forcePixels method Null safety

  1. @protected
void forcePixels(
  1. double value
)

Change the value of pixels to the new value, and notify any customers, but without honoring normal conventions for changing the scroll offset.

This is used to implement jumpTo. It can also be used adjust the position when the dimensions of the viewport change. It should only be used when manually implementing the logic for honoring the relevant conventions of the class. For example, ScrollPositionWithSingleContext introduces ScrollActivity objects and uses forcePixels in conjunction with adjusting the activity, e.g. by calling ScrollPositionWithSingleContext.goIdle, so that the activity does not immediately set the value back. (Consider, for instance, a case where one is using a DrivenScrollActivity. That object will ignore any calls to forcePixels, which would result in the rendering stuttering: changing in response to forcePixels, and then changing back to the next value derived from the animation.)

To cause the position to jump or animate to a new value, consider jumpTo or animateTo.

This should not be called during layout (e.g. when setting the initial scroll offset). Consider correctPixels if you find you need to adjust the position during layout.

Implementation

@protected
void forcePixels(double value) {
  assert(hasPixels);
  assert(value != null);
  _impliedVelocity = value - pixels;
  _pixels = value;
  notifyListeners();
  SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
    _impliedVelocity = 0;
  });
}