V2.0.0

Breaking Changes:

• Remove onOffsetChange in SmartRefresher,autoLoad in RefreshConfiguration,scrollController in RefreshController
• add argument to onTwoLevel(callback when closed)

features

• migrate null-safety
• add needCallback in requestRefresh and requestLoading for avoiding the callback of onRefresh or onLoading

Bug fix

• In NestedScrollView+ClampingScrollPhysics(Android ScrollBehaviour),header can be seen when fling to top.
• unMounted widget used crash error when fast rebuild in requestRefresh
• fix sliverRefreshBody layoutSize instead of -0.001,it will crash error when viewportMainAxis=0

Other

• Add assert to avoid invalid usage