AvoidExpandedOutsideFlexRule class

Warns when Expanded or Flexible is used outside Row, Column, or Flex.

Since: v2.3.10 | Updated: v4.13.0 | Rule version: v7

Alias: expanded_outside_flex, flexible_parent

Expanded and Flexible only work inside Flex widgets (Row, Column, Flex). Using them elsewhere causes runtime errors.

Why This Crashes

Expanded, Flexible, and Spacer work by writing FlexParentData onto their child's render object. Only RenderFlex — the render object behind Row, Column, and Flex — reads that data during layout. Every other parent render object either ignores or rejects it. When Flutter detects the mismatch it throws an unrecoverable ParentDataWidget error that cannot be caught by try-catch.

The most dangerous variant is when a reusable widget returns Expanded from its build() method. The widget appears to work when placed directly in a Row, but the moment anyone wraps it — with Padding, LimitedBox, GestureDetector, or any other widget — the Flex→Expanded parent chain breaks and the app crashes at runtime.

BAD - Inside non-Flex container:

Stack(
  children: [
    Expanded(child: Container()), // CRASH!
  ],
)

BAD - Wrapped by RenderObject widgets:

class _MyWidget extends StatelessWidget {
  Widget build(BuildContext context) => Expanded(child: Text('Hi'));
}
// Usage: Row(children: [Padding(child: _MyWidget())]) // CRASH!
// The Padding breaks the Flex→Expanded parent chain.

GOOD - Direct child of Flex:

Column(
  children: [
    Expanded(child: Container()),
  ],
)

GOOD - Assigned to variable, used in Flex:

final content = Expanded(child: Text('Hi'));
return Column(children: [content]); // OK - lint trusts variable usage

GOOD - Helper method returning Expanded:

List<Widget> _buildChildren() {
  return [Expanded(child: Text('Hi'))]; // OK - trusts helper methods
}
Widget build(BuildContext context) => Row(children: _buildChildren());

GOOD - Collection builders (List.generate, .map):

Column(
  children: List.generate(3, (i) => Expanded(child: Text('$i'))), // OK
)
Row(
  children: items.map((i) => Expanded(child: Text(i))).toList(), // OK
)

Trusted Patterns (No False Positives)

The rule trusts these patterns and does not report them:

Variable assignment: final x = Expanded(...);
Helper method returns: Expanded in return statements of non-build methods
Collection builders: Expanded inside List.generate() or .map() callbacks

When to Ignore

Use // ignore: avoid_expanded_outside_flex if the widget's build() returns Expanded and you're certain it's only used as a direct Flex child.

Design Guidance

Prefer adding Expanded at the call site rather than inside widget definitions. This makes the flex behavior explicit and avoids crashes when the widget is wrapped with Padding, GestureDetector, etc.

Inheritance

Object
SaropaLintRule
AvoidExpandedOutsideFlexRule

Constructors

AvoidExpandedOutsideFlexRule()

Properties

accuracyTarget → AccuracyTarget?: Optional accuracy target for this rule (for documentation and tooling). Does not enforce; used by reports and rule-audit scripts.
no setterinherited
applicableFileTypes → Set<FileType>?: The file types this rule applies to.
no setteroverride
canUseParsedResult → bool: Indicates whether this analysis rule can work with just the parsed information or if it requires a resolved unit.
no setterinherited
certIds → List<String>: CERT coding standard identifiers (e.g. STR02-C). Populate only where there is a clear mapping; leave empty for most rules initially.
no setterinherited
code → LintCode: The lint code for this rule.
no setterinherited
configAliases → List<String>: Alternate config keys that can be used to reference this rule.
no setterinherited
conflictingRules → List<String>: Curated opposite/competing rule names.
no setterinherited
cost → RuleCost: The estimated execution cost of this rule.
no setteroverride
cweIds → List<int>: CWE identifiers this rule helps prevent or detect. https://cwe.mitre.org/ — e.g. CWE-798 (Hardcoded Credentials).
no setterinherited
description → String: Short description suitable for display in console output and IDEs.
finalinherited
diagnosticCode → DiagnosticCode: The code to report for a violation.
no setterinherited
diagnosticCodes → List<DiagnosticCode>: The diagnostic codes associated with this analysis rule.
no setterinherited
documentationUrl → String: Returns the documentation URL for this rule.
no setterinherited
effectiveSeverity → DiagnosticSeverity?: Get the effective severity for this rule, considering overrides.
no setterinherited
exampleBad → String?: Short code example that VIOLATES this rule (shown in CLI walkthrough).
no setterinherited
exampleGood → String?: Short code example of COMPLIANT code (shown in CLI walkthrough).
no setterinherited
fixGenerators → List<SaropaFixGenerator>: Fix producer generators for this rule.
no setteroverride
hashCode → int: The hash code for this object.
no setterinherited
hyphenatedName → String: Returns the rule name in hyphenated format for display.
no setterinherited
impact → LintImpact: Expanded/Flexible outside Flex causes runtime crash.
no setteroverride
incompatibleRules → List<String>: A list of incompatible rule names.
no setterinherited
isDisabled → bool: Check if this rule is disabled via configuration.
no setterinherited
maximumLineCount → int: Maximum line count for this rule to run.
no setterinherited
minimumLineCount → int: Minimum line count for this rule to run.
no setterinherited
name → String: The rule name.
finalinherited
owasp → OwaspMapping?: OWASP categories this rule helps prevent.
no setterinherited
pubspecVisitor → PubspecVisitor?: A visitor that visits a Pubspec to perform analysis.
no setterinherited
relatedRules → List<String>: Curated "see also" rule names for discoverability in docs/IDE tooling.
no setterinherited
reporter ← DiagnosticReporter: Sets the DiagnosticReporter for the CompilationUnit currently being visited.
no getterinherited
requiredPatterns → Set<String>?: String patterns that must be present in the file for this rule to run.
no setterinherited
requiresAsync → bool: Whether this rule only applies to async code.
no setterinherited
requiresBlocImport → bool: Whether this rule only applies to files that import Bloc.
no setterinherited
requiresClassDeclaration → bool: Whether this rule only applies to files with class declarations.
no setterinherited
requiresFlutterImport → bool: Whether this rule only applies to files that import Flutter.
no setterinherited
requiresImports → bool: Whether this rule only applies to files with imports.
no setterinherited
requiresMainFunction → bool: Whether this rule only applies to files with a main() function.
no setterinherited
requiresProviderImport → bool: Whether this rule only applies to files that import Provider.
no setterinherited
requiresRiverpodImport → bool: Whether this rule only applies to files that import Riverpod.
no setterinherited
requiresWidgets → bool: Whether this rule only applies to Flutter widget code.
no setterinherited
ruleStatus → RuleStatus: Lifecycle status. Default RuleStatus.ready. Use RuleStatus.beta for new or heuristic-heavy rules; RuleStatus.deprecated for sunset.
no setterinherited
ruleType → RuleType?: Semantic type of this rule. Default null = unspecified (legacy). When set, used for quality gates, accuracy targets, and reporting.
no setteroverride
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited
skipExampleFiles → bool: Whether to skip example files (example/**).
no setterinherited
skipFixtureFiles → bool: Whether to skip fixture files (fixture/, fixtures/).
no setterinherited
skipGeneratedCode → bool: Whether to skip generated files (*.g.dart, *.freezed.dart, *.gen.dart).
no setterinherited
skipTestFiles → bool: Whether to skip test files (*_test.dart, test/**).
no setterinherited
state → RuleState: The state of this analysis rule.
finalinherited
supersedesRules → List<String>: Rule names this rule supersedes/replaces.
no setterinherited
tags → Set<String>: Tags for filtering and discovery (e.g. in docs, IDE, or CI). Examples: 'performance', 'accessibility', 'suspicious', 'convention'.
no setteroverride
testRelevance → TestRelevance: How this rule relates to test files.
no setterinherited

Methods

noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
registerNodeProcessors(RuleVisitorRegistry registry, RuleContext ruleContext) → void: Registers node processors in the given registry.
inherited
reportAtNode(AstNode? node, {List<Object> arguments = const [], List<DiagnosticMessage>? contextMessages}) → Diagnostic?: Reports a diagnostic at node with message arguments and contextMessages.
inherited
reportAtOffset(int offset, int length, {List<Object> arguments = const [], List<DiagnosticMessage>? contextMessages}) → Diagnostic: Reports a diagnostic at offset, with length, with message arguments and contextMessages.
inherited
reportAtPubNode(PubspecNode node, {List<Object> arguments = const [], List<DiagnosticMessage> contextMessages = const []}) → Diagnostic: Reports a diagnostic at Pubspec node, with message arguments and contextMessages.
inherited
reportAtSourceRange(SourceRange sourceRange, {List<Object> arguments = const [], List<DiagnosticMessage>? contextMessages}) → Diagnostic: Reports a diagnostic at sourceRange, with message arguments and contextMessages.
inherited
reportAtToken(Token token, {List<Object> arguments = const [], List<DiagnosticMessage>? contextMessages}) → Diagnostic?: Reports a diagnostic at token, with message arguments and contextMessages.
inherited
runWithReporter(SaropaDiagnosticReporter reporter, SaropaContext context) → void: Override this method to implement your lint rule.
override
shouldSkipFile(String path) → bool: Check if a file path should be skipped based on context settings.
inherited
toString() → String: A string representation of this object.
inherited

Operators

operator ==(Object other) → bool: The equality operator.
inherited