Add MultiOutput

This output sends events to different other outputs in parallel.
Credits go @gmpassos!

Author: haarts

lib/logger.dart

@@ -9,6 +9,7 @@ export 'src/filters/production_filter.dart';
 export 'src/outputs/console_output.dart';
 export 'src/outputs/stream_output.dart';
 export 'src/outputs/memory_output.dart';
+export 'src/outputs/multi_output.dart';
 
 export 'src/printers/pretty_printer.dart';
 export 'src/printers/logfmt_printer.dart';

lib/src/outputs/multi_output.dart

@@ -0,0 +1,24 @@
+import 'package:logger/src/log_output.dart';
+import 'package:logger/src/logger.dart';
+
+/// Logs simultaneously to multiple [LogOutput] outputs.
+class MultiOutput extends LogOutput {
+  List<LogOutput> _outputs;
+
+  MultiOutput(List<LogOutput> outputs) {
+    _outputs = _normalizeOutputs(outputs);
+  }
+
+  List<LogOutput> _normalizeOutputs(List<LogOutput> outputs) {
+    if (outputs == null) return [];
+
+    outputs.removeWhere((o) => o == null);
+
+    return outputs;
+  }
+
+  @override
+  void output(OutputEvent event) {
+    _outputs.forEach((o) => o.output(event));
+  }
+}

test/outputs/multi_output_test.dart

@@ -0,0 +1,39 @@
+import 'package:logger/logger.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Multiple outputs are populated with the same events', () {
+    final output1 = MemoryOutput(bufferSize: 2);
+    final output2 = MemoryOutput(bufferSize: 2);
+
+    final multiOutput = MultiOutput([output1, output2]);
+
+    final event0 = OutputEvent(Level.info, []);
+    multiOutput.output(event0);
+
+    expect(output1.buffer.length, 1);
+    expect(output2.buffer.length, 1);
+    expect(output1.buffer.elementAt(0), equals(output2.buffer.elementAt(0)));
+    expect(output1.buffer.elementAt(0), equals(event0));
+
+    final event1 = OutputEvent(Level.info, []);
+    multiOutput.output(event1);
+
+    expect(output1.buffer.length, 2);
+    expect(output2.buffer.length, 2);
+    expect(output1.buffer.elementAt(0), equals(output2.buffer.elementAt(0)));
+    expect(output1.buffer.elementAt(0), equals(event0));
+    expect(output1.buffer.elementAt(1), equals(output2.buffer.elementAt(1)));
+    expect(output1.buffer.elementAt(1), equals(event1));
+  });
+
+  test('passing null does not throw an exception', () {
+    final output = MultiOutput(null);
+    output.output(OutputEvent(Level.info, []));
+  });
+
+  test('passing null in the list does not throw an exception', () {
+    final output = MultiOutput([null]);
+    output.output(OutputEvent(Level.info, []));
+  });
+}