Button press doesn't work during parsing - objective-c

Question

I cannot make back button (previous page in navigation) work during html parsing process. What I am trying to do is parsing an html and get a pdf file on viewDidAppear. During that time I want back button to be enabled because parsing can take long time. So users can decide to leave parsing process.
here is my code:
-(void)viewDidAppear:(BOOL)animated
{
[self performSelector:#selector(getPDFUrl) withObject:nil afterDelay:0];
}
-(void) getPDFUrl
{
NSURL *programURL = [NSURL URLWithString:#"http://www.example.com/somepdf/"];
NSData *programHtmlData;
#try
{
programHtmlData = [NSData dataWithContentsOfURL:programURL];
}
#catch(NSException* ex)
{}
// 2
TFHpple *programHTMLParser = [TFHpple hppleWithHTMLData:programHtmlData];
NSString *studiosXpathQueryString =
#"//div[#class='ultra_wrapper']/div[#class='container columns extra_pad boxed_lay centered']/div[#id='prk_ajax_container']/div[#id='centered_block']/div[#id='main_block']/div[#id='content']/div[#id='main']/div[#class='twelve columns sidebarized']/div[#class='prk_no_composer']/p/a";
NSArray *programNodes = [programHTMLParser searchWithXPathQuery:studiosXpathQueryString];
NSMutableArray *activities = [[NSMutableArray alloc] init];
Tutorial *tutorial;
if (programNodes.count > 0) {
for (TFHppleElement *element in programNodes)
{
#try
{
tutorial = [[Tutorial alloc] init];
tutorial.url = [element objectForKey:#"href"];
}
#catch(NSException* ex)
{
}
}
NSURL *targetURL = [NSURL URLWithString:tutorial.url];
webView.scalesPageToFit=YES;
NSURLRequest *request = [NSURLRequest requestWithURL:targetURL];
[webView loadRequest:request];
}
else
{
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"warning"
message:#"warning!"
delegate:self
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[alert show];
}
}

Answer 1

[self performSelector:#selector(getPDFUrl) withObject:nil afterDelay:0];
will execute on main thread. Therefore Blocking your UI.
You should use
[self performSelectorInBackground:#selector(getPDFUrl) withObject:nil];
Or you can use NSOperationQueue or just NSOperation or more simple way NSBlockOperation.
NSBlockOperation *op = [NSBlockOperation blockOperationWithBlock:^{
NSLog(#"dasds");
}];
[op start];