Twitter图标没有显示在ios 7中的uiactivitycontroller

Question

我在设置菜单中正确配置了twitter，我的问题只是在UIActivityViewController上显示了twitter字符串，任何人都可以帮助我如何显示twitter图标？

NSString *shareString = @"Welcome.";
UIImage *shareImage = [UIImage imageNamed:@"welcome.png"];
NSURL *shareUrl = [NSURL URLWithString:@"http://www.welcome.com"];

NSArray *activityItems = [NSArray arrayWithObjects:shareString, shareImage, shareUrl, nil];

UIActivityViewController *activityViewController = [[UIActivityViewController alloc] initWithActivityItems:activityItems applicationActivities:nil];
activityViewController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
activityViewController.excludedActivityTypes = @[UIActivityTypePostToWeibo, UIActivityTypeAssignToContact];
[self presentViewController:activityViewController animated:YES completion:nil];