Red Light, Green Light

A bee, a ladybug and a snail race across the stage — but only while the referee says they may. This is the playground game, and it is also the first project where sprites send each other messages.

Nothing here needs a downloaded file.

The problem messages solve

The referee has to start and stop three racers. You could give the referee a list of every racer and call a method on each one. That works until you add a fourth racer, and then you have to remember to add it to the list too.

Messages turn it around. The referee shouts and does not care who is listening:

broadcast [go v]

Any sprite that wants to can listen:

when I receive [go v]

Add a fourth racer and it starts listening on its own. The referee never changes.

Step 1: The referee

The referee flips between stop and go every couple of seconds and announces it. broadcast sends the message to every sprite on the stage.

import org.openpatch.scratch.*;
 
public class Referee extends Sprite {
  private boolean green = false;
 
  public Referee() {
    this.addCostume("sign");
    this.setSize(45);
    this.setPosition(-250, 110);
  }
 
  public void run() {
    if (this.getTimer().everyMillis(2200)) {
      this.green = !this.green;
      if (this.green) {
        this.say("Go!");
        this.broadcast("go");
      } else {
        this.say("Stop!");
        this.broadcast("stop");
      }
    }
  }
}

this.green = !this.green flips a true into a false and back again. It is a tidy way of saying "switch to the other one".

Step 2: A racer that listens

whenIReceive is called on every sprite whenever anybody broadcasts. The message arrives as text, so the racer checks which one it was.

import org.openpatch.scratch.*;
 
public class Racer extends Sprite {
  private String creature;
  private double speed;
  private boolean running = false;
 
  public Racer(String creature, double y, double speed) {
    this.creature = creature;
    this.speed = speed;
    this.addCostume(creature);
    this.addCostume(creature + "_move");
    this.setSize(45);
    this.setPosition(-260, y);
  }
 
  public void whenIReceive(String message) {
    if (message.equals("go")) {
      this.running = true;
    }
    if (message.equals("stop")) {
      this.running = false;
      this.switchCostume(this.creature);
    }
  }
 
  public void run() {
    if (!this.running) {
      return;
    }
    this.changeX(this.speed);
    if (this.getTimer().everyMillis(150)) {
      this.nextCostume();
    }
    if (this.getX() > 170) {
      this.say("I win!");
      this.running = false;
    }
  }
}

The racer never mentions the referee, and the referee never mentions the racer. Neither knows the other exists.

Step 3: One class, three racers

Look at the constructor: Racer(String creature, double y, double speed). Until now your sprites have been made with new Something() and nothing else. Giving the constructor parameters means one class can produce three different racers.

import org.openpatch.scratch.*;
 
public class RaceStage extends Stage {
  public RaceStage() {
    super(600, 340);
    this.addBackdrop("background");
 
    this.add(new Referee());
    this.add(new Racer("bee", 60, 2.2));
    this.add(new Racer("ladybug", 0, 1.6));
    this.add(new Racer("snail", -60, 1.0));
  }
 
  public static void main(String[] args) {
    new RaceStage();
  }
}

The costume name is a parameter too, and because every creature in the built-in library has a matching _move costume, creature + "_move" gives the racer its second picture without you naming it twice.

In Scratch this would have been three separate sprites, each with its own copy of the same scripts, and a change to one meaning three edits.

The finished project

If something will not run, you can compare against the finished version. It needs no assets, only the library itself.

Things to try

• Add a fourth racer — frog and fishGreen both have _move costumes. Notice that the referee needs no change at all.
• Broadcast a "reset" message that sends everyone back to the start.
• Make the referee wait a random time instead of always 2.2 seconds, so the stop cannot be predicted.