RangeMaintainingScrollPhysics class

Scroll physics that attempt to keep the scroll position in range when the contents change dimensions suddenly.

This attempts to maintain the amount of overscroll or underscroll already present, if the scroll position is already out of range and the extents have decreased, meaning that some content was removed. The reason for this condition is that when new content is added, keeping the same overscroll would mean that instead of showing it to the user, all of it is being skipped by jumping right to the max extent.

If the scroll activity is animating the scroll position, sudden changes to the scroll dimensions are allowed to happen (so as to prevent animations from jumping back and forth between in-range and out-of-range values).

These physics should be combined with other scroll physics, e.g. BouncingScrollPhysics or ClampingScrollPhysics, to obtain a complete description of typical scroll physics. See applyTo.

Implementation details

Specifically, these physics perform two adjustments.

The first is to maintain overscroll when the position is out of range.

The second is to enforce the boundary when the position is in range.

If the current velocity is non-zero, neither adjustment is made. The assumption is that there is an ongoing animation and therefore further changing the scroll position would disrupt the experience.

If the extents haven't changed, then the overscroll adjustment is not made. The assumption is that if the position is overscrolled, it is intentional, otherwise the position could not have reached that position. (Consider ClampingScrollPhysics vs BouncingScrollPhysics for example.)

If the position itself changed since the last animation frame, then the overscroll is not maintained. The assumption is similar to the previous case: the position would not have been placed out of range unless it was intentional.

In addition, if the position changed and the boundaries were and still are finite, then the boundary isn't enforced either, for the same reason. However, if any of the boundaries were or are now infinite, the boundary is enforced, on the assumption that infinite boundaries indicate a lazy-loading scroll view, which cannot enforce boundaries while the full list has not loaded.

If the range was out of range, then the boundary is not enforced even if the range is not maintained. If the range is maintained, then the distance between the old position and the old boundary is applied to the new boundary to obtain the new position.

If the range was in range, and the boundary is to be enforced, then the new position is obtained by deferring to the other physics, if any, and then clamped to the new range.