ExtentScrollBehavior({double contentExtent: double.INFINITY, double containerExtent: 0.0, @required TargetPlatform platform })

Creates a scroll behavior for a scrollable widget with linear extent. We start with an INFINITE contentExtent so that we don't accidentally clamp a scrollOffset until we receive an accurate value in updateExtents.

The extents and the platform must not be null.

Source 

ExtentScrollBehavior({
  double contentExtent: double.INFINITY,
  double containerExtent: 0.0,
  @required TargetPlatform platform
}) : _contentExtent = contentExtent,
     _containerExtent = containerExtent,
     super(platform: platform);