onRefresh property
Callback invoked when pulled by refreshTriggerPullDistance.
If provided, must return a Future which will keep the indicator in the RefreshMode.refresh state until the Future completes.
Can be null, in which case a single frame of RefreshMode.armed state will be drawn before going immediately to the RefreshMode.done where the sliver will start retracting.
Implementation
final OnRefreshCallback? onRefresh;