OAuth, Tumblr and Scope - ios

Question

I'm trying to login to Tumblr via OAuth and a Mac App I begin to code.
I donwloaded gtm-oauth source code, everything is fine.
I just began with this code inside a view controller :
- (GTMOAuthAuthentication *)myCustomAuth {
GTMOAuthAuthentication *auth;
auth = [[[GTMOAuthAuthentication alloc] initWithSignatureMethod:kGTMOAuthSignatureMethodHMAC_SHA1
consumerKey:kConsumerKey
privateKey:kConsumerSecret] autorelease];
auth.serviceProvider = #"Custom Auth Service";
return auth;
}
- (void)viewController:(GTMOAuthViewControllerTouch *)viewController
finishedWithAuth:(GTMOAuthAuthentication *)auth
error:(NSError *)error {
NSLog(#"finishedWithAuth");
if (error != nil) {
NSLog(#"failed");
} else {
NSLog(#"done");
}
}
- (void)signInToCustomService {
GTMOAuthAuthentication *auth = [self myCustomAuth];
if (auth == nil) {
NSLog(#"A valid consumer key and consumer secret are required for signing in to Tumblr");
}
else
{
NSLog(#"Ok auth");
}
NSURL *requestURL = [NSURL URLWithString:#"http://www.tumblr.com/oauth/request_token"];
NSURL *accessURL = [NSURL URLWithString:#"http://www.tumblr.com/oauth/access_token"];
NSURL *authorizeURL = [NSURL URLWithString:#"http://www.tumblr.com/oauth/authorize"];
NSString *scope = #"http://api.tumblr.com"; // HERE I DON'T KNOW WHAT TO WRITE
GTMOAuthViewControllerTouch *viewController;
viewController = [[[GTMOAuthViewControllerTouch alloc] initWithScope:scope
language:nil
requestTokenURL:requestURL
authorizeTokenURL:authorizeURL
accessTokenURL:accessURL
authentication:auth
appServiceName:#"My App: Custom Service"
delegate:self
finishedSelector:#selector(viewController:finishedWithAuth:error:)] autorelease];
[self presentModalViewController:viewController animated:YES];
}
- (void)viewDidLoad
{
[super viewDidLoad];
[self signInToCustomService];
}
But nothing happens.
- (void)viewController:(GTMOAuthViewControllerTouch *)viewController
finishedWithAuth:(GTMOAuthAuthentication *)auth
error:(NSError *)error;
This method is never called.
Perhaps this is my scope variable. I don't know which value I have to write for it.
Thanks for your help !