How to listen only changed documents

I have used below code to listen Firestore collection change,

firestoreInstance
    .collection('Countries')
    .snapshots()
    .listen((result) async {

});

When a document change in Countries collection on Firestore. I expect result value should have only changed document. But result value has all the collection data.

How to get only changed document in result value.

Im using Flutter Firestore.

Here is your potential answer
There are 3 types of changes can happen for document

1. New Doc added
2. Doc updated
3. Doc deleted

Here is example how to handle all of them :

firestoreInstance
    .collection('Countries')
    .snapshots()
    .listen((QuerySnapshot snapshot) {
  snapshot.docChanges.forEach((DocumentChange change) {
    if (change.type == DocumentChangeType.added) {
      // Handle added document
      print("Added document: ${change.doc.data()}");
    } else if (change.type == DocumentChangeType.modified) {
      // Handle modified document
      print("Modified document: ${change.doc.data()}");
    } else if (change.type == DocumentChangeType.removed) {
      // Handle removed document
      print("Removed document: ${change.doc.data()}");
    }
  });
});

Another way is if you want to check for updated docs only

firestoreInstance
    .collection('Countries')
    .snapshots()
    .listen((QuerySnapshot snapshot) {
  var modifiedDocuments = snapshot.docChanges
      .where((change) => change.type == DocumentChangeType.modified)
      .map((change) => change.doc)
      .toList();

  // Process the modified documents
  modifiedDocuments.forEach((modifiedDocument) {
    // Do something with the modified document
    print('Modified document: ${modifiedDocument.data()}');
  });
});

Here is the reference for that

https://firebase.google.com/docs/reference/android/com/google/firebase/firestore/DocumentChange

For Flutter/Dart Firestore, you can know only changed document.
Here's an example in Flutter/Dart that demonstrates how to filter out only the changed documents.

  firestoreInstance
        .collection('Countries')
        .snapshots()
        .listen((event) {
      for (var change in event.docChanges) {
        switch (change.type) {
          case DocumentChangeType.added:
            print("New Countries: ${change.doc.data()}");
            break;
          case DocumentChangeType.modified:
            print("Modified Countries: ${change.doc.data()}");
            break;
          case DocumentChangeType.removed:
            print("Removed Countries: ${change.doc.data()}");
            break;
        }
      }
    });

Here is official documentation link
https://firebase.google.com/docs/firestore/query-data/listen#view_changes_between_snapshots