Completion blocks

In my viewController i create an instance of the class QuestionParser which will start a xml parser. My question is: IS there the possibility so the parser will not finish parsing before the program reaches compblock, thus returning nil argument or even an error ?

MenuViewController.m

QuestionParser *parser = [[QuestionParser alloc]init];
[parser parseCategories:^(NSArray *categoryName, NSArray *categoryID)
{
    if(categoryName)
    {
        NSlog(@"%@",categoryName);
    }
}];

QuestionParser.m

(void) parseCategories:(myCategoryCompletion) compblock {

[self parseCategories];

compblock(categoryName,categoryID); 

}

I am sorry i didn't provide all the info, and i will try to clarify the situation. in parseCategories function i do this:

-(void) parseCategories {

  NSLog(@"<> start parsing categories <>");

  categoryParser = [[NSXMLParser alloc]initWithContentsOfURL:[URL]];

  [categoryParser setDelegate:self];

  [categoryParser parse]; 
}

And i expect so the completion block does not end before my parser's delegate "didFinishDocument" reaches. Will this be the case ?

Now, while testing this functions, everything works ok, my parser delegates create the arrays i need from the xml and after that, it reaches the end on the block compblock(categoryName,categoryID) returning to the view controller what i need. I wander if the parser takes to long will it still work ?

You don't need to worry about that. The "parse" of NSXMLParser is synchronous. Unless the completion or failure returns, "parse" will be over.