I can't get data from firestore "Instance of 'Future<dynamic>'"

Question

I have this method the get all my documents from a given collection:

  getData() async {
    
    await databaseReference
        .collection("app").doc('usr').collection(_id).
        .get()
        .then((querySnapshot) {
      querySnapshot.docs.forEach((result) {
        return result.data();
      });
    })
}

What I want to get is all the documents from this collection, and not only the last. With the code above I get this when calling getData()

Instance of 'Future < dynamic>'

What I want to get:

[{name: Victor, age: 18}, {name: Tommy, age: 40}]

How can I reach it?

UPDATE

If I run the code below...:

await databaseReference
.collection("app").doc('usr').collection(_id)
        .get()
        .then((QuerySnapshot snapshot) {
      snapshot.docs.forEach((f) => print(f.data()));
    })

In the console it prints all the documents but separately (First prints one, after another):

I/flutter (16316): {name: Victor, age: 18}
I/flutter (16316): {name: Tommy, age: 40}

UPDATE 2 If I write what @Sahil Chadha and @kovalyovi suggest, and just print the list ... :

var items = List<dynamic>();
... my code....
snapshot.docs.forEach((f) => items.add(f.data()));
return items;

//returns exactly what I want

... It returns exactly what I want, but if I write return items and in the calling do var a = getData();, The A value is Instance of future. How can I have the result expected?

UPDATE 3 I forgot the await before getData(). Now it's working:

var a = await getData();
    print(a); //my expected result

  getData() async {
    var items = List<dynamic>();
      await databaseReference
.collection("app").doc('usr').collection(_id)
        .get()
        .then((QuerySnapshot snapshot) {
      snapshot.docs.forEach((f) => items.add(f.data()));
      })
return items
}

Answer 1

To be able to store the data you receive in a form of a list, you will need to initialize a list at the beginning of the method and then populate that list where you forEach the response. As you mentioned in the comments, I am posting an answer here for you:

getData() async {
  // initialize your list here
  var items = List<dynamic>();

  await databaseReference.collection("app").doc('usr').collection(_id).get().then((QuerySnapshot snapshot) {
    snapshot.docs.forEach(
      // add data to your list
      (f) => items.add(f.data()),
    );
  });
  return items;
}

Answer 2

Like there is no problem with the code. It will be healthier if you just model the data you get. You just did not mention that you were async where you were calling. Since it is future data, there must be a waiting event.

 Future<List<User>> getData() async {
   await databaseReference
    .collection("app").doc('usr').collection(_id).
    .get()
    .then((querySnapshot) {
       querySnapshot.docs.map((f) => 
         User.fromJson(f)).toList());
   })
  }

You should try two of them.

  List<User> loaddata=await getData();

--OR------

 var loadData;
  getData().then((result){
    List<User>=result;
   });

and you create the user model. It will be more revealing if you do it this way.

  import 'dart:convert';

  User userFromJson(String str) => User.fromJson(json.decode(str));

 String userToJson(User data) => json.encode(data.toJson());

class User {
   User({
     this.name,
     this.age,
  });

 String name;
 int age;

 factory User.fromJson(Map<String, dynamic> json) => User(
    name: json["name"],
    age: json["age"],
 );

 Map<String, dynamic> toJson() => {
    "name": name,
    "age": age,
  };
 }

If there is a problem in the code, you are calling the getData method in the build widget. So you have to call it from initstate.