Add unit tests and edit README.md

Author: Bonny BUN

README.md

@@ -107,6 +107,9 @@ for testing a firebase service that isn't fully mocked.
     - ```ref```
     - ```value```
     - ```exists```
+    - ```hasChild```
+    - ```child```
+    - ```children```
 
 
 ### Contributing

test/mock_data_snapshot_test.dart

@@ -20,4 +20,56 @@ void main() {
   test('Should return key that is the same as in reference', () {
     expect(MockDataSnapshot(reference, null).key, equals(reference.key));
   });
+
+  group("child tests", () {
+    late List<String> sampleList;
+    late Map<String, dynamic> sampleMap;
+    setUp(() {
+      sampleList = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j"];
+      sampleMap = {"a": 2, "b": 5, "c": 0};
+    });
+
+    test('Should return the same updated list', () async {
+      await reference.update({"list": sampleList});
+      final dataSnapshot = await reference.child("list").get();
+
+      expect(dataSnapshot.value, sampleList);
+      expect(dataSnapshot.hasChild("1"), true);
+      expect(dataSnapshot.hasChild("-1"), false);
+      expect(dataSnapshot.child("1").value, "b");
+      expect(dataSnapshot.child("-1").value, null);
+      var children = dataSnapshot.children.toList();
+      for (int i = 0; i < children.length; i++) {
+        expect(children[i].value, sampleList[i]);
+      }
+    });
+
+    test('Should return the same updated map', () async {
+      await reference.update({"map": sampleMap});
+
+      final MockDataSnapshot dataSnapshot =
+          await reference.child("map").get() as MockDataSnapshot;
+      expect(dataSnapshot.value, sampleMap);
+      expect(dataSnapshot.hasChild("b"), true);
+      expect(dataSnapshot.hasChild("foo"), false);
+      expect(dataSnapshot.child("b").value, 5);
+      expect(dataSnapshot.child("d").value, null);
+      var children = dataSnapshot.children.toList();
+      for (int i = 0; i < children.length; i++) {
+        expect(children[i].value, sampleMap[children[i].key]);
+      }
+    });
+
+    test('Should return the same single value', () async {
+      await reference.update({"value": 42});
+      var dataSnapshot = (await reference.child("value").get());
+      expect(dataSnapshot.value, 42);
+      expect(dataSnapshot.child("foo").value, null);
+
+      await reference.set(100);
+      dataSnapshot = (await reference.get());
+      expect(dataSnapshot.value, 100);
+      expect(dataSnapshot.children, []);
+    });
+  });
 }