Dart:从Firebase快照解码Json

这可能是一个微不足道的问题,我尝试从Firebase快照解码json,以下是json的结构:

firebaseKey1 : {img1:value1}

firebaseKey2 : {img2:value2}

firebaseKey3 : {img3:value3}

以下是我尝试过的:

 DatabaseReference firebaseRef = FirebaseDatabase.instance.reference();
 firebaseRef.child('...').once().then((DataSnapshot snapshot) {
      Map<dynamic,dynamic> map = snapshot.value;
      map.forEach((key, jsonString) {
            print('$key: $jsonString'); // jsonString = {img1:value1}

            //how to get value1 inside jsonString?
     });
 });