Question

I’m loading a rather simple XML file from a URL. I wanted to show the network activity indicator while the parser is loading the file, but obviously not while it is parsing.

Question: Does the initWithContentsOfUrl: method lock program execution while the document is loaded from the url? In other words, is the code below correct?

It seems obvious to me that this is okay, but I wanted to make 100% sure.

NSString* const urlString = @"...";
NSURL* url = [NSURL URLWithString:urlString];
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];
NSXMLParser* parser = [[NSXMLParser alloc] initWithContentsOfURL:url];
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];   

Answer 1

This article explains it pretty well http://akosma.com/2010/05/28/initwithcontentsofurl-methods-considered-harmful/:

The main problem with these methods, of course, is the fact that they
are synchronous; this means that the thread executing them (usually
the UI thread) will block completely until they return, and in most
applications this means that you are de-facto blocking the whole
application for an unknown amount of time. This means that no buttons
or UI widgets will react to input, no navigation will be possible, no
touch events will be delivered or executed, nothing will happen at all
until the network operation completes.

Even worse; when using initWithContentsOfURL:, there is no timeout,
there is no meaningful feedback for network failures, and no way for
the user to cancel the current network operation. This last factor
justifies by itself not using initWithContentsOfURL: at all; you must
never ship code that leads to a bad user experience. Your users will
resent this and will complain!