building-rotavo-21.js

// _onPointermove(e)
this._touchX = e.clientX;
this._touchY = e.clientY;
this._angle =
  // initial rotation of the element
  this._initialAngle
  // subtract the starting touch angle
  - this._initialTouchAngle
  // add the current touch angle
  + Math.atan2(this._touchY - this._centerY, this._touchX - this._centerX);
// Normalise value back into a 2π range
this._angle = (this._angle + TWO_PI) % TWO_PI;
// Done, update the value!
this.value = this._angle;