source property
Describes whether we should get from server or cache.
Setting to Source.serverAndCache (default value), causes 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.
Setting to Source.server causes 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).
Setting to Source.cache causes 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, DocumentReference.get() will return an error and QuerySnapshot.get() will return an empty QuerySnapshot with no documents.
Implementation
final Source source;