for-loop should wait for future-CodePudding

I have a list of userIDs and I want to get a value from the database for each user and write it to a new list. But the for loop doesn't wait for the future and throws the error "Unhandled Exception: RangeError (index): Invalid value: Valid value range is empty: 0"

List userIDs = ["gsdgsgsgda32", "gwerszhgda7h", "fsdgz675ehds"];
  Future <dynamic> getList() async {
    List items=[];
    for (var i = 0; i < userIDs.length; i  ) {
      items[i] = await getUserItems(userIDs[i]);
    }
    return items;
  }

  Future <String?> getUserItems(String? _userID) async {
    String? userItem=" ";
    final FirebaseApp testApp = Firebase.app();
    final FirebaseDatabase database = FirebaseDatabase.instanceFor(app: testApp);
    database.ref().child('users').child(_userID!).once().then((pdata) {
      userItem = pdata.snapshot.child('item').value as String?;
      });
    return userItem;
  }

CodePudding user response：

This is not problem with future. List items is empty so when you call items[0] = 3; there is no items[0] and you get RangeError. Proper way to add element to list is call items.add(3)

So your code should look like this:

List userIDs = ["gsdgsgsgda32", "gwerszhgda7h", "fsdgz675ehds"];
  Future <dynamic> getList() async {
    List items=[];
    for (var i = 0; i < userIDs.length; i  ) {
      final item = await getUserItems(userIDs[i]);
      items.add(item); 
    }
    return items;
  }

  Future <String?> getUserItems(String? _userID) async {
    String? userItem=" ";
    final FirebaseApp testApp = Firebase.app();
    final FirebaseDatabase database = FirebaseDatabase.instanceFor(app: testApp);
    database.ref().child('users').child(_userID!).once().then((pdata) {
      userItem = pdata.snapshot.child('item').value as String?;
      });
    return userItem;
  }

CodePudding user response：

Try to use it like this

 Future <dynamic> getList() async {
        List items=[];
        userIDs.forEach((item) async {
          items.add(await getUserItems(item));
        });
        return items;
      }