How to access query from itemBuilder?

问题

I am make random chat app use FirestoreAnimatedList (FirebaseAnimatedList for Firestore).

I want show user avatar only one time if same user send many message so not have same avatar show many time.

My solution is use index for check if last message send by same user.

So I need check like :

if (snapshot[index - 1][‘user’] == user) {
  return true;
}

But my snapshot is DocumentSnapshot so cannot call [index - 1].

Maybe I must get list from query? How to access so can call index on list and query like this?

Thanks!

Here example:

body: FirestoreAnimatedList(
        query: firestore.collection('messages').snapshots(),
        itemBuilder: (
          BuildContext context,
          DocumentSnapshot snapshot,
          Animation<double> animation,
          int index,
        ) {
          return FadeTransition(
            opacity: animation,
            child: MessageItem(
              index: index,
              document: snapshot,
              onTap: _removeMessage,
            ),
          );
        },

回答1:

You can just store the users in a Map.

In your widget's class, you will have to create a variable to hold the map:

final Map<int, dynamic> users = {};

Then, you can populate the map in the itemBuilder callback:

itemBuilder: (
          BuildContext context,
          DocumentSnapshot snapshot,
          Animation<double> animation,
          int index,
        ) {
          users[index] = snapshot['user']; // added line
          return FadeTransition(
            opacity: animation,
            child: MessageItem(
              index: index,
              document: snapshot,
              onTap: _removeMessage,
            ),
          );
        },

Now, you can do what you wanted:

if (users[index - 1] == user) {
  return true;
}

来源：https://stackoverflow.com/questions/55199281/how-to-access-query-from-itembuilder