/**
 * CLASS: OpenScrollWorld
 *
 * DESCRIPTION: creates an unlimited scrolling world;

 * a Fish object will be followed or scolling will be key controlled;

 * all images used are part of the greenfoot image set;
 * their filenames are shown within the code;

 * to avoid jumping when changing actors,
 * a limit is set on the speed of scrolling
 */
 

import greenfoot.*;

 

public class OpenScrollWorld extends World
{
    public static final int WIDE = 800; // world width (viewport)
    public static final int HIGH = 600; // world height (viewport)
   
    Scroller scroller; // the object that performs the scrolling
    Actor scrollActor; // the selected actor to stay in view
   
    // creates an unlimited scrolling world
    public OpenScrollWorld()
    {
        super(WIDE, HIGH, 1, false); // creates an unbounded world
        GreenfootImage image = new GreenfootImage("wet-blue.jpg"); // background img
        scroller = new Scroller(this, image); // creates Scroller object
        scrollActor = new Fish(); // creates and sets an actor to stay in view
        scrollActor.setImage("fish3.png"); // sets an image to the actor
        addObject(scrollActor, WIDE/2, HIGH/2); // adds the actor in middle of world
        Actor fish = new Fish(); // creates a second actor
        fish.setImage("fish2.png"); // sets an image to this second actor
        addObject(fish, WIDE/2, 100); // adds the second actor into world
        fish = new Fish(); // creates a third actor
        fish.setImage("fish1.png"); // sets an image to this third actor
        addObject(fish, WIDE/2, 500); // adds the third actor into world
        scroller.scroll(0, 0); // sets the initial background image
    }
   
    public void act()
    {
        if (scrollActor != null) scroll(); else keyScroll(); // perform scrolling
        if (Greenfoot.mouseClicked(this))

             scrollActor = null; // actor de-selection
    }
   
    // scroll by arrow keys
    private void keyScroll()
    {
        // determine scrolling offsets and scroll
        int dsx = 0, dsy = 0;
        if (Greenfoot.isKeyDown("right")) dsx++;
        if (Greenfoot.isKeyDown("left")) dsx--;
        if (Greenfoot.isKeyDown("down")) dsy++;
        if (Greenfoot.isKeyDown("up")) dsy--;
        int rate = 2; // scroll speed
        scroller.scroll(dsx*rate, dsy*rate);
    }

   
    // perform speed-governed center screen actor follow scrolling
    private void scroll()
    {
        int rate = 3; // speed limit of scrolling
        // determine offsets and scroll
        int dsx = scrollActor.getX()-WIDE/2;
        int dsy = scrollActor.getY()-HIGH/2;
        int signX = (int)Math.signum(dsx), signY = (int)Math.signum(dsy);
        dsx = signX*Math.min(Math.abs(dsx), rate);
        dsy = signY*Math.min(Math.abs(dsy), rate);
        scroller.scroll(dsx, dsy);
    }
}

​

​

The following can be used to allow some actor movement before scrolling begins:\

​

// perform speed-governed roaming actor follow scrolling
private void scroll()
{
    // roaming limits of actor
    int loX = 100;
    int hiX = WIDE-100;
    int loY = 100;
    int hiY = HIGH-100;
    // determine offsets and scroll
    int dsx = 0;
    int dsy = 0;
    if (scrollActor.getX() < loX) dsx = scrollActor.getX()-loX;
    if (scrollActor.getX() > hiX) dsx = scrollActor.getX()-hiX;
    if (scrollActor.getY() < loY) dsy = scrollActor.getY()-loY;
    if (scrollActor.getY() > hiY) dsy = scrollActor.getY()-hiY;
    int rate = 3; // speed limit of scrolling
    int signX = (int)Math.signum(dsx), signY = (int)Math.signum(dsy);
    dsx = signX*Math.min(Math.abs(dsx), rate);
    dsy = signY*Math.min(Math.abs(dsy), rate);
    scroller.scroll(dsx, dsy);
}

​

The code used in my Fish class follows:

​

import greenfoot.*;

​

public class Fish extends Actor
{
    public void act()
    {
        if (((OpenScrollWorld)getWorld()).scrollActor == this) // if selected
        {
            // turning
            if (Greenfoot.isKeyDown("right")) turn(1);
            if (Greenfoot.isKeyDown("left")) turn(-1);
            // moving
            if (Greenfoot.isKeyDown("up")) move(2);
        }
        else if (Greenfoot.mouseClicked(this)) // else if selecting
        {
            ((OpenScrollWorld)getWorld()).scrollActor = this; // set selection
           
        }
        else // not selected
        {
            turn(1);
            move(2);
        }
    }
}