code: function( world ){
  
              world.on( 'add:body', this.trackBody, this );
              world.on( 'remove:body', this.untrackBody, this );
              world.on( 'integrate:positions', this.sweep, this, 1 );
  
              // add current bodies
              var bodies = world.getBodies();
              for ( var i = 0, l = bodies.length; i < l; ++i ){
  
                  this.trackBody({ body: bodies[ i ] });
              }
          }
docs: extended