prefer_async_callback

v0.4.0 Warning Fix Type Annotations

Flags uses of Future<void> Function() that can be replaced with the AsyncCallback typedef from package:flutter/foundation.dart. The typedef is shorter, more readable, and is the standard Flutter convention for no-argument async void callbacks.

Why use this rule

AsyncCallback is a well-known typedef in the Flutter framework. Using it instead of the verbose Future<void> Function() makes code more concise and consistent with the rest of the Flutter ecosystem. The quick fix automatically replaces the type and adds the necessary import.

See also: AsyncCallback typedef

Don’t

class BadWidget {
  final Future<void> Function() onTap;
  final Future<void> Function()? onLongPress;

  const BadWidget(this.onTap, this.onLongPress);
}

void badParameter(Future<void> Function() callback) {}

Future<void> Function() badReturnType() => () async {};

List<Future<void> Function()> callbacks = [];

Do

class GoodWidget {
  final AsyncCallback onTap;
  final AsyncCallback? onLongPress;

  const GoodWidget(this.onTap, this.onLongPress);
}

// Function types with different return types or parameters are fine:
Future<int> Function() goodFutureInt = () async => 0;
Future<void> Function(int value) goodWithParams = (_) async {};
void Function() goodVoidCallback = () {};

Configuration

This rule is in the opinionated preset, so it is on with preset: opinionated, or by name:

many_lints.yaml

rules:
  prefer_async_callback: true

To turn it off again:

many_lints.yaml

rules:
  prefer_async_callback: false

To keep the rule on but skip certain paths, use per-rule exclude.

Related rules

• prefer_void_callback — Use ‘VoidCallback’ instead of ‘void Function()’.
• prefer_equatable_mixin — Prefer using EquatableMixin instead of extending Equatable.
• prefer_explicit_function_type — Prefer explicit function type annotations over the bare ‘Function’ type.
• prefer_explicit_parameter_names — Name the parameters of a function type.