_onMouseMove(/*object*/ event) {
    const x = event.clientX;
    const y = event.clientY;

    this._deltaX += (x - this._x);
    this._deltaY += (y - this._y);

    if (this._animationFrameID === null) {
      // The mouse may move faster then the animation frame does.
      // Use `requestAnimationFramePolyfill` to avoid over-updating.
      this._animationFrameID =
        requestAnimationFramePolyfill(this._didMouseMove);
    }

    this._x = x;
    this._y = y;
    event.preventDefault();
  }
Calls onMove passed into constructor and updates internal state.