# Migrate `of` to non-nullable return values, and add `maybeOf` > To eliminate nullOk parameters to help with API clarity in the face of null safety. :::important These breaking change docs are accurate, as of the release under which they are published. Over time, the workarounds described here might become inaccurate. We don't, in general, keep these breaking change docs up to date as of each release. The [breaking change index file](/release/breaking-changes) lists the docs created for each release. ::: ## Summary This migration guide describes conversion of code that uses various static `of` functions to retrieve information from a context that used to return nullable values, but now return non-nullable values. ## Context Flutter has a common pattern of allowing lookup of some types of widgets (typically [`InheritedWidget`][]s, but also others) using static member functions that are typically called `of`. When non-nullability was made the default, it was then desirable to have the most commonly used APIs return a non-nullable value. This is because saying `Scrollable.of(context)` and then still requiring an `!` operator or `?` and a fallback value after that call felt awkward, and was not idiomatic for non-nullable Dart code. A lot of this migration was performed when we eliminated `nullOk` parameters in a [previous migration][], but some `of` methods were missed in that migration, and some were subsequently added with nullable return types, counter to our common pattern. In this migration, the affected `of` accessors were split into two calls: one that returned a non-nullable value and threw an exception when the sought-after value was not present (still called `of`), and one that returned a nullable value that didn't throw an exception, and returned null if the value was not present (a new method called `maybeOf`). ## Description of change The change modified these static `of` APIs to return non-nullable values. If a value is not found, they will also now assert in debug mode, and throw an exception in release mode. * [`AutofillGroup.of`] * [`DefaultTabController.of`] * [`DefaultTextHeightBehavior.of`] * [`Form.of`] * [`HeroControllerScope.of`] * [`Material.of`] * [`Overlay.of`] * [`PageStorage.of`] * [`PrimaryScrollController.of`] * [`RenderAbstractViewport.of`] * [`RestorationScope.of`] * [`Scrollable.of`] * [`ScrollNotificationObserver.of`] This change also introduced new static `maybeOf` APIs alongside the above functions, which return a nullable version of the same value, and simply return null if the value is not found, without throwing any exceptions. * [`AutofillGroup.maybeOf`] * [`DefaultTabController.maybeOf`] * [`DefaultTextHeightBehavior.maybeOf`] * [`Form.maybeOf`] * [`HeroControllerScope.maybeOf`] * [`Material.maybeOf`] * [`Overlay.maybeOf`] * [`PageStorage.maybeOf`] * [`PrimaryScrollController.maybeOf`] * [`RenderAbstractViewport.maybeOf`] * [`RestorationScope.maybeOf`] * [`Scrollable.maybeOf`] * [`ScrollNotificationObserver.maybeOf`] ## Migration guide To modify your code to use the new form of the APIs, first convert all instances of the original static `of` functions (where its nullability is important) to use the `maybeOf` form instead. Code before migration: ```dart ScrollController? controller = Scrollable.of(context); ``` Code after migration: ```dart ScrollController? controller = Scrollable.maybeOf(context); ``` Then, for instances where the code calls the `of` API followed by an exclamation point, just remove the exclamation point: it can no longer return a nullable value. Code before migration: ```dart ScrollController controller = Scrollable.of(context)!; ``` Code after migration: ```dart ScrollController controller = Scrollable.of(context); ``` The following can also be helpful: * [`unnecessary_non_null_assertion`][] (linter message) identifies places where an `!` operator should be removed * [`unnecessary_null_checks`][] (analysis option) identifies places where the `?` operator isn't needed * [`unnecessary_null_in_if_null_operators`][] identifies places where a `??` operator isn't needed * [`unnecessary_nullable_for_final_variable_declarations`][] (analysis option) finds unnecessary question mark operators on `final` and `const` variables ## Timeline In stable release: 3.7 ## References API documentation: * [`Material.of`][] Relevant PRs: * [Add `maybeOf` for all the cases when `of` returns nullable][] * [Add `Overlay.maybeOf`, make `Overlay.of` return a non-nullable instance][] [previous migration]: /release/breaking-changes/eliminating-nullok-parameters [`unnecessary_non_null_assertion`]: https://dart.dev/tools/diagnostic-messages#unnecessary_non_null_assertion [`unnecessary_null_checks`]: https://dart.dev/tools/linter-rules#unnecessary_null_checks [`unnecessary_null_in_if_null_operators`]: https://dart.dev/tools/linter-rules#unnecessary_null_in_if_null_operators [`unnecessary_nullable_for_final_variable_declarations`]: https://dart.dev/tools/linter-rules#unnecessary_nullable_for_final_variable_declarations [`AutofillGroup.maybeOf`]: https://api.flutter.dev/flutter/widgets/AutofillGroup/maybeOf.html [`AutofillGroup.of`]: https://api.flutter.dev/flutter/widgets/AutofillGroup/of.html [`DefaultTabController.maybeOf`]: https://api.flutter.dev/flutter/material/DefaultTabController/maybeOf.html [`DefaultTabController.of`]: https://api.flutter.dev/flutter/material/DefaultTabController/of.html [`DefaultTextHeightBehavior.maybeOf`]: https://api.flutter.dev/flutter/widgets/DefaultTextHeightBehavior/maybeOf.html [`DefaultTextHeightBehavior.of`]: https://api.flutter.dev/flutter/widgets/DefaultTextHeightBehavior/of.html [`Form.maybeOf`]: https://api.flutter.dev/flutter/widgets/Form/maybeOf.html [`Form.of`]: https://api.flutter.dev/flutter/widgets/Form/of.html [`HeroControllerScope.maybeOf`]: https://api.flutter.dev/flutter/widgets/HeroControllerScope/maybeOf.html [`HeroControllerScope.of`]: https://api.flutter.dev/flutter/widgets/HeroControllerScope/of.html [`InheritedWidget`]: https://api.flutter.dev/flutter/widgets/InheritedWidget-class.html [`Material.maybeOf`]: https://api.flutter.dev/flutter/material/Material/maybeOf.html [`Material.of`]: https://api.flutter.dev/flutter/material/Material/of.html [`Overlay.maybeOf`]: https://api.flutter.dev/flutter/widgets/Overlay/maybeOf.html [`Overlay.of`]: https://api.flutter.dev/flutter/widgets/Overlay/of.html [`PageStorage.maybeOf`]: https://api.flutter.dev/flutter/widgets/PageStorage/maybeOf.html [`PageStorage.of`]: https://api.flutter.dev/flutter/widgets/PageStorage/of.html [`PrimaryScrollController.maybeOf`]: https://api.flutter.dev/flutter/widgets/PrimaryScrollController/maybeOf.html [`PrimaryScrollController.of`]: https://api.flutter.dev/flutter/widgets/PrimaryScrollController/of.html [`RenderAbstractViewport.maybeOf`]: https://api.flutter.dev/flutter/rendering/RenderAbstractViewport/maybeOf.html [`RenderAbstractViewport.of`]: https://api.flutter.dev/flutter/rendering/RenderAbstractViewport/of.html [`RestorationScope.maybeOf`]: https://api.flutter.dev/flutter/widgets/RestorationScope/maybeOf.html [`RestorationScope.of`]: https://api.flutter.dev/flutter/widgets/RestorationScope/of.html [`Scrollable.maybeOf`]: https://api.flutter.dev/flutter/widgets/Scrollable/maybeOf.html [`Scrollable.of`]: https://api.flutter.dev/flutter/widgets/Scrollable/of.html [`ScrollNotificationObserver.maybeOf`]: https://api.flutter.dev/flutter/widgets/ScrollNotificationObserver/maybeOf.html [`ScrollNotificationObserver.of`]: https://api.flutter.dev/flutter/widgets/ScrollNotificationObserver/of.html [Add `maybeOf` for all the cases when `of` returns nullable]: https://github.com/flutter/flutter/pull/114120 [Add `Overlay.maybeOf`, make `Overlay.of` return a non-nullable instance]: https://github.com/flutter/flutter/pull/110811