build-item-main.dart

//New 
Widget _buildItem(UserModel user, [int index]) {
  return ListTile(
    key: ValueKey<UserModel>(user),
    title: Text(user.firstName),
    subtitle: Text(user.lastName),
    leading: CircleAvatar(
      backgroundImage: NetworkImage(user.profileImageUrl),
    ),
    onLongPress: index != null ? () => deleteUser(index) : null,
  );
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text("Animated List Demo"),
      centerTitle: true,
      leading: IconButton(
        icon: Icon(Icons.add),
        onPressed: addUser,
      ),
    ),
    body: SafeArea(
      child: AnimatedList(
        key: _listKey,
        initialItemCount: listData.length,
        itemBuilder: (BuildContext context, int index, Animation animation) {
          return FadeTransition(
            opacity: animation,
            child: _buildItem(listData[index], index), //Change
          );
        },
      ),
    ),
  );
}