Dynamically controlled particle system with Toxiclibs

Hi all,

I'm working on modifying the Attraction2D example from the Toxiclibs library to be controlled by gestures from a Leap Motion sensor, as opposed to the mouse in the example.

I'm doing all my gesture recognition in an Open Frameworks app, and sending that over OSC.

When a "Gesture 0" event occurs, I call the method below to remove the gestureAttractor from the physics object:

void resetAttraction() {
  if (gestureAttractor != null){
      physics.removeBehavior(gestureAttractor);
      println("ATTRACTOR NULL");
     } else {
        println("not null");
     }
}

If a "Gesture 1" event occurs, I call this method to create a new gestureAttractor, and add it back to the physics object:

void addAttraction(){ if (gestureAttractor == null) { println("ATTRACTOR NULL"); position1.set(340, 191); gestureAttractor = new AttractionBehavior2D(position1, 250, 0.9f); physics.addBehavior(gestureAttractor); } else { println("not null"); } }

What seems to happen consistently is whenever the gesture state changes, I'll get a "Concurrent Modification Exception" crash at physics.update(); in the draw method.

I'm sure it has something to do with the way the lifecycle of these objects are handled, but I haven't been able to determine anything yet - anyone have any ideas?

Below is the entirety of the sketch:

import toxi.geom.*;
import toxi.physics2d.*;
import toxi.physics2d.behaviors.*;

import oscP5.*;
import netP5.*;

OscP5 oscP5;

int NUM_PARTICLES = 750;

VerletPhysics2D physics;
//AttractionBehavior2D mouseAttractor;
AttractionBehavior2D gestureAttractor;


//Vec2D mousePos;
Vec2D position1;


boolean isGestureAttractorAdded;

void setup() {
  size(680, 382,P3D);
  // setup physics with 10% drag
  physics = new VerletPhysics2D();
  physics.setDrag(0.05f);
  physics.setWorldBounds(new Rect(0, 0, width, height));
  // the NEW way to add gravity to the simulation, using behaviors
  physics.addBehavior(new GravityBehavior2D(new Vec2D(0, 0.15f)));

  // start oscP5, listening for incoming messages at port 12000
  oscP5 = new OscP5(this, 6000);

  position1 = new Vec2D(340, 191);

  addAttraction();

  //gestureAttractor = new AttractionBehavior2D(position1, 250, 0.9f);
  //physics.addBehavior(gestureAttractor);
}

void addParticle() {
  VerletParticle2D p = new VerletParticle2D(Vec2D.randomVector().scale(5).addSelf(width / 2, 0));
  physics.addParticle(p);
  // add a negative attraction force field around the new particle
  physics.addBehavior(new AttractionBehavior2D(p, 20, -1.2f, 0.01f));
}

void draw() {
  background(255,0,0);
  noStroke();
  fill(255);
  if (physics.particles.size() < NUM_PARTICLES) {
    addParticle();
  }
  physics.update();
  for (VerletParticle2D p : physics.particles) {
    ellipse(p.x, p.y, 5, 5);
  }
}

void mousePressed() {
  //position1 = new Vec2D(mouseX, mouseY);
   //create a new positive attraction force field around the mouse position (radius=250px)
  //gestureAttractor = new AttractionBehavior2D(position1, 250, 0.9f);
  //physics.addBehavior(gestureAttractor);

  //println(physics.behaviors);
}

void mouseDragged() {
  // update mouse attraction focal point
  //position1.set(mouseX, mouseY);
}

void mouseReleased() {
  // remove the mouse attraction when button has been released
  //physics.removeBehavior(gestureAttractor);
}


///// OSC RECEIVING

void oscEvent(OscMessage theOscMessage) {
  /* check if theOscMessage has the address pattern we are looking for. */

  if (theOscMessage.checkAddrPattern("/gesture_classification") == true)  {
    /* check if the typetag is the right one. */
    if(theOscMessage.checkTypetag("i")) {
      /* parse theOscMessage and extract the values from the osc message arguments. */
      int gestureClassLabel = theOscMessage.get(0).intValue();
      println(" Gesture is: ", gestureClassLabel);

      if (gestureClassLabel == 0){
        resetAttraction();
      } else if (gestureClassLabel == 1) {
        addAttraction();
      } else if (gestureClassLabel == 2) {
          //physics.removeBehavior(gestureAttractor);
      }
    }
  }

}

//////METHODS FOR SETTING POSITION / REMOVAL OF ATTRACTORS...

void resetAttraction() {
  if (gestureAttractor != null){
      physics.removeBehavior(gestureAttractor);
      println("ATTRACTOR NULL");
     } else {
        println("not null");
     }
 }

void addAttraction(){
     if (gestureAttractor == null) {
         println("ATTRACTOR NULL");
         position1.set(340, 191);
         gestureAttractor = new AttractionBehavior2D(position1, 250, 0.9f);
         physics.addBehavior(gestureAttractor);
     } else {
       println("not null");
     }
}