getSnapshot abstract method
      
Future<TargetedWidgetSnapshot?> 
getSnapshot(
    
- Offset location,
 - Object key,
 - ValueGetter<
Widget?> widgetBuilder 
Returns snapshot for given location and key. If widget is already
registered for the key, the snapshot will be provided immediately.
Otherwise the widgetBuilder will be called to create the widget and
snapshot will be provided as soon as the widget is laid out.
If widgetBuilder returns null or null widget is registered for given
key, the default child widget will be used as snapshot.
Implementation
Future<TargetedWidgetSnapshot?> getSnapshot(
  Offset location,
  Object key,
  ValueGetter<Widget?> widgetBuilder,
);