thread_save.m

/*
* Example: needed to save a pdf document and show that pdf data in main thread. So using the example: 
*/

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    //load your data here.
 dispatch_async(dispatch_get_main_queue(), ^{
    //update UI in main thread.
  });
});

/*
* I ended using this code: 
*/


//Thread save
 __block NSURL *url = nil;
 
 dispatch_async(dispatch_get_global_queue(0, 0), ^{
    //load data here.
    [self savePDF:myData withDocumentName:documentName onComplete:^(NSURL *urlDocument){
          url = urlDocument;
     }];
     dispatch_async(dispatch_get_main_queue(), ^{
        //update UI in main thread.
        [self presentPdfReaderViewController:url withBlock:^(BOOL success, NSDictionary *dict) {
            success ? [self setSendResultWithBoolean:success messageError:dict[SUCCESS] command:command] :   [self setSendResultWithBoolean:success messageError:dict[ERROR] command:command];
        }];
      });
    });