Waiting for result of Future<DocumentSnapshot> in Flutter Firestore - firebase

Question

I have a cloud FireStore database with 2 fields.
imageUrl (url of a remote file)
user (reference field of a document in users collection)
Below is how I get the documents from the images collection.
class ImagePost {
final String imageUrl;
final User user;
const ImagePost(
{this.imageUrl,
this.user});
factory ImagePost.fromDocument(DocumentSnapshot document) {
User userInfo;
DocumentReference userReference = document['user'];
Future<DocumentSnapshot> userRef = userReference.get();
userRef.then((document) {
userInfo = User.fromJSON(document.data);
});
ImagePost post = new ImagePost(
imageUrl: document['imageUrl'],
user: userInfo // ==> always null while returning
);
return post;
}
}
When getting the reference user document, the post object always contains null value for user field. I expect the user object to be populated.
But the user value is retrieved late and not returned along with the post object.
How can I ensure that user value is retrieve before returning the post value?

Answer 1

That's because the get() method return a Future and you need to use async 'await' in order to wait for the response , but is not posible to use it in your constructor.
Just create a method (not constructor) and use like this :
Future<ImagePost> getImagePostFromDocument(DocumentSnapshot document) async {
DocumentReference userReference = document['user'];
DocumentSnapshot userRef = await userReference.get();
User userInfo = User.fromJSON(userRef);
ImagePost post = new ImagePost(
imageUrl: document['imageUrl'],
user: userInfo
);
return post;
}
I recommend you to call it form a FutureBuilder