2分钟
06 applyPhysicsToUserOffset
ClampingScrollPhysics 默认是没有重载 applyPhysicsToUserOffset 方法的，当 parent == null 时，用户的滑动 offset 是什么就返回什么：
double applyPhysicsToUserOffset(ScrollMetrics position, double offset) {
if (parent == null)
return offset;
return parent.applyPhysicsToUserOffset(position, offset);
}BouncingScrollPhysics 中对 applyPhysicsToUserOffset 方法进行了 override ，其中 用户没有达到边界前，依旧返回默认的 offset，当用户到达边界时，通过算法来达到模拟溢出阻尼效果。
///摩擦因子
double frictionFactor(double overscrollFraction) => 0.52 * math.pow(1 - overscrollFraction, 2);
@override
double applyPhysicsToUserOffset(ScrollMetrics position, double offset) {
assert(offset != 0.0);
assert(position.minScrollExtent <= position.maxScrollExtent);
if (!position.outOfRange)
return offset;
final double overscrollPastStart = math.max(position.minScrollExtent - position.pixels, 0.0);
final double overscrollPastEnd = math.max(position.pixels - position.maxScrollExtent, 0.0);
final double overscrollPast = math.max(overscrollPastStart, overscrollPastEnd);
final bool easing = (overscrollPastStart > 0.0 && offset < 0.0)
|| (overscrollPastEnd > 0.0 && offset > 0.0);
final double friction = easing
// Apply less resistance when easing the overscroll vs tensioning.
? frictionFactor((overscrollPast - offset.abs()) / position.viewportDimension)
: frictionFactor(overscrollPast / position.viewportDimension);
final double direction = offset.sign;
return direction * _applyFriction(overscrollPast, offset.abs(), friction);
}
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