Logger

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.

Exercise 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').

Show example 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');
}

Test yourself

Match the logger hierarchy

Logger('a.b')

Child logger
'.'

Hierarchy separator
parent

Parent
child

Child
What does Logger('a.b') represent?
- A single logger
- Two separate loggers
- A hierarchical child logger
- Sibling loggers
Show answer

A hierarchical child logger — Logger('a.b') is a child logger of 'a'.

Creating a child logger

final dbLog = ____('uygulama.veritabani');

Show answer

final dbLog = Logger('uygulama.veritabani');

Next lesson: Handler

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