createBallisticSimulation method

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

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.

This method can potentially be called in every frame, even in the middle of what the user perceives as a single ballistic scroll. For example, in a ListView when previously off-screen items come into view and are laid out, this method may be called with a new ScrollMetrics.maxScrollExtent. The method implementation should ensure that when the same ballistic scroll motion is still intended, these calls have no side effects on the physics beyond continuing that motion.

Generally this is ensured by having the Simulation conform to a physical metaphor of a particle in ballistic flight, where the forces on the particle depend only on its position, velocity, and environment, and not on the current time or any internal state. This means that the time-derivative of Simulation.dx should be possible to write mathematically as a function purely of the values of Simulation.x, Simulation.dx, and the parameters used to construct the Simulation, independent of the time.

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 double target = _getTargetPixels(position, velocity);
  if (target != position.pixels) {
    return ScrollSpringSimulation(spring, position.pixels, target, velocity,
        tolerance: tolerance);
  }
  return null;
}