Add G/KG/Pounds to a new weight comparison class

lib/src/weight.dart

@@ -0,0 +1,30 @@
+class Weight implements Comparable<Weight> {
+  /*
+   * The value of this weight object in grams.
+   */
+  final double _grams;
+
+  /// pounds to kg
+  static const double _pounds_to_kg = 0.45359237;
+
+  Weight.fromPounds({double value: 0.0}) : _grams = (value * _pounds_to_kg) * 1000;
+  Weight.fromGrams({double value: 0.0}) : _grams = value;
+  Weight.fromKilograms({double value: 0.0}) : _grams = value * 1000;
+
+  double get inPounds => _returnValue((_grams / 1000) / _pounds_to_kg);
+  double get inGrams => _returnValue(_grams);
+  double get inKiloGrams => _returnValue(_grams / 1000);
+
+  double _returnValue(num value) => double.parse(value.toString());
+
+  /**
+     * Compares this Weight to [other], returning zero if the values are equal.
+     *
+     * Returns a negative integer if this `Weight` is less than
+     * [other], or a positive integer if it is heavier.
+     *
+     * A negative `Weight` is always considered less than a positive one.
+     *
+     */
+  int compareTo(Weight other) => _grams.compareTo(other._grams);
+}

lib/units.dart

@@ -7,4 +7,5 @@ library units;
 export 'src/speed.dart';
 export 'src/length.dart';
 export 'src/angle.dart';
-export 'src/temperature.dart';
+export 'src/temperature.dart';
+export 'src/weight.dart';

test/test_all.dart

@@ -4,10 +4,12 @@ import 'angle_test.dart' as angle;
 import 'length_test.dart' as length;
 import 'speed_test.dart' as speed;
 import 'temperature_test.dart' as temperature;
+import 'weight_test.dart' as weight;
 
 void main() {
     group('angle', angle.main);
     group('length', length.main);
     group('speed', speed.main);
     group('temperature', temperature.main);
+    group('weight', weight.main);
 }

test/weight_test.dart

@@ -0,0 +1,46 @@
+import 'package:units/units.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Weight tests', () {
+    test('Having -1.0 should match return', () {
+      expect(new Weight.fromGrams(value: -1.0).inKiloGrams, equals(-0.001));
+      expect(new Weight.fromGrams(value: -1.0).inPounds,
+          equals(-0.002204622621848776));
+    });
+
+    test('Having 1 gram should match return', () {
+      expect(new Weight.fromGrams(value: 1.0).inGrams, equals(1.0));
+      expect(new Weight.fromGrams(value: 1.0).inKiloGrams, equals(0.001));
+      expect(new Weight.fromGrams(value: 1.0).inPounds, equals(0.002204622621848776));
+    });
+
+    test('Having 1200 gram should match return', () {
+      expect(new Weight.fromGrams(value: 1200.0).inGrams, equals(1200));
+      expect(new Weight.fromGrams(value: 1200.0).inKiloGrams, equals(1.2));
+      expect(new Weight.fromGrams(value: 1200.0).inPounds, equals(2.6455471462185307));
+    });
+  });
+
+  group('Weight comparison tests', () {
+    Weight weight1;
+    Weight weight2;
+
+    setUp(() {
+      weight1 = Weight.fromGrams(value: 1200.0);
+      weight2 = Weight.fromPounds(value: 3.0); // ~1.3kg
+    });
+
+    test('Comparing same weight, should return 0 (same weight)', () {
+      expect(weight1.compareTo(weight1), equals(0));
+    });
+
+    test('Comparing weight, weight 1 is lighter than weight 2', () {
+      expect(weight1.compareTo(weight2), lessThan(0));
+    });
+
+    test('Comparing weight, weight 2 is heavier than weight 1', () {
+      expect(weight2.compareTo(weight1), greaterThan(0));
+    });
+  });
+}