Question

I am dispatching a queue to download some flickr photos on a separate thread (in viewWillAppear). When I log the contents of the array inside the block, it shows everything perfectly:

    dispatch_queue_t photoDowonload=dispatch_queue_create("photoDownload", NULL);
dispatch_async(photoDowonload, ^{

    NSArray *photoList=[FlickrFetcher topPlaces]; //downloads flickr data
    self.listOfCities=photoList;
    NSLog(@"inside block: %@", self.listOfCities); //shows contents

});

but when I try to log the array that was set inside the block outside the block, it returns null.

    dispatch_queue_t photoDowonload=dispatch_queue_create("photoDownload", NULL);
dispatch_async(photoDowonload, ^{

    NSArray *photoList=[FlickrFetcher topPlaces];
    self.listOfCities=photoList;

});

    NSLog(@"after block: %@", self.listOfCities); //returns null

What’s the problem here? self.listOfCities is set up as NSArray property so once it’s set in the block, it should be accessible outside of it.

Answer 1

The code in the block is run asynchronously. So the code after the block is run before the code in the block has had a chance to run (or certainly complete at least).