[De]serialize empty Document instances (#1262)

* [De]serialize empty Document instances

Still TODO:
- non-empty
- NoDocument
- ErrorHandling

1 files changed, 7 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/nanopb/reader.h b/Firestore/core/src/firebase/firestore/nanopb/reader.h
index 7d77a4d..093e20b 100644
--- a/Firestore/core/src/firebase/firestore/nanopb/reader.h
+++ b/Firestore/core/src/firebase/firestore/nanopb/reader.h
@@ -80,6 +80,11 @@ class Reader {
    *
    * Call this method when reading a nested message. Provide a function to read
    * the message itself.
+   *
+   * @param read_message_fn Function to read the submessage. Note that this
+   * function is expected to check the Reader's status (via
+   * Reader::status().ok()) and if not ok, to return a placeholder/invalid
+   * value.
    */
   template <typename T>
   T ReadNestedMessage(const std::function<T(Reader*)>& read_message_fn);
@@ -130,14 +135,14 @@ T Reader::ReadNestedMessage(const std::function<T(Reader*)>& read_message_fn) {
   // Implementation note: This is roughly modeled on pb_decode_delimited,
   // adjusted to account for the oneof in FieldValue.
 
-  if (!status_.ok()) return T();
+  if (!status_.ok()) return read_message_fn(this);
 
   pb_istream_t raw_substream;
   if (!pb_make_string_substream(&stream_, &raw_substream)) {
     status_ =
         util::Status(FirestoreErrorCode::DataLoss, PB_GET_ERROR(&stream_));
     pb_close_string_substream(&stream_, &raw_substream);
-    return T();
+    return read_message_fn(this);
   }
   Reader substream(raw_substream);