Retrieving firestore document is initially empty-CodePudding

I've been having trouble retrieving array contents from a firebase document using a method. Upon testing, the array initially prints [], but as the code goes on it prints the correct values? I don't know if my approach is correct so any suggestions will be appreciated.

Future method

Future<List<dynamic>> retrieveMovieList(String uid) async{
    List<dynamic> movies = [];
    final docRef = await _firestore.collection('users').doc(uid);
    docRef.get().then(
          (DocumentSnapshot doc) async{
        final data = doc.data() as Map<String, dynamic>;
        movies = await data['movies'];
        // If i print movies here it works. (await is an experiment, it also doesnt work without it)
      },
      one rror: (e) => print("Error getting document: $e"),
    );
    return movies;
  }

Method being called

List<dynamic> list = await data.retrieveMovieList(uid);
List<Widget> movies = await load.getMoviesById(list);

When the list is being printed all I get is [] initially. Any help, or can anyone point out what I'm doing wrong?

CodePudding user response：

The problem is that your code reaches return movies; before movies = await data['movies']; happens, and the reason is that the code inside your callback runs asynchronously and you are not awaiting it.

What you need to do is

Future<List<dynamic>> retrieveMovieList(String uid) async{
    try {
        List<dynamic> movies = [];
        final docRef = await _firestore.collection('users').doc(uid);
        final doc = await docRef.get(); // here we need to await
        final data = doc.data() as Map<String, dynamic>;
    
        movies = data['movies']; // you had await movies but I don't think you need await

        return movies;
    } catch (e) {
        print("Error getting document: $e")
    }
  }

CodePudding user response：

the return movies; will be executed immediately since it's outside the then, it will wait for what's inside it code block, but runs other code in parallel, instead use a full method with await/async:

 Future<List<dynamic>> retrieveMovieList(String uid) async{
    List<dynamic> movies = [];
    final docRef = await _firestore.collection('users').doc(uid);
    final docSnap = await docRef.get();

    final data = docSnap.data() as Map<String, dynamic>;
    movies = data['movies'] as List;
      },
      one rror: (e) => print("Error getting document: $e"),
    );
    return movies;
  }

or you can simply include the return movies; inside the then code block