如何检测firestore中文档内的字段删除？

Question

Here's the code we can use to know if a document is added, modified or removed:

db.collection("cities")
        .whereEqualTo("state", "CA")
        .addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot snapshots,
                                @Nullable FirebaseFirestoreException e) {
                if (e != null) {
                    Log.w(TAG, "listen:error", e);
                    return;
                }

                for (DocumentChange dc : snapshots.getDocumentChanges()) {
                    switch (dc.getType()) {
                        case ADDED:
                            Log.d(TAG, "New city: " + dc.getDocument().getData());
                            break;
                        case MODIFIED:
                            Log.d(TAG, "Modified city: " + dc.getDocument().getData());
                            break;
                        case REMOVED:
                            Log.d(TAG, "Removed city: " + dc.getDocument().getData());
                            break;
                    }
                }

            }
        });

Is there a way for a client to know if another client deleted a field inside a document?

Answer 1

The Firestore SDK does not provide any direct way to detect this situation. You will have to write code to compare the prior snapshot with the new snapshot to see if any fields were added, changed or removed. Your solution will have to:

• Retain the QuerySnapshot from the last invocation of your listener and compare that to the current snapshot
• Iterate each new DocumentSnapshot contained in the QuerySnapshot, and find the matching prior DocumentSnapshot from the retained QuerySnapshot.
• Diff the Map<String, Object> from the data of both current and prior DocumentSnapshot to find the specific change

This is obviously a non-trivial amount of code, and will be highly dependent on the actual field data you want to compare.