UI无法立即显示微调器

Question

Hey, I'm creating an iOS app that requires the user to login using a username and password, this username and password is then sent to a web server to be authenticated. While it sends the request and waits for the response I show a UIActivityIndicator.

The problem I get is that sometimes the UIActivityIndicator appears straight away on the button press and other times it appears only after a few seconds have passed. Am guessing its because the OS is executing another thread before updating the UI but I would like to know if anyone knows for sure what could be causing this sometime delay or is it just that my code is at fault.

My code below:

-(void)loginPressed:(id)sender{

    if ([self validateForm]) {

        [self startLoadingAnimation:@"CONNECTING"];

        [Session theSession].authenicatedUsername = usernameTextField.text;
        [Session theSession].authenicatedPassword = passwordTextField.text;

        usernameTextField.text = @"";
        passwordTextField.text = @"";

        [self setUserDetailsBasedOnUserID:[Session theSession].authenicatedUsername password:[Session theSession].authenicatedPassword]; 

    }else {

        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"MISSING_LOGIN_DETAILS_TITLE", @"")
                                                        message:NSLocalizedString(@"MISSING_LOGIN_MESSAGE", @"")
                                                       delegate:self
                                              cancelButtonTitle:NSLocalizedString(@"OK_BUTTON", @"")
                                              otherButtonTitles:nil];

        [alert show];
        [alert release];
    }
}

Thanks

Answer 1

You should do networking asynchronously, otherwise, you'll block the main (UI) thread while potentially waiting a long time for the server to respond (you don't know what kind of connection your users have etc.).

If you absolutely can't do this either in a background thread or using NSURLConnection's asynchronous interface, you can first let your runloop return to update the UI (start the activity indicator) and then start the task, like this:

- (void)loginPressed:(id)sender {
  [activityIndicator startAnimating];
  [self performSelector:@selector(doLogin) withObject:nil afterDelay:0.0];
}

- (void)doLogin {
  //perform your task here...
}

But as I said, this is generally bad practice...