Question

When I load my Master View it automatically loads a JSON feed with blog posts.

I have a refresh button on the top bar of my Master View. I have already connected it successfully to an IBAction and when clicked, I can output a string to log.

I am trying to get my view to reload the JSON feed when I click on the refresh button but that doesn’t work.

What am I doing wrong?

My ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UICollectionViewController {
    NSArray *posts;
}

- (void)fetchPosts;

- (IBAction)refresh:(id)sender;
@end

My ViewController.m

...

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self fetchPosts];
}

- (IBAction)refresh:(id)sender {

    [self fetchPosts];
}

- (void)fetchPosts
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString: @"http://website.com/app/"]];

        NSError* error;

        posts = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];

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

Answer 1

The post is not being updated as you expect as it is being captured inside of the async block. If I remember correctly, instance variables are copied once passed into a block, so changes to them aren’t reflected outside of the async block unless they have the __block modifier.

Try this,

- (void)fetchPosts
{
    __block NSArray *blockPosts = posts;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString: @"http://website.com/app/"]];

        NSError* error;

        blockPosts = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];

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