Cloud Firestore: retrieving cached data via direct get?

Question

Retrieving data from the server may take some seconds. Is there any way to retrieve cached data in the meantime, using a direct get?

The onComplete seems to be called only when the data is retrieved from the server:

db.collection("cities").whereEqualTo("state", "CA").get()
        .addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
            @Override
            public void onComplete(@NonNull Task<DocumentSnapshot> task) {
                if (task.isSuccessful()) {
                ...
                }
            }
        });

Is there any callback for the cached data?

Answer 1

Now it is possible to load data only from cached version. From docs

You can specify the source option in a get() call to change the default behavior.....you can fetch from only the offline cache.

If it fails, then you can again try for the online version.

Example:

DocumentReference docRef = db.collection("cities").document("SF");

// Source can be CACHE, SERVER, or DEFAULT.
Source source = Source.CACHE;

// Get the document, forcing the SDK to use the offline cache
docRef.get(source).addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
        if (task.isSuccessful()) {
            // Document found in the offline cache
            DocumentSnapshot document = task.getResult();
            Log.d(TAG, "Cached document data: " + document.getData());
        } else {
            Log.d(TAG, "Cached get failed: ", task.getException());
            //try again with online version
        }
    }
});