prefer_void_callback

v0.4.0 Warning Fix Type Annotations

Flags void Function() written where the VoidCallback typedef from dart:ui 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 void return. void Function(int) and int Function() are left alone — there is no typedef for them.

Don’t

A callback field on a widget, spelled out longhand:

class DismissButton {
  const DismissButton({required this.onPressed, this.onLongPress});

  final void Function() onPressed;
  final void Function()? onLongPress;
}

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

void onNextFrame(void Function() callback) {}

void Function() dismissAction(String routeName) => () {};

And in a collection of teardown work:

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

Do

import 'dart:ui';

class DismissButton {
  const DismissButton({required this.onPressed, this.onLongPress});

  final VoidCallback onPressed;
  final VoidCallback? onLongPress;
}

void onNextFrame(VoidCallback callback) {}

VoidCallback dismissAction(String routeName) => () {};

final List<VoidCallback> disposers = [];

In a Flutter file VoidCallback usually needs no import of its own — it is re-exported by package:flutter/foundation.dart and by material.dart.

Shapes that are never reported

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

// Takes an argument — that family is `ValueChanged<T>`.
void Function(int index) onSelected = (_) {};

// Returns a value.
int Function() itemCount = () => 0;

// Asynchronous — that one is `AsyncCallback`; see prefer_async_callback.
Future<void> Function() refresh = () async {};

See also: VoidCallback typedef

Configuration

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

many_lints.yaml

rules:
  prefer_void_callback: true

To turn it off again:

many_lints.yaml

rules:
  prefer_void_callback: false

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

Related rules

• 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.
• prefer_typedefs_for_callbacks — Name a multi-parameter function type with a typedef.
• prefer_async_callback — Use ‘AsyncCallback’ instead of ‘Future<void> Function()’.