Nothing happens after calling [table reloadData] when called with dispatch_async

Question

    - (void) setRooms:(NSArray *)newRooms
    {
        NSLog(@"Main thread(cp3)... %d", [rooms count]);

        rooms = newRooms;

        [table reloadData];

        NSLog(@"Main thread(cp4)... %d", [rooms count]);
    }

- (void) parseJSONWithURL:(NSURL *)jsonURL
{
    [UIApplication sharedApplication].networkActivityIndicatorVisible = YES;

    NSLog(@"Main thread(cp1)...%d", [rooms count]);

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);

    dispatch_async(queue, ^{

        NSLog(@"Background thread(cp1)...%d", [rooms count]);

        NSError *error = nil;

        // Request the data and store in a string.
        NSString *resp = [NSString stringWithContentsOfURL:jsonURL
                                                  encoding:NSASCIIStringEncoding
                                                     error:&error];

        // Convert the String into an NSData object.
        NSData *data = [resp dataUsingEncoding:NSASCIIStringEncoding];

        // Parse that data object using NSJSONSerialization without options.
        NSDictionary *json = [[NSDictionary alloc] init];
        json = [NSJSONSerialization JSONObjectWithData:data
                                               options:kNilOptions
                                                 error:&error];

        // Return to the main thread to update the UI elements
        dispatch_sync(dispatch_get_main_queue(), ^{

            NSLog(@"Main thread(cp2)...%d", [rooms count]);

            [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;

            [self setRooms:[json valueForKey:@"Rooms"]];
        });

        NSLog(@"Background thread(cp2)...%d", [rooms count]);
    });

    NSLog(@"Main thread(cp5)...%d", [rooms count]);
}

Hi @AbdullahShafique please note that I also have two chained ifs’… the first if checks the link availability and the second checks parse error. The else’s are the corresponding failure blocks — Anaximandro Furtado, Jul 23 '13 at 08:22

Satheesh · Answer 1 · 2013-07-23T08:44:06.410

0

Try this

[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);

dispatch_async(queue, ^{

NSError *error = nil;

// Request the data and store in a string.
NSString *resp = [NSString stringWithContentsOfURL:jsonURL
                                          encoding:NSASCIIStringEncoding
                                             error:&error];
if (error == nil) {

    // Convert the String into an NSData object.
    NSData *data = [resp dataUsingEncoding:NSASCIIStringEncoding];

    // Parse that data object using NSJSONSerialization without options.
    NSDictionary *json = [[NSDictionary alloc] init];
    json = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];

             dispatch_sync(dispatch_get_main_queue(), ^{

                [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
                _rooms = [json valueForKey:@"Rooms"];
                [_table reloadData];
            });
      }
   });

or try

[self performSelectorOnMainThread:@selector(udpateAfterFetch:) withObject: [json valueForKey:@"Rooms"] waitUntilDone:YES];

-(void)udpateAfterFetch:(NSArray or whatever *) yourObject
{
   _rooms = yourObject;

   [_table reloadData];

   [UIApplication sharedApplication].networkActivityIndicatorVisible = NO;

}

The table is not being reloaded because any UI update should happen in the main thread and it is not happening so in ur lines of code.

edited Jul 23 '13 at 08:44

answered Jul 23 '13 at 06:35

Satheesh

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Hi @satheeshwaran, thank you for your answer. I tried both ideas and still not working... In my initial code I was doing the UI update by returning to the main thread in a asynchronous way while you suggest doing it in a synchronous way - that doesn't work. I try to use the selector trick with the background scope or with returning to the main thread scope and still with no luck. – Anaximandro Furtado Jul 23 '13 at 08:14
By the way this unexpected behaviour is while I’m running it on iOS Simulator. As you know, Apple Developer page has been down, so I’m not able to access the provisioning portal and test it on real device. Not sure if this is only because I’m on the iOS Simulator... – Anaximandro Furtado Jul 23 '13 at 08:29
Try the edited part now, this has nothing to do with simulator. – Satheesh Jul 23 '13 at 08:44
again thank you for your time @satheeshwaran, but I’m still stuck… Would it help if told you that I’m calling the function that runs that piece of code from viewDidLoad callback will this helps? – Anaximandro Furtado Jul 23 '13 at 23:13
Calling it from viewDidLoad isn't a problem, I suspect two things here the table reload data part and the table datasource part. NSLog the datasource array before reloading the tableview and also keep a break point in tableView's data source methods - numberOfRowsAtIndexPath and cellForRowAtIndexPath an check whether they are being called at least once. – Satheesh Jul 24 '13 at 06:12
It's not clear but is numberOfRowsInSection returning 0 zero rows? – Satheesh Jul 24 '13 at 08:04
On the edited code above, I was expecting to see NSLog("numberOfRowsInSection: 9”) between the NSLog(“Main thread(cp3)... 0”) and NSLog(“Main thread(cp4)... 9”) that didn’t happen… – Anaximandro Furtado Jul 24 '13 at 08:09
So your table was not reloaded for sure! – Satheesh Jul 24 '13 at 08:40
Which array's count are you returning in numberOfRows method? Are you using the same in cellForRow. Its a dumb question but though some times we do like that. – Satheesh Jul 24 '13 at 09:14
The "rooms" is actually a property of the view and this way is alwasy the same variable... – Anaximandro Furtado Jul 24 '13 at 17:36
Dear @satheeshwaran, I finally found the issue: I forgot to link the "IBOutlet UITableView *table” to the correspondent storyboard element. Sorry for wasting your time and million of thanks. – Anaximandro Furtado Jul 24 '13 at 22:12
@AnaximandroFurtado Ha ha ha, hilarious dude. Its ok we do silly mistakes all the time. – Satheesh Jul 25 '13 at 06:17

Nothing happens after calling [table reloadData] when called with dispatch_async

1 Answers1