Configures the behavior of get() calls on DocumentReference and Query. By
providing a Source value, these methods can be configured to fetch results only from the
server, only from the local cache, or attempt to fetch results from the server and fall back to
the cache (which is the default).
Causes Cloud Firestore to immediately return a value from the cache, ignoring the server
completely (implying that the returned value may be stale with respect to the value on the
server). If there is no data in the cache to satisfy the get() call, get() will return an error and get() will return an empty
QuerySnapshot with no documents.
Causes Cloud Firestore to try to retrieve an up-to-date (server-retrieved) snapshot, but fall back to returning cached data if the server can't be reached.
Causes Cloud Firestore to avoid the cache, generating an error if the server cannot be reached. Note that the cache will still be updated if the server request succeeds. Also note that latency-compensation still takes effect, so any pending write operations will be visible in the returned data (merged into the server-provided data).