how to login some other user on facebook in your app?

i am publishing some story on the user's facebook wall in my app. its working fine but i wanna know if some other user wanna login in my app with ot开发者_开发百科her facebook account how can he do that. i mean how can i logout an existing user and allow other user to login. my code in AppDelegate is:

     - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
         [self.window addSubview:navigationController.view];
         [self.window makeKeyAndVisible];
         facebook = [[Facebook alloc] initWithAppId:@"app_id"];

         NSArray* permissions =  [[NSArray arrayWithObjects:
                      @"publish_stream", nil] retain];

        [facebook authorize:permissions delegate:self];
     return YES;
 }

   - (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url {

        return [facebook handleOpenURL:url]; 
  } 

and my code in DetailViewController is:

  AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

  SBJSON *jsonWriter = [[SBJSON new] autorelease];
  NSDictionary* attachment = [NSDictionary dictionaryWithObjectsAndKeys:
                        [sTitle stringByReplacingOccurrencesOfString:@"\"" withString:@"\\\""], @"name",
                        [sSummary stringByReplacingOccurrencesOfString:@"\"" withString:@"\\\""], @"description",
                        [sLink stringByReplacingOccurrencesOfString:@"\"" withString:@"\\\""], @"href", nil];
      NSString *attachmentStr = [jsonWriter stringWithObject:attachment];
      NSMutableDictionary* params = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                           @"XXX", @"api_key",
                           @"Share on Facebook",  @"user_message_prompt",
                           attachmentStr, @"attachment",
                           nil];

     [appDelegate.facebook dialog:@"stream.publish" andParams:params andDelegate:self];

once app is installed it asks for username and password for facebook. and then it publishes stories to only that user. what about if i wanna change user in my app. where should i write

   [facebook logout:self];

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You'll need to provide the options for login/ logout. This post will help you how to do it.