UI issues while using blocks - objective-c

Question

I am using MBProgressHUD to display a loading indicator on my app. When i want to change something at UI or i open a new screen inside block, i only see a blank white screen. Does anyone has an idea what am i missing in my code?
-(void)doSomething:(id)sender
{
HUD = [[MBProgressHUD alloc] initWithView:self.view];
[self.view addSubview:HUD];
HUD.delegate = self;
HUD.labelText = #"Please wait";
[HUD showAnimated:YES whileExecutingBlock:^{
NSDictionary* dictReturn = [ServerApi getItemDetails::itemId userId:userId;
NewScreenController* vc = [[NewScreenController alloc]init];
[self presentViewController:vc animated:YES completion:nil];
}];
}

Answer 1

MBProgressHUD doesn't execute the block in the main thread. That's why you shouldn't change UI there at all.
You should use the method with a completionBlock instead.
- (void)doSomething:(id)sender {
HUD = [[MBProgressHUD alloc] initWithView:self.view];
[self.view addSubview:HUD];
HUD.delegate = self;
HUD.labelText = #"Please wait";
dispatch_block_t executionBlock = ^{
self.dictReturn = [ServerApi getItemDetails:itemId userId:userId];
};
void (^completionBlock)() = ^{
NewScreenController *vc = [[NewScreenController alloc] init];
[self presentViewController:vc animated:YES completion:nil];
};
[HUD showAnimated:YES
whileExecutingBlock:executionBlock
completionBlock:completionBlock];
}