Returning an NSArray to an ivar from a completion Handler

Question

I am kind of stuck since 2 days. I am trying to show a UITableView with a timeline of twitter-user with the new Twitter-Fabric-Kit (I followed this guide: https://dev.twitter.com/twitter-kit/ios/show-tweets). I tried to do this: Get the Tweet-IDs from a certain user via JSON and put it in an instance variable. This part is working. The problem - I figured this out so far - is that the array is passed to the instance variable in a completion handler but it seems to be running asynchronous. I already tried to solve this with GCD but I had no luck. Maybe someone can point in the right direction or even that my solution is crap and that there is a much better way ;-)

#import "TweetTableViewController.h"

static NSString * const TweetTableReuseIdentifier = @"TwitterCell";

@interface TweetTableViewController ()

@property (nonatomic, strong) NSArray *tweetArray;
@property (nonatomic, strong) TWTRTweetTableViewCell *prototypeCell;

@end

@implementation TweetTableViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self createTweetArray];
//  .... tableView setup is here ...    

//    If I uncomment the next line the whole thing works!
//    NSArray *test = @[@"517330786857795584", @"516652595021352960"];

    __weak typeof(self) weakSelf = self;
    [[[Twitter sharedInstance]APIClient]loadTweetsWithIDs:self.tweetArray completion:^(NSArray *tweets, NSError *error) {
        if (tweets) {
            typeof(self) strongSelf = weakSelf;
            strongSelf.tweetArray = tweets;
            [strongSelf.tableView reloadData];
        } else {
            NSLog(@"Failed to load tweet: %@", [error localizedDescription]);
        }
    }];
}

- (void)createTweetArray {
        NSMutableArray *tempArray = [NSMutableArray array];
        [[Twitter sharedInstance]logInGuestWithCompletion:^(TWTRGuestSession *guestSession, NSError *error) {
            if (guestSession) {
                NSString *timeline = @"https://api.twitter.com/1.1/statuses/user_timeline.json";
                NSDictionary *parameters = @{@"user_id" : @"345330869"};
                NSError *clientError;
                NSURLRequest *request = [[[Twitter sharedInstance]APIClient]URLRequestWithMethod:@"GET"
                                                                                             URL:timeline
                                                                                      parameters:parameters
                                                                                           error:&clientError];
                if (request) {
                    [[[Twitter sharedInstance]APIClient]sendTwitterRequest:request completion:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
                        if (data) {
                            NSError *jsonError;
                            NSArray *json = [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];
                            for (NSDictionary *dic in json) {
                                [tempArray addObject:[dic objectForKey:@"id_str"]];
                            }
                            [self handleJSONData:tempArray];
                        } else {
                            NSLog(@"Error: %@", connectionError);
                        }
                    }];
                } else {
                    NSLog(@"Error: %@", clientError);
                }
            }
        }];
}

- (void)handleJSONData:(NSMutableArray *)array {
    self.tweetArray = [NSArray arrayWithArray:array];
    NSLog(@" %@", self.tweetArray);
}

When I run my app with this code I got the following logs in the console:

2014-11-10 22:08:41.447 [65550:3869433] numberOfSectionsInTableView
2014-11-10 22:08:41.448 [65550:3869433] numberOfSectionsInTableView
2014-11-10 22:08:41.448 [65550:3869433] numberOfRowsInSection: 0
2014-11-10 22:08:41.448 [65550:3869433] numberOfSectionsInTableView
2014-11-10 22:08:41.448 [65550:3869433] numberOfRowsInSection: 0
2014-11-10 22:08:41.449 [65550:3869433] numberOfSectionsInTableView
2014-11-10 22:08:41.450 [65550:3869433] numberOfRowsInSection: 0
2014-11-10 22:08:43.426 [65550:3869433] (
    517330786857795584,
    516652595021352960,
    514841221907243008,
    513704220600848384,
    513598624706875392,
    513598609603178496,
    513598594017132544,
    513598581471969280,
    513598566502502400,
    513229247117545472,
    511424250138611713,
    509025219584212992,
    505655616338407424,
    500357644226285568,
    474861008314322944,
    469938745438109697,
    468465457750888449,
    464706315924041728,
    464704566492418048,
    457148030731694080
)

The self.tweetArray array should be used for the numberOfRowsInSection as well as the data source, but the block seems to finish to late. As mentioned before I tried dispatch_async and dispatch_sync but nothing seemed to work. My last thought was a [self.tableView reloadData] in the handleJSONData: method but that crashed my app. With a static Array the tableView is working...

Thanks for your input, I am really stuck here...

Answer 1

Set a breakpoint in your handleJSONData: method and verify that it is running on the main queue.

If it is not running on the main queue you can't update the UI, for example call reloadData on self.tableView

To run your update on the main queue you can use GCD:

dispatch_async(dispatch_get_main_queue(),
^{
   [self.tableView reloadData]
});

or NSOperationQueue:

[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    [self.tableView reloadData]
}];

Answer 2

A completion handler will always finish after the code that invokes it finishes. That's how async code works. Yes, you should rebuild your data structures that feed your tableview's data source in the completion handler and then call reloadData. If that's crashing it's because of a bug, not because it's the wrong thing to do.