diff --git a/lib/src/observable_map.dart b/lib/src/observable_map.dart
index a99610c..54e40e2 100644
--- a/lib/src/observable_map.dart
+++ b/lib/src/observable_map.dart
@@ -56,6 +56,9 @@
     return result;
   }
 
+  /// Creates a new observable map wrapping [other].
+  ObservableMap.spy(Map<K, V> other) : _map = other;
+
   @override
   Iterable<K> get keys => _map.keys;
 
diff --git a/test/observable_map_test.dart b/test/observable_map_test.dart
index a424ce1..04aeccc 100644
--- a/test/observable_map_test.dart
+++ b/test/observable_map_test.dart
@@ -362,6 +362,21 @@
       });
     });
   });
+
+  group('Updates delegate as a spy', () {
+    Map delegate;
+    ObservableMap map;
+
+    setUp(() {
+      delegate = {};
+      map = new ObservableMap.spy(delegate);
+    });
+
+    test('[]=', () {
+      map['a'] = 42;
+      expect(delegate, {'a': 42});
+    });
+  });
 }
 
 _lengthChange(map, int oldValue, int newValue) =>