first version of a logging library.

lib/logging/logging.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * Provides APIs for debugging and error logging. This library introduces
+ * abstractions similar to those used in other languages, such as the Closure JS
+ * Logger and java.util.logging.Logger.
+ */
+#library('logging');
+
+/**
+ * Whether to allow fine-grain logging and configuration of loggers in a
+ * hierarchy. When false, all logging is merged in the root logger.
+ */
+bool hierarchicalLoggingEnabled = false;
+
+/**
+ * Level for the root-logger. This will be the level of all loggers if
+ * [hierarchicalLoggingEnabled] is false.
+ */
+Level _rootLevel = Level.INFO;
+
+
+/**
+ * Use a [Logger] to log debug messages. [Logger]s are named using a
+ * hierarchical dot-separated name convention.
+ */
+class Logger {
+  /** Simple name of this logger. */
+  final String name;
+
+  /** The full name of this logger, which includes also the parent's names. */

[gram, 2012/06/29 19:55:21]

Remove 'also'

[Siggi Cherem (dart-lang), 2012/06/29 20:16:52]

Done.

+  String get fullName() =>
+      (parent == null || parent.name == '') ? name : '${parent.fullName}.$name';

[gram, 2012/06/29 19:55:21]

What is I have three levels A, B, C, and A is named 'A', B has name '', and C
has name 'C'. Now C's fullname would be 'C', which seems wrong. It's a weird
case I agree, but I think if you use hierarhical logging you are very likely
going to give each level a name and the empty name should not be treated
specially. 

[Siggi Cherem (dart-lang), 2012/06/29 20:16:52]

as we discussed in person - this shouldn't be a problem because the hierarchy is
constructed from the names and you can't build the hierarchy by hand.

There is a cornercase when names start with a '.' making (new Logger('.foo') ===
new Logger('foo')). I'll add a fix for that case.

+
+  /** Parent of this logger in the hierarchy of loggers. */
+  final Logger parent;
+
+  /** Logging [Level] used for entries generated on this logger. */
+  Level _level;
+
+  /** Children in the hierarchy of loggers, indexed by their simple names. */
+  Map<String, Logger> children;
+
+  /** Handlers used to process log entries in this logger. */
+  List<LoggerHandler> _handlers;
+
+  /**
+   * Singleton constructor. Calling `new Logger(name)` will return the same
+   * actual instance whenever it is called with the same string name.
+   */
+  factory Logger(String name) {
+    if (_loggers == null) _loggers = <Logger>{};
+    if (_loggers.containsKey(name)) return _loggers[name];

[gram, 2012/06/29 19:55:21]

you could have an "else" here.

[Siggi Cherem (dart-lang), 2012/06/29 20:16:52]

true. as we briefly discussed :), left as is to make the code shorter.

+
+    // Split hierarchical names (separated with '.').
+    int dot = name.lastIndexOf('.');
+    Logger parent = null;
+    String thisName;
+    if (dot == -1) {
+      if (name != '') parent = new Logger('');
+      thisName = name;
+    } else {
+      parent = new Logger(name.substring(0, dot));
+      thisName = name.substring(dot + 1);
+    }
+    final res = new Logger._internal(thisName, parent);
+    _loggers[name] = res;
+    return res;
+  }
+
+  Logger._internal(this.name, this.parent)
+      : children = new Map<String, Logger>() {
+    if (parent != null) parent.children[name] = this;
+  }
+
+  /**
+   * Effective level considering the levels established in this logger's parents
+   * (when [hierarchicalLoggingEnabled] is true).
+   */
+  Level get level() {
+    if (hierarchicalLoggingEnabled) {
+      if (_level != null) return _level;
+      if (parent != null) return parent.level;
+    }
+    return _rootLevel;
+  }
+
+  /** Override the level for this particular [Logger] and its children. */
+  Level set level(value) {
+    if (hierarchicalLoggingEnabled && parent != null) {
+      _level = value;
+    } else {
+      if (parent != null) {
+        throw new UnsupportedOperationException(
+            'Please set "hierarchicalLoggingEnabled" to true if you want to '
+            'change the level on a non-root logger.');
+      }
+      _rootLevel = value;
+    }
+  }
+
+  /**
+   * Returns an event manager for this [Logger]. You can listen for log messages
+   * by adding a [LoggerHandler] to an event from the event manager, for
+   * instance:
+   *    logger.on.record.add((record) { ... });
+   */
+  LoggerEvents get on() => new LoggerEvents(this);
+
+  /** Adds a handler to listen whenever a log record is added to this logger. */
+  void _addHandler(LoggerHandler handler) {
+    if (hierarchicalLoggingEnabled || parent == null) {
+      if (_handlers == null) {
+        _handlers = new List<LoggerHandler>();
+      }
+      _handlers.add(handler);
+    } else {
+      root._addHandler(handler);
+    }
+  }
+
+  /** Remove a previously added handler. */
+  void _removeHandler(LoggerHandler handler) {
+    if (hierarchicalLoggingEnabled || parent == null) {
+      if (_handlers == null) return;
+      int index = _handlers.indexOf(handler);
+      if (index != -1) _handlers.removeRange(index, 1);
+    } else {
+      root._removeHandler(handler);
+    }
+  }
+
+  /** Removes all handlers previously added to this logger. */
+  void _clearHandlers() {
+    if (hierarchicalLoggingEnabled || parent == null) {
+      _handlers = null;
+    } else {
+      root._clearHandlers();
+    }
+  }
+
+  /** Whether a message for [value]'s level is loggable in this logger. */
+  bool isLoggable(Level value) => (value >= level);
+
+  /**
+   * Adds a log record for a [message] at a particular [logLevel] if
+   * `isLoggable(logLevel)` is true. Use this method to create log entries for
+   * user-defined levels. To record a message at a predefined level (e.g.
+   * [Level.INFO], [Level.WARNING], etc) you can use their specialized methods
+   * instead (e.g. [info], [warning], etc).
+   */
+  // TODO(sigmund): add support for logging exceptions.
+  void log(Level logLevel, String message) {
+    if (isLoggable(logLevel)) {
+      var record = new LogRecord(logLevel, message, fullName);
+      if (hierarchicalLoggingEnabled) {
+        var target = this;
+        while (target != null) {
+          target._publish(record);
+          target = target.parent;
+        }
+      } else {
+        root._publish(record);
+      }
+    }
+  }
+
+  /** Log message at level [Level.FINEST]. */
+  void finest(String message) => log(Level.FINEST, message);
+
+  /** Log message at level [Level.FINER]. */
+  void finer(String message) => log(Level.FINER, message);
+
+  /** Log message at level [Level.FINE]. */
+  void fine(String message) => log(Level.FINE, message);
+
+  /** Log message at level [Level.CONFIG]. */
+  void config(String message) => log(Level.CONFIG, message);
+
+  /** Log message at level [Level.INFO]. */
+  void info(String message) => log(Level.INFO, message);
+
+  /** Log message at level [Level.WARNING]. */
+  void warning(String message) => log(Level.WARNING, message);
+
+  /** Log message at level [Level.SEVERE]. */
+  void severe(String message) => log(Level.SEVERE, message);
+
+  /** Log message at level [Level.SHOUT]. */
+  void shout(String message) => log(Level.SHOUT, message);
+
+  void _publish(LogRecord record) {
+    if (_handlers != null) {
+      _handlers.forEach((h) => h(record));
+    }
+  }
+
+  /** Top-level root [Logger]. */
+  static get root() => new Logger('');
+
+  /** All [Logger]s in the system. */
+  static Map<String, Logger> _loggers;
+}
+
+
+/** Handler callback to process log entries as they are added to a [Logger]. */
+typedef void LoggerHandler(LogRecord);
+
+
+/** Event manager for a [Logger] (holds events that a [Logger] can fire). */
+class LoggerEvents {
+  final Logger _logger;
+
+  LoggerEvents(this._logger);
+
+  /** Event fired when a log record is added to a [Logger]. */
+  LoggerHandlerList get record() => new LoggerHandlerList(_logger);
+}
+
+
+/** List of handlers that will be called on a logger event. */
+class LoggerHandlerList {
+  Logger _logger;
+
+  LoggerHandlerList(this._logger);
+
+  void add(LoggerHandler handler) => _logger._addHandler(handler);
+  void remove(LoggerHandler handler) => _logger._removeHandler(handler);
+  void clear() => _logger._clearHandlers();
+}
+
+
+/**
+ * [Level]s to control logging output. Logging can be enabled to include all
+ * levels above certain [Level]. [Level]s are ordered using an integer
+ * value [Level.value]. The predefined [Level] constants below are sorted as
+ * follows (in descending order): [Level.SHOUT], [Level.SEVERE],
+ * [Level.WARNING], [Level.INFO], [Level.CONFIG], [Level.FINE], [Level.FINER],
+ * [Level.FINEST], and [Level.ALL].
+ *
+ * We recommend using one of the predefined logging levels. If you define your
+ * own level, make sure you use a value between those used in [Level.ALL] and
+ * [Level.OFF].
+ */
+class Level implements Comparable, Hashable {
+
+  // TODO(sigmund): mark name/value as 'const' when the language supports it.
+  final String name;
+
+  /**
+   * Unique value for this level. Used to order levels, so filtering can exclude
+   * messages whose level is under certain value.
+   */
+  final int value;
+
+  const Level(this.name, this.value);
+
+  /** Special key to turn on logging for all levels ([value] = 0). */
+  static final Level ALL = const Level('ALL', 0);
+
+  /** Special key to turn off all logging ([value] = 2000). */
+  static final Level OFF = const Level('OFF', 2000);
+
+  /** Key for highly detailed tracing ([value] = 300). */
+  static final Level FINEST = const Level('FINEST', 300);
+
+  /** Key for fairly detailed tracing ([value] = 400). */
+  static final Level FINER = const Level('FINER', 400);
+
+  /** Key for tracing information ([value] = 500). */
+  static final Level FINE = const Level('FINE', 500);
+
+  /** Key for static configuration messages ([value] = 700). */
+  static final Level CONFIG = const Level('CONFIG', 700);
+
+  /** Key for informational messages ([value] = 800). */
+  static final Level INFO = const Level('INFO', 800);
+
+  /** Key for potential problems ([value] = 900). */
+  static final Level WARNING = const Level('WARNING', 900);
+
+  /** Key for serious failures ([value] = 1000). */
+  static final Level SEVERE = const Level('SEVERE', 1000);
+
+  /** Key for extra debugging loudness ([value] = 1200). */
+  static final Level SHOUT = const Level('SHOUT', 1200);
+
+  bool operator ==(Level other) => other != null && value == other.value;
+  bool operator <(Level other) => value < other.value;
+  bool operator <=(Level other) => value <= other.value;
+  bool operator >(Level other) => value > other.value;
+  bool operator >=(Level other) => value >= other.value;
+  int compareTo(Level other) => value - other.value;
+  int hashCode() => value;
+  String toString() => name;
+}
+
+
+/**
+ * A log entry representation used to propagate information from [Logger] to
+ * individual [Handler]s.
+ */
+class LogRecord {
+  final Level level;
+  final String message;
+
+  /** Logger where this record is stored. */
+  final String loggerName;
+
+  /** Time when this record was created. */
+  final Date time;
+
+  /** Unique sequence number greater than all log records created before it. */
+  final int sequenceNumber;
+
+  static int _nextNumber = 0;
+
+  /** Associated exception (if any) when recording errors messages. */
+  Exception exception;
+
+  /** Associated exception message (if any) when recording errors messages. */
+  String exceptionText;
+
+  LogRecord(
+      this.level, this.message, this.loggerName,
+      [time, this.exception, this.exceptionText]) :
+    this.time = (time == null) ? new Date.now() : time,
+    this.sequenceNumber = LogRecord._nextNumber++;
+}