main.dart

FutureBuilder(
    future: dbRef.once(),
    builder: (context, AsyncSnapshot<DataSnapshot> snapshot) {
        if (snapshot.hasData) {
        lists.clear();
        Map<dynamic, dynamic> values = snapshot.data.value;
        values.forEach((key, values) {
            lists.add(values);
        });
        return new ListView.builder(
            shrinkWrap: true,
            itemCount: lists.length,
            itemBuilder: (BuildContext context, int index) {
                return Card(
                child: Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                    Text("Name: " + lists[index]["name"]),
                    Text("Age: "+ lists[index]["age"]),
                    Text("Type: " +lists[index]["type"]),
                    ],
                ),
                );
            });
        }
        return CircularProgressIndicator();
    })