ScaleUpdateDetails({Point focalPoint: Point.origin, double scale: 1.0 })

Creates details for GestureScaleUpdateCallback.

The focalPoint and scale arguments must not be null. The scale argument must be greater than or equal to zero.

Source 

ScaleUpdateDetails({ this.focalPoint: Point.origin, this.scale: 1.0 }) {
  assert(focalPoint != null);
  assert(scale != null && scale >= 0.0);
}