OnRefreshCallback typedef
        OnRefreshCallback =
     Future<void>  Function()
    
    
A callback function that's invoked when the EasyRefreshSliverRefreshControl is
pulled a refreshTriggerPullDistance. Must return a Future. Upon
completion of the Future, the EasyRefreshSliverRefreshControl enters the
RefreshMode.done state and will start to go away.
Implementation
typedef OnRefreshCallback = Future<void> Function();