cacheExtent property Null safety

1. flutter_json_widgets
2. widget
3. CustomScrollView
4. cacheExtent property

CustomScrollView class

1. Constructors
2. CustomScrollView
3. fromJson
4. Properties
5. anchor
6. cacheExtent
7. center
8. clipBehavior
9. copyWith
10. hashCode
11. key
12. keyboardDismissBehavior
13. physics
14. primary
15. restorationId
16. reverse
17. runtimeType
18. scrollDirection
19. semanticChildCount
20. shrinkWrap
21. slivers
22. Methods
23. map
24. mapOrNull
25. maybeMap
26. maybeWhen
27. noSuchMethod
28. toJson
29. toString
30. when
31. whenOrNull
32. Operators
33. operator ==