Firestore: Remove copies in FieldValueInternal to reduce global refs consumption. (#1111)

Author: dconeybe

firestore/src/android/field_value_android.cc

@@ -156,7 +156,7 @@ FieldValueInternal::FieldValueInternal(GeoPoint value)
   object_ = GeoPointInternal::Create(env, value);
 }
 
-FieldValueInternal::FieldValueInternal(std::vector<FieldValue> value)
+FieldValueInternal::FieldValueInternal(const std::vector<FieldValue>& value)
     : cached_type_(Type::kArray) {
   Env env = GetEnv();
   Local<ArrayList> list = ArrayList::Create(env, value.size());
@@ -167,7 +167,7 @@ FieldValueInternal::FieldValueInternal(std::vector<FieldValue> value)
   object_ = list;
 }
 
-FieldValueInternal::FieldValueInternal(MapFieldValue value)
+FieldValueInternal::FieldValueInternal(const MapFieldValue& value)
     : cached_type_(Type::kMap) {
   Env env = GetEnv();
   Local<HashMap> map = HashMap::Create(env);

firestore/src/android/field_value_android.h

@@ -64,8 +64,14 @@ class FieldValueInternal {
   FieldValueInternal(const uint8_t* value, size_t size);
   explicit FieldValueInternal(DocumentReference value);
   explicit FieldValueInternal(GeoPoint value);
-  explicit FieldValueInternal(std::vector<FieldValue> value);
-  explicit FieldValueInternal(MapFieldValue value);
+  // Deviate from the iOS signatures of the following two constructors. The iOS
+  // versions take values into which the caller moves, to elide a copy. In
+  // Android, this actually *costs* an extra copy when calling from
+  // `DocumentReferenceInternal::Set()`, doubling the number of global
+  // references needed. Using const references in Android avoids this extra,
+  // costly copy (https://github.com/firebase/quickstart-unity/issues/1303).
+  explicit FieldValueInternal(const std::vector<FieldValue>& value);
+  explicit FieldValueInternal(const MapFieldValue& value);
 
   Type type() const;
 

release_build_files/readme.md

@@ -634,6 +634,12 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
+### Upcoming
+-   Changes
+    - Firestore (Android): Reduce the number of JNI global references consumed
+      when creating or updating documents
+      ([#1111](https://github.com/firebase/firebase-cpp-sdk/pull/1111)).
+
 ### 10.0.0
 -   Changes
     - General (Android): Update to Firebase Android BoM version 31.0.0.