so, it places that background above the other

     * giving the illusion of an infinitely moving background.

     */

    private void update() {

        background1.yPos += 5 * (int) screenRatioHeight;

        background2.yPos += 5 * (int) screenRatioHeight;

        if(background1.yPos > screenHeight) {

            background1.yPos = -screenHeight;

        }

        if(background2.yPos > screenHeight) {

            background2.yPos = -screenHeight;

        }

    }

    /*

     * draws both backgrounds onto the screen given their initialized

     */

    private void draw() {

        if(getHolder().getSurface().isValid()){

            Canvas canvas = getHolder().lockCanvas();

            canvas.drawBitmap(background1.background, background1.xPos, background1.yPos, paint);

            canvas.drawBitmap(background2.background, background2.xPos, background2.yPos, paint);

            getHolder().unlockCanvasAndPost(canvas);

        }

    }

    /*

     * https://developer.android.com/reference/java/lang/Thread#start()

     * initializes a thread and starts it, automatically calling the overridden run() method above.

     */

    public void resume() {

        isRunning = true;

        thread = new Thread(this);

        thread.start();

    }

    /*

     * https://developer.android.com/reference/java/lang/Thread#join()

     * waits for thread to die if game is paused

     */

    public void pause() {

        try {

            isRunning = false;

            thread.join();

        } catch (InterruptedException e) {

            throw new RuntimeException(e);

        }

    }

}

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