Logger onLevelChanged notification (#138)

Notify when the log level is changed. 
Logger `onLevelChanged` broadcasts a stream of level values.
Fixes #139

Author: desistefanova

CHANGELOG.md

@@ -1,5 +1,6 @@
-## 1.1.2-wip
+## 1.2.0-wip
 
+* Add notification when the log level is changed. Logger `onLevelChanged` broadcasts a stream of level values.
 * Require Dart 2.19.
 
 ## 1.1.1

README.md

@@ -33,6 +33,14 @@ First, set the root `Level`. All messages at or above the current level are sent
 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:
+
+```dart
+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

lib/src/logger.dart

@@ -55,6 +55,9 @@ class Logger {
   /// root [Logger].
   StreamController<LogRecord>? _controller;
 
+  /// Controller used to notify when the log level of this logger is changed.
+  StreamController<Level?>? _levelChangedController;
+
   /// Create or find a Logger by name.
   ///
   /// Calling `Logger(name)` will return the same instance whenever it is called
@@ -139,7 +142,26 @@ class Logger {
       throw UnsupportedError(
           'Cannot set the level to `null` on a logger with no parent.');
     }
+    final isLevelChanged = _level != value;
     _level = value;
+    if (isLevelChanged) {
+      _levelChangedController?.add(value);
+    }
+  }
+
+  /// Returns a stream of level values set to this [Logger].
+  ///
+  /// You can listen for set levels using the standard stream APIs,
+  /// for instance:
+  ///
+  /// ```dart
+  /// logger.onLevelChanged.listen((level) { ... });
+  /// ```
+  /// A state error will be thrown if the level is changed
+  /// inside the callback.
+  Stream<Level?> get onLevelChanged {
+    _levelChangedController ??= StreamController<Level?>.broadcast(sync: true);
+    return _levelChangedController!.stream;
   }
 
   /// Returns a stream of messages added to this [Logger].

pubspec.yaml

@@ -1,5 +1,5 @@
 name: logging
-version: 1.1.2-wip
+version: 1.2.0-wip
 description: >-
   Provides APIs for debugging and error logging, similar to loggers in other
   languages, such as the Closure JS Logger and java.util.logging.Logger.

test/logging_test.dart

@@ -714,5 +714,32 @@ void main() {
       expect(records[1].error, isNotNull);
       expect(records[2].error, isNull);
     });
+
+    test('listen for level changed', () {
+      final levels = <Level?>[];
+      root.level = Level.ALL;
+      root.onLevelChanged.listen(levels.add);
+      root.level = Level.SEVERE;
+      root.level = Level.WARNING;
+      expect(levels, hasLength(2));
+    });
+
+    test('onLevelChanged is not emited if set the level to the same value', () {
+      final levels = <Level?>[];
+      root.level = Level.ALL;
+      root.onLevelChanged.listen(levels.add);
+      root.level = Level.ALL;
+      expect(levels, hasLength(0));
+    });
+
+    test('setting level in a loop throws state error', () {
+      root.level = Level.ALL;
+      root.onLevelChanged.listen((event) {
+        // Cannot fire new event. Controller is already firing an event
+        expect(() => root.level = Level.SEVERE, throwsStateError);
+      });
+      root.level = Level.WARNING;
+      expect(root.level, Level.SEVERE);
+    });
   });
 }