let world;

//list to track fixed objects

let boundaries = [];

//list to store all the box objects

let boxes = [];

​

function setup() {

  createCanvas(560, 390);

​

  //initialize box2d physics and create the world

  world = createWorld();

​

  //add fixed boundaries

  boundaries.push(new Boundary(width / 4, height - 5, width / 2 - 50, 10));

  boundaries.push(new Boundary(3 * width / 4, height - 50, width / 2 - 50, 10));

​

  let b = new Box(width / 2, 30);

  boxes.push(b);

}

​

function draw() {

  background(220);

​

  //we must always step through time

  let timeStep = 1.0 / 30;

  //second and third arguments are velocity and position iterations

  world.Step(timeStep, 10, 10);

​

  //boxes fall from the top every so often

  if (random(1) < 0.2) {

    let b = new Box(width / 2, 30);

    boxes.push(b);

  }

​

  //display all the boundaries

  for (let i = 0; i < boundaries.length; i++) {

    boundaries[i].display();

  }

​

  //display all the box objects

  for (let i = boxes.length - 1; i >= 0; i--) {

    boxes[i].display();

    if (boxes[i].done()) {

      boxes.splice(i, 1);

    }

  }

}