Is there a way to compare two list and check for the value that are the same

Question

I am create a social media type app I want to create a follower and following list like Instagram, I want when I go on some else profile and click on their followers or following list it shows a list of all the users that is following that person and if I am also following someone in the list it give me the option to unfollow that person and if I am not following the same person it gives me the option to follow them.

The code below is what I was to check if I am following any one in profile user followers list, What I have done is I query the data from firestore of all the user id the I am following and the same for user profile that is currently open and store the list in two separate variable and what it should do is check both list and if a user Id is in both list it means I am also following that user and should show that I am following that user and give the option to unfollow but what happens is instead of showing that I am only following the user who's id is in both list it show that I am following every one.

checkfollowers(BuildContext context, String? id) async {
    final auth = Provider.of<AuthState>(context, listen: false);
    List<String> followingList = [];
    List<String> myFollowingList = [];

    try {
      final QuerySnapshot following = await _firestore
          .collection('following')
          .doc(auth.getCurrentUser.uid)
          .collection('userFollowing')
          .get();

      QuerySnapshot userfollowing = await _firestore
          .collection('followers')
          .doc(id)
          .collection('userFollowers')
          .get();

      following.docs.forEach((element) {
        myFollowingList.add(element.id);
      });
      userfollowing.docs.forEach((element) {
        followingList.add(element.id);
      });

      // followingList.where((item) => myFollowingList.contains(item));

      check(value) => myFollowingList.contains(value);
      isFollowedByMe = followingList.any(check);
      notifyListeners();

      print(followingList);
      print(myFollowingList);
    } catch (err) {
      print(err.toString() + 'this error is coming from profileState');
    }
  }

below code is how I build the follower/following list

 final FirebaseFirestore _firestore = FirebaseFirestore.instance;
  UserModel? users;

  @override
  void initState() {
    final profileState = Provider.of<ProfileState>(context, listen: false);
    profileState.checkfollowers(context, widget.proFileId);
    super.initState();
  }

  userComponent(UserModel? model, BuildContext context) {
    final profileState = Provider.of<ProfileState>(context);
    final auth = Provider.of<AuthState>(context);

    return Container(
      margin: EdgeInsets.symmetric(horizontal: 10),
      padding: EdgeInsets.only(top: 10, bottom: 10),
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: [
          GestureDetector(
            onTap: () {
              Navigator.push(
                context,
                MaterialPageRoute(
                  builder: (context) =>
                      ProfilePage(profileId: '${model?.userId}'),
                ),
              );
            },
            child: Row(
              children: [
                Container(
                  width: 60,
                  height: 60,
                  child: CircleAvatar(
                    radius: 50,
                    backgroundImage: NetworkImage('${model?.profilePic}'),
                  ),
                ),
                SizedBox(width: 10),
                Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  mainAxisSize: MainAxisSize.min,
                  children: [
                    Text('${model?.userName}',
                        style: TextStyle(
                            color: Colors.black, fontWeight: FontWeight.w500)),
                    SizedBox(
                      height: 5,
                    ),
                    Text(
                      '${model?.displayName}',
                      style: TextStyle(
                        color: Colors.grey[500],
                      ),
                    ),
                  ],
                )
              ],
            ),
          ),
          auth.getCurrentUser.uid == model?.userId
              ? Container()
              : GestureDetector(
                  onTap: () {
                    if (auth.getCurrentUser.uid == model?.userId) {
                      print('you can not follow your self');
                    } else if (profileState.isFollowedByMe == true) {
                      profileState.setIsFollowedByMeToFalse();
                      profileState.handleUnFollow(context, model?.userId);
                    } else if (profileState.isFollowedByMe == false) {
                      profileState.setIsFollowedByMeToTrue();
                      profileState.handleFollow(context, model?.userId);
                    }
                  },
                  child: AnimatedContainer(
                    height: 35,
                    width: 110,
                    duration: Duration(milliseconds: 300),
                    decoration: BoxDecoration(
                      color: profileState.isFollowedByMe == true
                          ? AppColors.white
                          : AppColors.pinkaccent,
                      borderRadius: BorderRadius.circular(5),
                      border: Border.all(
                        color: Colors.grey.shade700,
                      ),
                    ),
                    child: Center(
                      child: Text(
                        profileState.isFollowedByMe == true
                            ? 'UnFollow'
                            : 'Follow',
                        style: TextStyle(
                            color: profileState.isFollowedByMe == true
                                ? Colors.black
                                : Colors.white),
                      ),
                    ),
                  ),
                )
        ],
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        elevation: 0,
        backgroundColor: Colors.white,
        title: Container(
          height: 38,
          child: TextField(
            onChanged: (value) {},
            decoration: InputDecoration(
                filled: true,
                fillColor: Colors.white,
                contentPadding: EdgeInsets.all(0),
                prefixIcon: Icon(
                  Icons.search,
                  color: Colors.grey.shade500,
                ),
                border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(50),
                    borderSide: BorderSide.none),
                hintStyle: TextStyle(fontSize: 14, color: Colors.grey.shade500),
                hintText: "Search users"),
          ),
        ),
      ),
      body: StreamListWrapper(
        stream: _firestore
            .collection('followers')
            .doc(widget.proFileId)
            .collection('userFollowers')
            .snapshots(),
        itemBuilder: (context, DocumentSnapshot snapshot) {
          var data = snapshot.data() as Map<String, dynamic>;
          users = UserModel.fromJson(data);
          return userComponent(users, context);
        },
        text: '${widget.user?.userName} as no Followers',
      ),
    );

Answer 1

It would be simpler (and more performant) to use the Set data structure rather than List. The intersection method on Set returns the items contained in both sets.

for example:

void main() {
  Set<String> followingSet = {'Jeff', 'Mike', 'Joe', 'Jess'};
  Set<String> myFollowingSet = {'Jess', 'Matt', 'Mike', 'Frank'};
  Set<String> usersInBothSets = followingSet.intersection(myFollowingSet);
  print(usersInBothSets); // {Mike, Jess}
}