Question

Having issue with prefetching data and displaying on uitableview. So basically I want to block the main UI thread so that I can get data from web. I am using serial dispatch queue for synchronizing. Also, dispatch queue block is executing another block that fetches data from web. The code for executing is written in viewdidload:

dispatch_queue_t queue= dispatch_queue_create("myQueue", NULL);


CMStore *store = [CMStore defaultStore];

// Begin to fetch all of the items
dispatch_async(queue, ^{

[store allObjectsOfClass:[Inventory class]
       additionalOptions:nil
                callback:^(CMObjectFetchResponse *response) {

                    //block execution to fetch data

                }];
});
dispatch_async(queue, ^{
//load data on local data structure


    [self.tableView reloadData];
});

Answer 1

You should NEVER perform any UI-related code anywhere other than in the main thread/queue.

Always perform every UI-related code (like reloadData on an UITableView) on the main thread/queue. In your example I guess you should also reload the tableview only when your data has been fetched, so in the completion block, and not before the callback has been called.

// Begin to fetch all of the items
dispatch_async(queue, ^{
   [store allObjectsOfClass:[Inventory class]
       additionalOptions:nil
                callback:^(CMObjectFetchResponse *response) {

                // block execution to fetch data
                ...
                // load data on local data structure
                ...

                // Ask the main queue to reload the tableView
                dispatch_async(dispatch_get_main_queue(), ^{
                    // Alsways perform such code on the main queue/thread
                    [self.tableView reloadData];
                });
    }];
});