null_closures

Do not pass null as an argument where a closure is expected.

This rule is available as of Dart 2.0.0.

Rule sets: recommended, flutter

This rule has a quick fix available.

Details

DON’T pass null as an argument where a closure is expected.

Often a closure that is passed to a method will only be called conditionally, so that tests and “happy path” production calls do not reveal that null will result in an exception being thrown.

This rule only catches null literals being passed where closures are expected in the following locations:

Constructors

• From dart:async
  • Future at the 0th positional parameter
  • Future.microtask at the 0th positional parameter
  • Future.sync at the 0th positional parameter
  • Timer at the 0th positional parameter
  • Timer.periodic at the 1st positional parameter
• From dart:core
  • List.generate at the 1st positional parameter

Static functions

• From dart:async
  • scheduleMicrotask at the 0th positional parameter
  • Future.doWhile at the 0th positional parameter
  • Future.forEach at the 0th positional parameter
  • Future.wait at the named parameter cleanup
  • Timer.run at the 0th positional parameter

Instance methods

• From dart:async
  • Future.then at the 0th positional parameter
  • Future.complete at the 0th positional parameter
• From dart:collection
  • Queue.removeWhere at the 0th positional parameter
  • `Queue.retain
  • Iterable.firstWhere at the 0th positional parameter, and the named parameter orElse
  • Iterable.forEach at the 0th positional parameter
  • Iterable.fold at the 1st positional parameter
  • Iterable.lastWhere at the 0th positional parameter, and the named parameter orElse
  • Iterable.map at the 0th positional parameter
  • Iterable.reduce at the 0th positional parameter
  • Iterable.singleWhere at the 0th positional parameter, and the named parameter orElse
  • Iterable.skipWhile at the 0th positional parameter
  • Iterable.takeWhile at the 0th positional parameter
  • Iterable.where at the 0th positional parameter
  • List.removeWhere at the 0th positional parameter
  • List.retainWhere at the 0th positional parameter
  • String.replaceAllMapped at the 1st positional parameter
  • String.replaceFirstMapped at the 1st positional parameter
  • String.splitMapJoin at the named parameters onMatch and onNonMatch

BAD:

[1, 3, 5].firstWhere((e) => e.isOdd, orElse: null);

GOOD:

[1, 3, 5].firstWhere((e) => e.isOdd, orElse: () => null);

Usage

To enable the null_closures rule, add null_closures under linter > rules in your analysis_options.yaml file:

linter:
  rules:
    - null_closures