prefer_async_callback

v0.4.0 Warning Fix Type Annotations

Flags Future<void> Function() written where the AsyncCallback typedef from package:flutter/foundation.dart would say the same thing. The quick fix replaces the type and adds the import.

Only the exact shape is reported: no parameters, and a Future<void> return. Future<int> Function() and Future<void> Function(int) are left alone — there is no typedef for them.

Don’t

A callback field on a widget, spelled out longhand:

class UploadButton {
  const UploadButton({required this.onPressed, this.onCancel});

  final Future<void> Function() onPressed;
  final Future<void> Function()? onCancel;
}

The same type in a function signature and a return type:

void retryOnFailure(Future<void> Function() action) {}

Future<void> Function() saveDraftAction(String draftId) =>
    () async {};

And in a collection of pending work:

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

Do

import 'package:flutter/foundation.dart';

class UploadButton {
  const UploadButton({required this.onPressed, this.onCancel});

  final AsyncCallback onPressed;
  final AsyncCallback? onCancel;
}

void retryOnFailure(AsyncCallback action) {}

AsyncCallback saveDraftAction(String draftId) => () async {};

final List<AsyncCallback> pendingUploads = [];

Shapes that are never reported

AsyncCallback is exactly Future<void> Function(), so anything with a parameter or a different return type has no typedef to swap in:

// A result to await, not just completion.
Future<int> Function() fetchCount = () async => 0;

// Takes an argument.
Future<void> Function(int index) removeAt = (_) async {};

// Synchronous — that one is `VoidCallback`; see prefer_void_callback.
void Function() dismiss = () {};

See also: AsyncCallback typedef

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.