LibGDX Stage and Viewport management for Actors?

Question

So I've done research about how to use Stage, Actor and Viewports etc. To optimize the game for most devices running Android. I'm pretty sure that the viewport is configured or the background image is just being drawn over the screen without actually using the viewport. That is fine though. So I'm basically telling the viewport to draw an image over the whole screen and it seems that that applies to actors also. So I guess the viewport is trying to draw the actor over the whole screen.

When testing the app on emulators with different screen sizes the actor is not resized (probably because of the viewport setting), but how can I get the actor to resize so that the actor fits in the world? The world will be the whole screen of the device.

Do I need to use two viewports where one is for UI (background so far) and another for the game? If I need to do that wouldn't the UI viewport need to be the screen size and the Game viewport holding the world size would have size constants? If so, then I don't have the whole screen size as the world, but drawing the Game viewport using the screen width and height would probably result in drawing the actor over the whole screen again, right?

And I've seen in some tutorials that they often set a constant unit of the world width and world height. Wouldn't that fail to fit all devices since they have a specific constant for width and height?

I want all of my Actors to be resized so that they fit inside the world (screen size).

UPDATE TO CLARIFY MY QUESTION

As you see the background is scaled up and down due to being drawn as the width and height of the screen, I guess, but I need the actors to also be scaled up or down. The actors can't remain the same size for all screens.

MyGdxGame

public class MyGdxGame extends ApplicationAdapter {

private OrthographicCamera camera;
private Stage stage;
private Viewport viewport;
private Texture textureBackground;

@Override
public void create () {
    camera = new OrthographicCamera(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    camera.position.set(Gdx.graphics.getWidth() / 2, Gdx.graphics.getHeight() / 2, 0);

    final float GAME_WORLD_WIDTH = Gdx.graphics.getWidth();
    final float GAME_WORLD_HEIGHT = Gdx.graphics.getHeight();
    viewport = new FitViewport(GAME_WORLD_WIDTH, GAME_WORLD_HEIGHT, camera);

    stage = new Stage(viewport);
    MyActor actor = new MyActor(new Texture("badlogic.jpg"));
    actor.setPosition(0, 0);
    stage.addActor(actor);
    stage.getViewport().apply();
    Gdx.input.setInputProcessor(stage);

    textureBackground = new Texture("TheWorld.png");
}

@Override
public void resize(int width, int height) {
    stage.getViewport().update(width, height);
    stage.getCamera().position.set(Gdx.graphics.getWidth() / 2, Gdx.graphics.getHeight() / 2, 0);
}

@Override
public void render () {
    Gdx.gl.glClearColor(1, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    stage.getBatch().setProjectionMatrix(stage.getCamera().combined);
    stage.getCamera().update();

    stage.getBatch().begin();
    stage.getBatch().draw(textureBackground, 0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
    stage.getBatch().end();

    stage.act(Gdx.graphics.getDeltaTime());
    stage.draw();
}
}

MyActor

package com.mygdx.game;

import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Batch;
import com.badlogic.gdx.scenes.scene2d.Actor;

public class MyActor extends Actor {

    private Texture texture;
    public MyActor(Texture texture) {
        this.texture = texture;
    }

    @Override
    public void draw(Batch batch, float parentAlpha) {
        batch.draw(texture, getX(), getY());
    }
}

Images:

Research links:

Using two viewports for UI and Game

Viewports and World units

LibGDX Viewport Tutorial

Answer 1

I had take your code and change it a little bit.
I hope this will solve your problem otherwise I do not know what exactly your problem is.

I will recommend you to not use Gdx.graphics.getHeight/getWidth.
With the camera, you can use your own unit in this example 100 and the stage.getBatch().setProjectionMatrix(stage.getCamera().combined); will do the calculation for you. More about the camera and Matrix are on this site: github.com/libgdx/libgdx/wiki/Orthographic-camera

Code:

public class MyGdxGame extends ApplicationAdapter {
    private OrthographicCamera camera;
    private Stage stage;
    private Viewport viewport;
    private Texture textureBackground;

    private static final int WORLD_WIDTH = 100;
    private static final int WORLD_HEIGHT = 100;

    @Override
    public void create () {
        camera = new OrthographicCamera(WORLD_WIDTH,WORLD_HEIGHT);
        camera.position.set(camera.viewportWidth / 2, camera.viewportHeight / 2, 0);

        viewport = new StretchViewport(WORLD_WIDTH, WORLD_HEIGHT, camera);

        stage = new Stage(viewport);
        MyActor actor = new MyActor(new Texture("badlogic.jpg"));
        actor.setPosition(0, 0);
        stage.addActor(actor);
        stage.getViewport().apply();
        Gdx.input.setInputProcessor(stage);

        textureBackground = new Texture("TheWorld.png");
    }

    @Override
    public void resize(int width, int height) {
        stage.getViewport().update(width, height);
        stage.getCamera().viewportWidth = WORLD_WIDTH;
        stage.getCamera().viewportHeight = WORLD_HEIGHT * height / width;
        stage.getCamera().position.set(stage.getCamera().viewportWidth / 2, stage.getCamera().viewportHeight / 2, 0);
        stage.getCamera().update();
    }

    @Override
    public void render () {
        Gdx.gl.glClearColor(1, 0, 0, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        stage.getBatch().setProjectionMatrix(stage.getCamera().combined);
        stage.getCamera().update();

        stage.getBatch().begin();
        stage.getBatch().draw(textureBackground, 0, 0, 100,100);
        stage.getBatch().end();

        stage.act(Gdx.graphics.getDeltaTime());
        stage.draw();
    }
}


public class MyActor extends Actor {

    private Texture texture;
    public MyActor(Texture texture) {
        this.texture = texture;
    }

    @Override
    public void draw(Batch batch, float parentAlpha) {
//        batch.draw(texture, getX(), getY());
        batch.draw(texture, 50, 50, 20, 20);
    }
}