adding converters between DocumentKey (#900)

* naively remove FSTPath import and source/test files.

* port FieldPath, part I

* port FieldPath, part II

* port ResourcePath, part I

* port ResourcePath, part II

* the grand commit to fix build errors

* use testutil:: helper instead of those from FSTHelpers

* fix test and lint

* use c_str in errmsg directly

* fix

* fix

* make code clean

* fix integration test I missed

* fix to avoid naming collision in preprocessor

* address changes

* address changes

* address changes

* fix: fieldMask are actually shared with different context.

* address changes

* add converter function between two DocumentKey implementations

* add unit test

* address changes

* fix lint

Author: zxu123

Firestore/Example/Tests/Model/FSTDocumentKeyTests.mm

@@ -18,8 +18,10 @@
 
 #import <XCTest/XCTest.h>
 
+#include "Firestore/core/src/firebase/firestore/model/document_key.h"
 #include "Firestore/core/src/firebase/firestore/model/resource_path.h"
 
+using firebase::firestore::model::DocumentKey;
 using firebase::firestore::model::ResourcePath;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -56,6 +58,15 @@ - (void)testComparison {
   XCTAssertEqual(NSOrderedDescending, [ab compare:a]);
 }
 
+- (void)testConverter {
+  const ResourcePath path{"rooms", "firestore", "messages", "1"};
+  FSTDocumentKey *objcKey = [FSTDocumentKey keyWithPath:path];
+  XCTAssertEqualObjects(objcKey, (FSTDocumentKey *)(DocumentKey{objcKey}));
+
+  DocumentKey cpp_key{path};
+  XCTAssertEqual(cpp_key, DocumentKey{(FSTDocumentKey *)(cpp_key)});
+}
+
 @end
 
 NS_ASSUME_NONNULL_END

Firestore/Source/Model/FSTDocumentKey.mm

@@ -101,7 +101,7 @@ + (BOOL)isDocumentKey:(const ResourcePath &)path {
   return path.size() % 2 == 0;
 }
 
-- (const firebase::firestore::model::ResourcePath &)path {
+- (const ResourcePath &)path {
   return _path;
 }
 

Firestore/core/src/firebase/firestore/model/document_key.h

@@ -21,6 +21,10 @@
 #include <memory>
 #include <string>
 
+#if defined(__OBJC__)
+#import "Firestore/Source/Model/FSTDocumentKey.h"
+#endif  // defined(__OBJC__)
+
 #include "Firestore/core/src/firebase/firestore/model/resource_path.h"
 #include "absl/strings/string_view.h"
 
@@ -43,6 +47,16 @@ class DocumentKey {
   /** Creates a new document key, taking ownership of the given path. */
   explicit DocumentKey(ResourcePath&& path);
 
+#if defined(__OBJC__)
+  DocumentKey(FSTDocumentKey* key)  // NOLINT(runtime/explicit)
+      : path_(std::make_shared<ResourcePath>(key.path)) {
+  }
+
+  operator FSTDocumentKey*() const {
+    return [FSTDocumentKey keyWithPath:path()];
+  }
+#endif
+
   /**
    * Creates and returns a new document key using '/' to split the string into
    * segments.
@@ -69,6 +83,11 @@ class DocumentKey {
     return path_ ? *path_ : Empty().path();
   }
 
+#if defined(__OBJC__)
+  // Helper function to convert to FSTDocumentKey during the C++ migration.
+  FSTDocumentKey* ToFSTDocumentKey() const;
+#endif  // defined(__OBJC__)
+
  private:
   // This is an optimization to make passing DocumentKey around cheaper (it's
   // copied often).