This migration guide describes conversion of code that uses various staticof functions to retrieve information from a context that used to return nullable values, but now return non-nullable values.

Flutter has a common pattern of allowing lookup of some types of widgets (typicallyInheritedWidgets, but also others) using static member functions that are typically calledof.

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 sayingScrollable.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 eliminatednullOk parameters in aprevious migration, but someof methods were missed in that migration, and some were subsequently added with nullable return types, counter to our common pattern.

In this migration, the affectedof 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 calledof), 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 calledmaybeOf).

The change modified these staticof 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 staticmaybeOf 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

To modify your code to use the new form of the APIs, first convert all instances of the original staticof functions (where its nullability is important) to use themaybeOf form instead.

Code before migration:

ScrollController?controller=Scrollable.of(context);

Code after migration:

ScrollController?controller=Scrollable.maybeOf(context);

Then, for instances where the code calls theof API followed by an exclamation point, just remove the exclamation point: it can no longer return a nullable value.

Code before migration:

ScrollControllercontroller=Scrollable.of(context)!;

Code after migration:

ScrollControllercontroller=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 onfinal andconst variables

In stable release: 3.7

API documentation:

• Material.of

Relevant PRs:

• AddmaybeOf for all the cases whenof returns nullable
• AddOverlay.maybeOf, makeOverlay.of return a non-nullable instance