// Daniel Shiffman

// http://natureofcode.com

​

// Class to describe the spring joint (displayed as a line)

​

// Constructor

class Spring {

  constructor(x, y) {

    // At first it doesn't exist

    this.mouseJoint = null;

  }

​

  // If it exists we set its target to the mouse location

  update(x, y) {

    if (this.mouseJoint !== null) {

      // Always convert to world coordinates!

      let mouseWorld = scaleToWorld(x, y);

      this.mouseJoint.SetTarget(mouseWorld);

    }

  }

​

  display() {

    if (this.mouseJoint !== null) {

​

      let posA = this.mouseJoint.GetAnchorA();

      let posB = this.mouseJoint.GetAnchorB();

​

      // We can get the two anchor points

      let v1 = scaleToPixels(posA.x, posA.y);

      let v2 = scaleToPixels(posB.x, posB.y);

      // And just draw a line

      stroke(200);

      strokeWeight(2);

​

      line(v1.x, v1.y, v2.x, v2.y);

    }

  }

​

  // This is the key function where

  // we attach the spring to an x,y location

  // and the Box object's location

  bind(x, y, box) {

    // Define the joint

    let md = new box2d.b2MouseJointDef();

    // Body A is just a fake ground body for simplicity (there isn't anything at the mouse)

    md.bodyA = world.CreateBody(new box2d.b2BodyDef()); //world.GetGroundBody();

    // Body 2 is the box's boxy

    md.bodyB = box.body;

    // Get the mouse location in world coordinates

    let mp = scaleToWorld(x, y);

    // And that's the target

    //println(mp.x + " " + mp.y);

    md.target = mp;

    //println(md.target.x + " " + md.target.y);

​

    // Some stuff about how strong and bouncy the spring should be

    md.maxForce = 1000.0 * box.body.m_mass;

    md.frequencyHz = 5.0;

    md.dampingRatio = 0.9;

​

    // Make the joint!

    this.mouseJoint = world.CreateJoint(md);

  }

​

  destroy() {

    // We can get rid of the joint when the mouse is released

    if (this.mouseJoint !== null) {

      world.DestroyJoint(this.mouseJoint);

      this.mouseJoint = null;

    }

  }

}