createBallisticSimulation method Null safety

  1. @override
Simulation? createBallisticSimulation(
  1. ScrollMetrics position,
  2. double velocity
)
override

Returns a simulation for ballistic scrolling starting from the given position with the given velocity.

This is used by ScrollPositionWithSingleContext in the ScrollPositionWithSingleContext.goBallistic method. If the result is non-null, ScrollPositionWithSingleContext will begin a BallisticScrollActivity with the returned value. Otherwise, it will begin an idle activity instead.

The given position is only valid during this method call. Do not keep a reference to it to use later, as the values may update, may not update, or may update to reflect an entirely unrelated scrollable.

Implementation

@override
Simulation? createBallisticSimulation(ScrollMetrics position, double velocity) {
  // If we're out of range and not headed back in range, defer to the parent
  // ballistics, which should put us back in range at a page boundary.
  if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) ||
      (velocity >= 0.0 && position.pixels >= position.maxScrollExtent)) {
    return super.createBallisticSimulation(position, velocity);
  }
  final Tolerance tolerance = this.tolerance;
  final double target = _getTargetPixels(position, tolerance, velocity);
  if (target != position.pixels) {
    return ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance);
  }
  return null;
}