Port `Precondition` to C++ (#1040)

* port FieldMask to C++

* address changes

* address changes

* fix test

* address change

* Port transform operations (FSTTransformOperation, FSTServerTimestampTransform) to C++

* address changes

* address changes

* address changes

* implement `FieldTransform` in C++

* port `FieldTransform`

* make `fieldTransforms` shared inside `context`

* Implement Precondition in C++ w/o test yet

* add unit test for `Precondition`

* port `Precondition`

* address changes

* address changes

* fix bugs for integration test

* address changes

* fix lint

* address changes

* address changes

4 files changed, 23 insertions, 15 deletions

diff --git a/Firestore/Source/API/FIRDocumentReference.mm b/Firestore/Source/API/FIRDocumentReference.mm
index 67e401d..9fb4541 100644
--- a/Firestore/Source/API/FIRDocumentReference.mm
+++ b/Firestore/Source/API/FIRDocumentReference.mm
@@ -41,11 +41,13 @@
 #import "Firestore/Source/Util/FSTUsageValidation.h"
 
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
+#include "Firestore/core/src/firebase/firestore/model/precondition.h"
 #include "Firestore/core/src/firebase/firestore/model/resource_path.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
 namespace util = firebase::firestore::util;
 using firebase::firestore::model::DocumentKey;
+using firebase::firestore::model::Precondition;
 using firebase::firestore::model::ResourcePath;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -168,7 +170,7 @@ NS_ASSUME_NONNULL_BEGIN
                                  ? [self.firestore.dataConverter parsedMergeData:documentData]
                                  : [self.firestore.dataConverter parsedSetData:documentData];
   return [self.firestore.client
-      writeMutations:[parsed mutationsWithKey:self.key precondition:[FSTPrecondition none]]
+      writeMutations:[parsed mutationsWithKey:self.key precondition:Precondition::None()]
           completion:completion];
 }
 
@@ -180,8 +182,7 @@ NS_ASSUME_NONNULL_BEGIN
         completion:(nullable void (^)(NSError *_Nullable error))completion {
   FSTParsedUpdateData *parsed = [self.firestore.dataConverter parsedUpdateData:fields];
   return [self.firestore.client
-      writeMutations:[parsed mutationsWithKey:self.key
-                                 precondition:[FSTPrecondition preconditionWithExists:YES]]
+      writeMutations:[parsed mutationsWithKey:self.key precondition:Precondition::Exists(true)]
           completion:completion];
 }
 
@@ -191,7 +192,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)deleteDocumentWithCompletion:(nullable void (^)(NSError *_Nullable error))completion {
   FSTDeleteMutation *mutation =
-      [[FSTDeleteMutation alloc] initWithKey:self.key precondition:[FSTPrecondition none]];
+      [[FSTDeleteMutation alloc] initWithKey:self.key precondition:Precondition::None()];
   return [self.firestore.client writeMutations:@[ mutation ] completion:completion];
 }
 
diff --git a/Firestore/Source/API/FIRWriteBatch.mm b/Firestore/Source/API/FIRWriteBatch.mm
index b1cfa09..df6e5e0 100644
--- a/Firestore/Source/API/FIRWriteBatch.mm
+++ b/Firestore/Source/API/FIRWriteBatch.mm
@@ -24,6 +24,10 @@
 #import "Firestore/Source/Model/FSTMutation.h"
 #import "Firestore/Source/Util/FSTUsageValidation.h"
 
+#include "Firestore/core/src/firebase/firestore/model/precondition.h"
+
+using firebase::firestore::model::Precondition;
+
 NS_ASSUME_NONNULL_BEGIN
 
 #pragma mark - FIRWriteBatch
@@ -69,8 +73,8 @@ NS_ASSUME_NONNULL_BEGIN
   [self validateReference:document];
   FSTParsedSetData *parsed = options.isMerge ? [self.firestore.dataConverter parsedMergeData:data]
                                              : [self.firestore.dataConverter parsedSetData:data];
-  [self.mutations addObjectsFromArray:[parsed mutationsWithKey:document.key
-                                                  precondition:[FSTPrecondition none]]];
+  [self.mutations
+      addObjectsFromArray:[parsed mutationsWithKey:document.key precondition:Precondition::None()]];
   return self;
 }
 
@@ -79,9 +83,8 @@ NS_ASSUME_NONNULL_BEGIN
   [self verifyNotCommitted];
   [self validateReference:document];
   FSTParsedUpdateData *parsed = [self.firestore.dataConverter parsedUpdateData:fields];
-  [self.mutations
-      addObjectsFromArray:[parsed mutationsWithKey:document.key
-                                      precondition:[FSTPrecondition preconditionWithExists:YES]]];
+  [self.mutations addObjectsFromArray:[parsed mutationsWithKey:document.key
+                                                  precondition:Precondition::Exists(true)]];
   return self;
 }
 
@@ -89,7 +92,7 @@ NS_ASSUME_NONNULL_BEGIN
   [self verifyNotCommitted];
   [self validateReference:document];
   [self.mutations addObject:[[FSTDeleteMutation alloc] initWithKey:document.key
-                                                      precondition:[FSTPrecondition none]]];
+                                                      precondition:Precondition::None()]];
   return self;
 }
 
diff --git a/Firestore/Source/API/FSTUserDataConverter.h b/Firestore/Source/API/FSTUserDataConverter.h
index a2f947a..ea20b3e 100644
--- a/Firestore/Source/API/FSTUserDataConverter.h
+++ b/Firestore/Source/API/FSTUserDataConverter.h
@@ -22,12 +22,12 @@
 #include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/field_mask.h"
 #include "Firestore/core/src/firebase/firestore/model/field_transform.h"
+#include "Firestore/core/src/firebase/firestore/model/precondition.h"
 
 @class FIRSetOptions;
 @class FSTObjectValue;
 @class FSTFieldValue;
 @class FSTMutation;
-@class FSTPrecondition;
 @class FSTSnapshotVersion;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -58,7 +58,8 @@ NS_ASSUME_NONNULL_BEGIN
  * field transforms) using the specified document key and precondition.
  */
 - (NSArray<FSTMutation *> *)mutationsWithKey:(const firebase::firestore::model::DocumentKey &)key
-                                precondition:(FSTPrecondition *)precondition;
+                                precondition:
+                                    (const firebase::firestore::model::Precondition &)precondition;
 
 @end
 
@@ -83,7 +84,8 @@ NS_ASSUME_NONNULL_BEGIN
  * field transforms) using the specified document key and precondition.
  */
 - (NSArray<FSTMutation *> *)mutationsWithKey:(const firebase::firestore::model::DocumentKey &)key
-                                precondition:(FSTPrecondition *)precondition;
+                                precondition:
+                                    (const firebase::firestore::model::Precondition &)precondition;
 
 @end
 
diff --git a/Firestore/Source/API/FSTUserDataConverter.mm b/Firestore/Source/API/FSTUserDataConverter.mm
index 6320e83..dff9235 100644
--- a/Firestore/Source/API/FSTUserDataConverter.mm
+++ b/Firestore/Source/API/FSTUserDataConverter.mm
@@ -38,6 +38,7 @@
 #include "Firestore/core/src/firebase/firestore/model/field_mask.h"
 #include "Firestore/core/src/firebase/firestore/model/field_path.h"
 #include "Firestore/core/src/firebase/firestore/model/field_transform.h"
+#include "Firestore/core/src/firebase/firestore/model/precondition.h"
 #include "Firestore/core/src/firebase/firestore/model/transform_operations.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 #include "absl/memory/memory.h"
@@ -48,6 +49,7 @@ using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::FieldMask;
 using firebase::firestore::model::FieldPath;
 using firebase::firestore::model::FieldTransform;
+using firebase::firestore::model::Precondition;
 using firebase::firestore::model::ServerTimestampTransform;
 using firebase::firestore::model::TransformOperation;
 
@@ -91,7 +93,7 @@ static NSString *const RESERVED_FIELD_DESIGNATOR = @"__";
 }
 
 - (NSArray<FSTMutation *> *)mutationsWithKey:(const DocumentKey &)key
-                                precondition:(FSTPrecondition *)precondition {
+                                precondition:(const Precondition &)precondition {
   NSMutableArray<FSTMutation *> *mutations = [NSMutableArray array];
   if (self.isPatch) {
     [mutations addObject:[[FSTPatchMutation alloc] initWithKey:key
@@ -132,7 +134,7 @@ static NSString *const RESERVED_FIELD_DESIGNATOR = @"__";
 }
 
 - (NSArray<FSTMutation *> *)mutationsWithKey:(const DocumentKey &)key
-                                precondition:(FSTPrecondition *)precondition {
+                                precondition:(const Precondition &)precondition {
   NSMutableArray<FSTMutation *> *mutations = [NSMutableArray array];
   [mutations addObject:[[FSTPatchMutation alloc] initWithKey:key
                                                    fieldMask:self.fieldMask