replacing FSTGetTokenResult by C++ Token implementation (#805)

* replacing Auth/FSTUser by C++ auth implementation

* address changes

* replacing FSTGetTokenResult by C++ Token implementation

* address changes

* address changes

* fix another const& v.s. dispatch bug

* fix more const& v.s. dispatch bug  zxu123 committed

* fix

* passing by value in callback

3 files changed, 30 insertions, 3 deletions

diff --git a/Firestore/core/src/firebase/firestore/auth/credentials_provider.h b/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
index 2a52c99..917f8e1 100644
--- a/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
+++ b/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
@@ -31,11 +31,11 @@ namespace auth {
 // `TokenErrorListener` is a listener that gets a token or an error.
 // token: An auth token as a string, or nullptr if error occurred.
 // error: The error if one occurred, or else nullptr.
-typedef std::function<void(const Token& token, const absl::string_view error)>
+typedef std::function<void(Token token, const absl::string_view error)>
     TokenListener;
 
 // Listener notified with a User change.
-typedef std::function<void(const User& user)> UserChangeListener;
+typedef std::function<void(User user)> UserChangeListener;
 
 /**
  * Provides methods for getting the uid and token for the current user and
diff --git a/Firestore/core/src/firebase/firestore/auth/token.cc b/Firestore/core/src/firebase/firestore/auth/token.cc
index 0618ddb..4ee1b69 100644
--- a/Firestore/core/src/firebase/firestore/auth/token.cc
+++ b/Firestore/core/src/firebase/firestore/auth/token.cc
@@ -21,7 +21,15 @@ namespace firestore {
 namespace auth {
 
 Token::Token(const absl::string_view token, const User& user)
-    : token_(token), user_(user) {
+    : token_(token), user_(user), is_valid_(true) {
+}
+
+Token::Token() : token_(), user_(User::Unauthenticated()), is_valid_(false) {
+}
+
+const Token& Token::Invalid() {
+  static const Token kInvalidToken;
+  return kInvalidToken;
 }
 
 }  // namespace auth
diff --git a/Firestore/core/src/firebase/firestore/auth/token.h b/Firestore/core/src/firebase/firestore/auth/token.h
index f3b7363..ff8d2f0 100644
--- a/Firestore/core/src/firebase/firestore/auth/token.h
+++ b/Firestore/core/src/firebase/firestore/auth/token.h
@@ -20,6 +20,7 @@
 #include <string>
 
 #include "Firestore/core/src/firebase/firestore/auth/user.h"
+#include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
 #include "absl/strings/string_view.h"
 
 namespace firebase {
@@ -45,6 +46,7 @@ class Token {
 
   /** The actual raw token. */
   const std::string& token() const {
+    FIREBASE_ASSERT(is_valid_);
     return token_;
   }
 
@@ -56,9 +58,26 @@ class Token {
     return user_;
   }
 
+  /**
+   * Whether the token is a valid one.
+   *
+   * ## Portability notes: Invalid token is the equivalent of nil in the iOS
+   * token implementation. We use value instead of pointer for Token instance in
+   * the C++ migration.
+   */
+  bool is_valid() const {
+    return is_valid_;
+  }
+
+  /** Returns an invalid token. */
+  static const Token& Invalid();
+
  private:
+  Token();
+
   const std::string token_;
   const User user_;
+  const bool is_valid_;
 };
 
 }  // namespace auth