Question

everybody.
I want to understand, how i shoud procceed situations when an asynchronous method has “didFinish:@selector(SEL)” parameter.
My code example is:

//
// Authentication check
- ( void )authenticationSuccess: ( GDataServiceTicket* ) ticket
         authenticatedWithError: ( NSError* ) error {

    if ( error == nil )
    {
        NSLog( @"authentication success" );
    }
    else 
    {
        NSLog( @"authentication error" );
    }
}
//

- ( void ) fetchFeedOfSpreadsheets {

    //create and authenticate to a google spreadsheet service 
    if ( !(mService) )
    {
        GDataServiceGoogleSpreadsheet *service = [self spreadsheetService];
        [mService autorelease];
        mService = [service retain];    
    }

    // check autentication success ( invoke "authenticationSuccess" method for debug success & error )
    [mService authenticateWithDelegate: self
               didAuthenticateSelector:@selector(authenticationSuccess:
                                                 authenticatedWithError:) ];


    // HERE I WANT TO MAKE A PAUSE AND WHAIT THE RESULT, EITHER I AUTHENTICATED OR NOT
    // AND MAKE AN "IF" STATEMENT TO CONTINTUE WORKING ON SERVER, OR RETURN ERROR


    //fetch retrieves the feed of spreadsheets entries 
    NSURL *feedURL = [ NSURL URLWithString: kGDataGoogleSpreadsheetsPrivateFullFeed ];
    GDataServiceTicket *ticket;
    ticket = [mService fetchFeedWithURL: feedURL
                               delegate: self
                      didFinishSelector: @selector(spreadsheetsTicket:finishedWithFeed:
                                                   error: ) ];

    // HERE I WANT TO WAIT SECOND TIME. I WANT "spreadsheetsTicket:        
    // finishedWithFeed:error:" TO PROCCEED ERROR AND PUT A FEED IN SOME NSARRAY OBJECT
    // AND AFTER THAT I WANT TO WORK WITH THAT NSARRAY RIGHT HERE
}

I’s clear, that i can push the code i want into the end of “authenticationSuccess” method section, but it’s also clear, that it’s a wrong a way to solve the proble. There a number of situations like this, where i call an asynchronous method with a selector parameter, and i want to find a solution providing me a flexible code writing.

Thanks in advance.

Answer 1

It’s a standard practice in Objective-C to put the code to be executed after the authentication in the authenticationSucess: method. You might not like it, but that is life.

Many people had the same complaint as you, so
on iOS 4 and later, there’s something called blocks which allow you to write the code to be executed after the authentication in the method which initiates the authentication, as in

[mService authenticateAndExecute:^{ 
                 code to be executed when successfully authenticated ;
          }            whenError:^{
                 code to be executed when authentication failed;
          } ]; 

But in this case you need to modify the API, which is possible by using categories. See this blog post by Mike Ash. He has many other posts on blocks on the same blog, which are also very instructive.