Add LogfmtPrinter

Author: haarts

lib/logger.dart

@@ -10,6 +10,7 @@ export 'src/outputs/stream_output.dart';
 export 'src/outputs/memory_output.dart';
 export 'src/printers/pretty_printer.dart';
 export 'src/printers/simple_printer.dart';
+export 'src/printers/logfmt_printer.dart';
 
 export 'src/log_output.dart'
     if (dart.library.io) 'src/outputs/file_output.dart';

lib/src/printers/logfmt_printer.dart

@@ -0,0 +1,35 @@
+import 'package:logger/src/log_printer.dart';
+import 'package:logger/src/logger.dart';
+
+/// Outputs a logfmt message:
+/// ```
+/// level=debug msg="hi there" time="2015-03-26T01:27:38-04:00" animal=walrus number=8 tag=usum
+/// ```
+class LogfmtPrinter extends LogPrinter {
+  static final levelPrefixes = {
+    Level.verbose: 'verbose',
+    Level.debug: 'debug',
+    Level.info: 'info',
+    Level.warning: 'warning',
+    Level.error: 'error',
+    Level.wtf: 'wtf',
+  };
+
+  @override
+  List<String> log(LogEvent event) {
+    var output = StringBuffer("level=${levelPrefixes[event.level]}");
+    if (event.message is String) {
+      output.write(' msg="${event.message}"');
+    } else if (event.message is Map) {
+      event.message.entries.forEach((entry) {
+        if (entry.value is num) {
+          output.write(' ${entry.key}=${entry.value}');
+        } else {
+          output.write(' ${entry.key}="${entry.value}"');
+        }
+      });
+    }
+
+    return [output.toString()];
+  }
+}

test/printers/logfmt_printer_test.dart

@@ -0,0 +1,44 @@
+import 'package:test/test.dart';
+import 'package:logger/logger.dart';
+
+void main() {
+  var printer = LogfmtPrinter();
+
+  test('includes level', () {
+    expect(
+      printer.log(LogEvent(
+        Level.debug,
+        "some message",
+        Exception("boom"),
+        StackTrace.current,
+      ))[0],
+      contains("level=debug"),
+    );
+  });
+
+  test('with a string message includes a msg key', () {
+    expect(
+        printer.log(LogEvent(
+          Level.debug,
+          "some message",
+          Exception("boom"),
+          StackTrace.current,
+        ))[0],
+        contains('msg="some message"'));
+  });
+
+  test('includes random key=value pairs', () {
+    var output = printer.log(LogEvent(
+      Level.debug,
+      {'a': 123, 'foo': 'bar baz'},
+      Exception("boom"),
+      StackTrace.current,
+    ))[0];
+
+    expect(output, contains("a=123"));
+    expect(output, contains('foo="bar baz"'));
+  });
+
+  test('handles a stacktrace', () {}, skip: "TODO");
+  test('handles an error/exception', () {}, skip: "TODO");
+}