Custom lint
- Dart course
- Expert
- Lesson 29/47
- 5 min
Learn to write custom lint rules.
Custom lint
custom_lint is a package that lets you write custom lint rules. It is built on the analyzer; each rule inspects an AST node.
A lint rule extends the LintRule class: name (rule name), message (warning text) and a run method (check logic).
Run it with dart pub run custom_lint. Rules walk all files in the project and report violations. An IDE plugin makes them work in real time.
Rule declaration
A lint rule declares these parts: name (unique identifier, e.g. avoid_print), message (violation text) and a run method.
The run method takes a ResolvedUnitResult (resolved source). Inside it you walk the AST and call reporter.reportError(...) on violations.
Rules are gathered in a plugin: class MyPlugin extends PluginLintPlugin. The plugin is enabled via analysis_options.yaml.
Practice Define a LintRule skeleton
Define a LintRule class with final String name; and final String message; fields, a LintRule(this.name, this.message); constructor and a void report() => print('[$name] $message'); method. In main create final rule = LintRule('avoid_print', 'do not use print()'); and call rule.report();.
Starter code
void main() {
// define a LintRule class (name, message fields; report method)
// create a rule; call report
}Sample solution
class LintRule {
final String name;
final String message;
LintRule(this.name, this.message);
void report() => print('[$name] $message');
}
void main() {
final rule = LintRule('avoid_print', 'do not use print()');
rule.report();
}Expected output
[avoid_print] do not use print()
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 LintRule inheritance
class AvoidPrintRule extends ____ { }class AvoidPrintRule extends LintRule { }Explanation: every custom lint rule extends the LintRule class.
2 Match the lint rule parts
| Term | Meaning |
|---|---|
LintRule |
Rule base |
name |
Rule name |
message |
Warning text |
AstVisitor |
Node visit |
3 What does a custom_lint rule extend?
- LintPlugin
- LintRule Correct answer
- Diagnostic
- AnalyzerConfig
Explanation: every rule extends the LintRule class.