Logger
- Dart course
- Advanced
- Lesson 63/83
- 5 min
Learn Logger instances and hierarchy.
Logger hierarchy
The Logger class works hierarchically with dot-separated names. Logger('uygulama') is a parent logger; Logger('uygulama.veritabani') is its child. The child inherits the parent's level.
Example: when Logger.root.level = Level.INFO; both Logger('uygulama') and Logger('uygulama.veritabani') record INFO and above. r.loggerName tells which logger the record came from. This is the clean way to split logs by module in large applications; each module uses its own child logger.
Parent and child logger
When a child logger writes a record the record also propagates to parent loggers. With Logger.root.onRecord.listen you capture all records in a single handler. With r.loggerName you tell which logger the record came from.
Example: final appLog = Logger('uygulama'); final dbLog = Logger('uygulama.veritabani');. The record appLog.info('started') has name 'uygulama'; dbLog.info('opened') has name 'uygulama.veritabani'. In the handler you print r.loggerName to see the source. This makes record tracking easier in large systems.
Practice Parent and child logger records
Add import 'package:logging/logging.dart';. In main add Logger.root.level = Level.INFO; and Logger.root.onRecord.listen((r) => print('${r.loggerName}: ${r.message}'));. Create final appLog = Logger('uygulama'); and final dbLog = Logger('uygulama.veritabani');. Call appLog.info('started') and dbLog.info('opened').
Starter code
// import the logging package
void main() {
// set root level and handler, log parent and child records
}Sample solution
import 'package:logging/logging.dart';
void main() {
Logger.root.level = Level.INFO;
Logger.root.onRecord.listen((r) => print('${r.loggerName}: ${r.message}'));
final appLog = Logger('uygulama');
final dbLog = Logger('uygulama.veritabani');
appLog.info('started');
dbLog.info('opened');
}Expected output
uygulama: started uygulama.veritabani: opened
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the logger hierarchy
| Term | Meaning |
|---|---|
Logger('a.b') |
Child logger |
'.' |
Hierarchy separator |
parent |
Parent |
child |
Child |
2 What does Logger('a.b') represent?
- A single logger
- Two separate loggers
- A hierarchical child logger Correct answer
- Sibling loggers
Explanation: Logger('a.b') is a child logger of 'a'.
3 Creating a child logger
final dbLog = ____('uygulama.veritabani');final dbLog = Logger('uygulama.veritabani');Explanation: Logger('uygulama.veritabani') creates a child logger.