(iPhone) how to wait a method to finished up, one of its arguments is calling selector..

Question

I'm stuck with this problem, looks like multithreading one but I'm quite new to this kind of topics. I need some help form experts!!!

[Problem Conditions] (1) Need to call a method which has 3 arguments, one argument is "@selector( myMethod: )"

(2) Need to call (1) for multiple times

(3) Need to make sure each of (1)'s selector is done in order to move on to next steps

(4) @selector( myMethod: ) is setting up xArray, an array of object X to make it simple

(5) So, logically I have one xArray with multithreads accessing it, and somehow need to process all elements of xArray...

[Thoughts]

performSelector isn't helpful because the one I need to put is a method with @selector argument...

[Pseudo-Code]

// The Starting Point of Alghorithm
- (void)initialCallerMethod {

  for(int i=0; i < [calendarArray count]; i++) {
    calendar = [calendarArray objectAtIndex:i];  
    // fetch the events feed
    NSString* alternateLink = [calendar alternateLink];
    NSURL* feedURL = [NSURL URLWithString:alternateLink];
    if (feedURL) {

      [self setEventFeed:nil];

      GDataQueryCalendar *query = [GDataQueryCalendar calendarQueryWithFeedURL:feedURL];
      [query setMaxResults:100]; 

      GDataServiceGoogleCalendar *service = [[[CalendarService alloc] init] calendarService];
      GDataServiceTicket *ticket;
      ticket = [service fetchFeedWithQuery:query 
        delegate:self
        didFinishSelector:@selector(calendarEventsTicket:finishedWithFeed:error:)];

      if ([self eventFetchError] == nil) {
        // query succeeded
 NSLog(@"Querry succeeded");
 [self howToDoThis];    
      }
    }
  }
}

// @selector's method with 3 arguments
- (void)calendarEventsTicket:(GDataServiceTicket *)ticket
            finishedWithFeed:(GDataFeedCalendarEvent *)feed
                       error:(NSError *)error {

 [self setEventFeed:feed];
}

//
// Somewhere I want to do something like this
//
- (void) howToDoThis {
  GDataFeedCalendarEvent* feed = [self eventFeed];
  NSArray *entries = [feed entries];

  // for now, I get's zero...
  NSLog(@"FEED ENTRIES COUNT: %d", [entries count]);

  for (int idx = 0; idx < [entries count]; idx++) {
    // to make it simple, I'm just accumulating elements of array
    id elm = [entries objectAtIndex:idx];
    [anArrayToSumUp addObject: elm ]; 
   }
}

I am truly overflowed... Please advice...

Katsumi

==== some progress, or struggles... 2009/10/29

Tim, I did some reading for NSInvocation and NSInvocationOperation. It sounds useful. Now, do you know how to pass "the address of selector"? you see, I can set target, selector and arguments with NSInvocation but how can I pass the @selector(...)'s address?

[Before using NSInvocation] ticket = [service fetchFeedWithQuery:query delegate:self didFinishSelector:@selector(calendarEventsTicket:finishedWithFeed:error:)];

[Trying to use NSInvocation, getting closer except for passing selector as an argument]

retInvo = [NSInvocation invocationWithMethodSignature: [self methodSignatureForSelector:@selector(finishMethod:withArray:)]]; [retInvo setTarget:self];

// * This is not OK * [retInvo setSelector:@selector(finishMethod:withArray:)]; // This is not OK

[retInvo setArgument:&calendar atIndex:2]; [retInvo setArgument:&events atIndex:3];

NSInvocationOperation* invoFinishOperation = [[NSInvocationOperation alloc] initWithInvocation:retInvo];

Answer 1

To deal with your need to call methods with selectors as arguments, I'd look at NSInvocation. An instance of NSInvocation is essentially a method call turned into an object - you specify a target and a series of arguments, then call invoke on the invocation and it gets run as though it were a straightfoward method call.

Answer 2

Waiting for these methods to return is going to make your UI appear to be hung. That's why they execute asynchronously and give you a callback. The method you supply as said callback is what should be responsible for telling the rest of your app that it's done waiting.