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 notitfications use:
Logger.root.onLevelChanged.listen((level) {
print('The new log level is $level');
});
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);
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 (√√)
For information about our publishing automation and release process, see https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.