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prefer-extracting-callbacks

added in: 1.6.0
style
⚙️
Pro+

Warns about inline callbacks in a widget tree and suggests to extract them to widget methods in order to make the build method more readable.

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.
note

You can fix this rule's issues using the "Extract Method" assist.

⚙️ Config​

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

Set ignored-named-arguments (default is empty) to ignore specific named parameters (example).

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

Example​

❌ Bad:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      style: ...,
      // LINT: Prefer extracting the callback to a separate widget method.
      onPressed: () {
        // 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() {
    ...
  }
}

Example with "allowed-line-count"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - prefer-extracting-callbacks:
        allowed-line-count: 3

✅ Good:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      style: ...,
      // Correct, less than 3 lines
      onPressed: () {
        // No more than 3 lines
      },
      child: ...
    );
  }
}

Example with "ignored-named-arguments"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - prefer-extracting-callbacks:
        ignored-named-arguments:
          - onPressed

✅ Good:

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