createConstraint-component.js

// Or inline before the <a-scene>.
      AFRAME.registerComponent('createConstraint', {
        // ...
		// Target (other) body for the constraint.
    target: { type: "selector" },

    // Offset of the hinge or point-to-point constraint, defined locally in the body. Used for hinge, coneTwist pointToPoint constraints.
    pivot: { type: "vec3" },
    targetPivot: { type: "vec3" },

    // An axis that each body can rotate around, defined locally to that body. Used for hinge constraints.
    axis: { type: "vec3", default: { x: 0, y: 1, z: 0 } },
    targetAxis: { type: "vec3", default: { x: 0, y: 1, z: 0 } }
  },

  init: function() {
    this.system = this.el.sceneEl.systems.physics;
    this.constraint = null;
  },

  remove: function() {
    if (!this.constraint) return;

    this.system.removeConstraint(this.constraint);
    this.constraint = null;
  },

  update: function() {
    const el = this.el,
      data = this.data;

    this.remove();

    if (!el.body || !data.target.body) {
      (el.body ? data.target : el).addEventListener("body-loaded", this.update.bind(this, {}), { once: true });
      return;
    }

    this.constraint = this.createConstraint();
    this.system.addConstraint(this.constraint);
  },
/**
   * @return {Ammo.btTypedConstraint}
   */
  createConstraint, function() {
    let constraint;
    const data = this.data,
      body = this.el.body,
      targetBody = data.target.body;

    const bodyTransform = body
      .getCenterOfMassTransform()
      .inverse()
      .op_mul(targetBody.getWorldTransform());
    const targetTransform = new Ammo.btTransform();
    targetTransform.setIdentity();

    switch (data.type) {
      case CONSTRAINT.LOCK: {
        constraint = new Ammo.btGeneric6DofConstraint(body, targetBody, bodyTransform, targetTransform, true);
        const zero = new Ammo.btVector3(0, 0, 0);
        //TODO: allow these to be configurable
        constraint.setLinearLowerLimit(zero);
        constraint.setLinearUpperLimit(zero);
        constraint.setAngularLowerLimit(zero);
        constraint.setAngularUpperLimit(zero);
        Ammo.destroy(zero);
        break;
      }
      //TODO: test and verify all other constraint types
      case CONSTRAINT.FIXED: {
        //btFixedConstraint does not seem to debug render
        bodyTransform.setRotation(body.getWorldTransform().getRotation());
        targetTransform.setRotation(targetBody.getWorldTransform().getRotation());
        constraint = new Ammo.btFixedConstraint(body, targetBody, bodyTransform, targetTransform);
        break;
      }
      case CONSTRAINT.SPRING: {
        constraint = new Ammo.btGeneric6DofSpringConstraint(body, targetBody, bodyTransform, targetTransform, true);
        //TODO: enableSpring, setStiffness and setDamping
        break;
      }
      case CONSTRAINT.SLIDER: {
        //TODO: support setting linear and angular limits
        constraint = new Ammo.btSliderConstraint(body, targetBody, bodyTransform, targetTransform, true);
        constraint.setLowerLinLimit(-1);
        constraint.setUpperLinLimit(1);
        // constraint.setLowerAngLimit();
        // constraint.setUpperAngLimit();
        break;
      }
      case CONSTRAINT.HINGE: {
        const pivot = new Ammo.btVector3(data.pivot.x, data.pivot.y, data.pivot.z);
        const targetPivot = new Ammo.btVector3(data.targetPivot.x, data.targetPivot.y, data.targetPivot.z);

        const axis = new Ammo.btVector3(data.axis.x, data.axis.y, data.axis.z);
        const targetAxis = new Ammo.btVector3(data.targetAxis.x, data.targetAxis.y, data.targetAxis.z);

        constraint = new Ammo.btHingeConstraint(body, targetBody, pivot, targetPivot, axis, targetAxis, true);

        Ammo.destroy(pivot);
        Ammo.destroy(targetPivot);
        Ammo.destroy(axis);
        Ammo.destroy(targetAxis);
        break;
      }
      case CONSTRAINT.CONE_TWIST: {
        const pivotTransform = new Ammo.btTransform();
        pivotTransform.setIdentity();
        pivotTransform.getOrigin().setValue(data.targetPivot.x, data.targetPivot.y, data.targetPivot.z);
        constraint = new Ammo.btConeTwistConstraint(body, pivotTransform);
        Ammo.destroy(pivotTransform);
        break;
      }
      case CONSTRAINT.POINT_TO_POINT: {
        const pivot = new Ammo.btVector3(data.pivot.x, data.pivot.y, data.pivot.z);
        const targetPivot = new Ammo.btVector3(data.targetPivot.x, data.targetPivot.y, data.targetPivot.z);

        constraint = new Ammo.btPoint2PointConstraint(body, targetBody, pivot, targetPivot);

        Ammo.destroy(pivot);
        Ammo.destroy(targetPivot);
        break;
      }
      default:
        throw new Error("[constraint] Unexpected type: " + data.type);
    }

    Ammo.destroy(bodyTransform);
    Ammo.destroy(targetTransform);

    return constraint;
  }
  
});