prefer-extracting-callbacks

added in: 1.6.0

style

⚙️

Warns about inline callbacks in a widget tree and suggests to extract them to widget methods in order to make a build method more readable. In addition extracting can help test those methods separately as well.

info

This rule will not trigger on:

• arrow functions like onPressed: () => _handler(...) in order to cover cases when a callback needs a variable from the outside;
• empty blocks.
• Flutter specific: arguments with functions returning Widget type (or its subclass) and with first parameter of type BuildContext. "Builder" functions is a common pattern in Flutter, for example, IndexedWidgetBuilder typedef is used in ListView.builder.

⚙️ Config

Set allowed-line-count (default is none) to configure the maximum number of lines after which the rule should trigger.

Set ignored-named-arguments (default is none) to ignore specific named parameters.

dart_code_metrics:
  ...
  rules:
    ...
    - prefer-extracting-callbacks:
        ignored-named-arguments:
          - onPressed
        allowed-line-count: 3

Example

❌ Bad:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      style: ...,
      onPressed: () { // LINT
        // Some
        // Huge
        // Callback
      },
      child: ...
    );
  }
}

✅ Good:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      style: ...,
      onPressed: () => handlePressed(context),
      child: ...
    );
  }

  void handlePressed(BuildContext context) {
    ...
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      style: ...,
      onPressed: handlePressed,
      child: ...
    );
  }

  void handlePressed() {
    ...
  }
}