gmail api full email access given the message id of email - ios

Question

I am trying to access the gmail messages in offline access mode. gmail access token I get from the server end.
I followed this link to here
I created my own authenticator class and subclass from GTMOAuth2Authentication and implemented the protocol as mentioned in the above link.
The canAutherize always fail and even though I make gmail query, It crashes in authorize function. Any help would be appreciated.
- (void)viewDidLoad {
[super viewDidLoad];
if ([self.subjectInfo length] > 0) {
[self.subjectLabel setText:self.subjectInfo];
}else {
[self.subjectLabel setText:#""];
}
self.service = [[GTLServiceGmail alloc] init];
[self fetchAccessToken];
// Initialize the Gmail API service & load existing credentials from the keychain if available.
}
// When the view appears, ensure that the Gmail API service is authorized, and perform API calls.
- (void)viewDidAppear:(BOOL)animated {
if (!self.service.authorizer.canAuthorize) {
// Not yet authorized, request authorization by pushing the login UI onto the UI stack.
// [self presentViewController:[self createAuthController] animated:YES completion:nil];
[self fetchAccessToken];
} else {
[self fetchMail:[self gmailMsgId]];
}
}
-(void) fetchAccessToken
{
[[APIManager sharedManager] fetchGmailToken:^(NSDictionary *resultDict) {
if (resultDict != nil) {
self.accessToken = [resultDict objectForKey:#"access_token"];
_authInst = [[GmailAuthenticator alloc] initWithAccessToken:self.accessToken];
self.service.authorizer = _authInst;
}
} failure:^(NSError *error) {
NSLog(#"fail to fetch access token");
}];
}
- (void)fetchMail:(NSString*)messageId {
GTLQueryGmail *query = [GTLQueryGmail queryForUsersMessagesGet];
query.messageId = messageId;
[self.service executeQuery:query
delegate:self
didFinishSelector:#selector(displayResultWithTicket:finishedWithObject:error:)];
}
- (void)displayResultWithTicket:(GTLServiceTicket *)ticket
finishedWithObject:(GTLGmailMessage *)messageResponse
error:(NSError *)error {
if (error == nil) {
NSLog(#"description is %#", [messageResponse description]);
} else {
NSLog(#"error : %#", [error description]);
}
}