Fetching JSON and blank TableView - ios

Question

I'm having trouble with my tableView. In practice, I have two tableViews and when I click on an element of the first tableView, I open the second tableView where I'm going to fetch a JSON (which works correctly because the dictionary contains all the right data). But the problem is that these downloaded data are not immediately displayed in the tableView, which remains empty. If I go back to the first tableView and reopen the same cell, the data magically appear.
So I thought it might be a problem because if I'm wrong the JSON requests are asynchronous, so I wrote the method:
dispatch_async (dispatch_get_main_queue (), ^ {
                     [self.tableView reloadData];
                 });
But it does not work anyway. Do you have any idea? This is my code:
self.arrayPM = [[NSMutableArray alloc] init];
if(![NSKeyedUnarchiver unarchiveObjectWithData:[[NSUserDefaults standardUserDefaults] objectForKey:#"dictpm"]]){
NSString *username = #"user";
NSString *password = #"pwd";
NSString *post = [NSString stringWithFormat:#"upmon_id=%#&user=%#&pwd=%#", pm_id, username, password];
NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
NSString *postLength = [NSString stringWithFormat:#"%lu" , (unsigned long)[postData length]];
NSURL *url = [NSURL URLWithString:#"urlblabla"];
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
[request setURL:url];
[request setHTTPMethod:#"POST"];
[request setValue:postLength forHTTPHeaderField:#"Content-Length"];
[request setValue:#"application/x-www-form-urlencoded" forHTTPHeaderField:#"Content-Type"];
[request setHTTPBody:postData];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler: ^(NSData *data, NSURLResponse *response, NSError *error) {
if (error) {
return;
}
if((long)[(NSHTTPURLResponse *)response statusCode] == 200) {
NSError *jsonError;
jsonResponse = [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];
dispatch_async(dispatch_get_main_queue(), ^{
[self.tableView reloadData];
});
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:jsonResponse];
[[NSUserDefaults standardUserDefaults] setObject:data forKey:#"dictpm"];
}
}];
[task resume];
}
else {
jsonResponse = [NSKeyedUnarchiver unarchiveObjectWithData:[[NSUserDefaults standardUserDefaults] objectForKey:#"dictpm"]];
for (int i = 0; i < [jsonResponse count]; i++)
{
PianoMonitoraggio *newPM = [[PianoMonitoraggio alloc] init];
bla bla bla, code
[self.arrayPM addObject:newPM];
}
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self.arrayPM];
[[NSUserDefaults standardUserDefaults] setObject:data forKey:#"arrayPM"];
[[NSUserDefaults standardUserDefaults]synchronize];
[self.tableView reloadData];
}