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Initializing

By default, the logging package does not do anything useful with the log messages. You must configure the logging level and add a handler for the log messages.

Here is a simple logging configuration that logs all messages via print.

Logger.root.level = Level.ALL; // defaults to Level.INFO
Logger.root.onRecord.listen((record) {
  print('${record.level.name}: ${record.time}: ${record.message}');
});

First, set the root Level. All messages at or above the current level are sent to the onRecord stream. Available levels are:

  • Level.OFF
  • Level.SHOUT
  • Level.SEVERE
  • Level.WARNING
  • Level.INFO
  • Level.CONFIG
  • Level.FINE
  • Level.FINER
  • Level.FINEST

Then, listen on the onRecord stream for LogRecord events. The LogRecord class has various properties for the message, error, logger name, and more.

To listen for changed level notifications use:

Logger.root.onLevelChanged.listen((level) {
  print('The new log level is $level');
});

Logging messages

Create a Logger with a unique name to easily identify the source of the log messages.

final log = Logger('MyClassName');

Here is an example of logging a debug message and an error:

var future = doSomethingAsync().then((result) {
  log.fine('Got the result: $result');
  processResult(result);
}).catchError((e, stackTrace) => log.severe('Oh noes!', e, stackTrace));

When logging more complex messages, you can pass a closure instead that will be evaluated only if the message is actually logged:

log.fine(() => [1, 2, 3, 4, 5].map((e) => e * 4).join("-"));

Available logging methods are:

  • log.shout(logged_content);
  • log.severe(logged_content);
  • log.warning(logged_content);
  • log.info(logged_content);
  • log.config(logged_content);
  • log.fine(logged_content);
  • log.finer(logged_content);
  • log.finest(logged_content);

Configuration

Loggers can be individually configured and listened to. When an individual logger has no specific configuration, it uses the configuration and any listeners found at Logger.root.

To begin, set the global boolean hierarchicalLoggingEnabled to true.

Then, create unique loggers and configure their level attributes and assign any listeners to their onRecord streams.

  hierarchicalLoggingEnabled = true;
  Logger.root.level = Level.WARNING;
  Logger.root.onRecord.listen((record) {
    print('[ROOT][WARNING+] ${record.message}');
  });

  final log1 = Logger('FINE+');
  log1.level = Level.FINE;
  log1.onRecord.listen((record) {
    print('[LOG1][FINE+] ${record.message}');
  });

  // log2 inherits LEVEL value of WARNING from `Logger.root`
  final log2 = Logger('WARNING+');
  log2.onRecord.listen((record) {
    print('[LOG2][WARNING+] ${record.message}');
  });


  // Will NOT print because FINER is too low level for `Logger.root`.
  log1.finer('LOG_01 FINER (X)');

  // Will print twice ([LOG1] & [ROOT])
  log1.fine('LOG_01 FINE (√√)');

  // Will print ONCE because `log1` only uses root listener.
  log1.warning('LOG_01 WARNING (√)');

  // Will never print because FINE is too low level.
  log2.fine('LOG_02 FINE (X)');

  // Will print twice ([LOG2] & [ROOT]) because warning is sufficient for all
  // loggers' levels.
  log2.warning('LOG_02 WARNING (√√)');

  // Will never print because `info` is filtered by `Logger.root.level` of
  // `Level.WARNING`.
  log2.info('INFO (X)');

Results in:

[LOG1][FINE+] LOG_01 FINE (√√)
[ROOT][WARNING+] LOG_01 FINE (√√)
[LOG1][FINE+] LOG_01 WARNING (√)
[ROOT][WARNING+] LOG_01 WARNING (√)
[LOG2][WARNING+] LOG_02 WARNING (√√)
[ROOT][WARNING+] LOG_02 WARNING (√√)

Libraries

logging