Log4Dart is a powerful and extensible logger for Dart inspired by Slf4J. It supports logging at ERROR, WARN, INFO and DEBUG levels and has easy support for configuring appenders and logging formats. It also supports a nested diagnostic context that allows you to eassly make session specific log traces accross deep calls.

Using log4dart

To use Log4Dart add the following to your pubspec.yaml file

  log4dart: any

install it by executing pub install and in your Dart program add one of the imports

// for client side logging (works when compiled to JS)
import "package:log4dart/log4dart.dart";

// for VM logging, allows you to log to files (does not compile to JS) 
import "package:log4dart/log4dart_vm.dart";

Now you are ready to use log4dart. The example below shows you how to use it in your code.

class MyClass {
    : _logger = LoggerFactory.getLogger("MyClass");

  someMethod() {
    _logger.info("a info message");
    _logger.warnFormat("%s %s", ["a warning message", "formated using c's sprintf syntax"]);
  final Logger _logger;

Log configuration

The logger is configured through the LogConfig API that can be accessed by logger name or wildcard via the LoggerFactory.config"logger-name" handle. Below are some examples:

// Disable info for all loggers 
LoggerFactory.config["*"].infoEnabled = false;
// Set the default logging format for all loggers
LoggerFactory.config["*"].logFormat = "[%d] %c %n:%x %m";
// Override logging levels for specifc loggers
LoggerFactory.config["MyClass"].debugEnabled = false;
LoggerFactory.config["MyClass"].infoEnabled = true;

For log formating log4dart supports many of the same options as is known from other loggers, such as:

  • c Output the level (category) of the logging event
  • d Output the date when the log message was recorded
  • m Output the actual logging message
  • n Output the name of the logger that recorded the log
  • x Output the context of the logger

Log4Dart defaults to logging to the console but also supports various appenders

  • ConsoleAppender Appender that logs to the console
  • FileAppender Appender that logs to a file
  • StringAppender Appender that logs to a string buffer

appenders are also configured through the LoggerFactory.config interface

// Use a file appender for a specifc logger
LoggerFactory.config["OtherClass"].appenders = [new FileAppender("/tmp/log.txt")];

to get output in multiple places, just add multiple appenders to the appenders list.

Diagnostic support

The logger supports nested diagnostic contexts which can be used to track application state like this

logger.putContext("context-name", "context-message");
try {
  // log messages from now gets added a context-message
  logger.debug("something important happend");
} finally {
  // stop logging with context-message

A running example of this can be seen in the context_log_test.dart class in the test folder.


Some missing stuff (feel free to add more):

  1. Generate DartDoc for Logger and Appender classes
  2. Create a Dart version of sprintf and use it for implementing the formatters
  3. When reflection arrives in Dart add ability to show the class/line where the log message originated

feel free to send in patched for these (or other features you miss).


BSD License (Same as Dart itself). See LICENSE file.



Logger that depends on VM functionality (file access).