Question

I have a strange issue, when it comes to parsing XML with NSXMLParser on the iPhone. When starting the app, I want to preload 4 table-views, that are populated by RSS-Feeds in the background.

When I init the table-views one-by-one, than loading, parsing and displaying all works like a charm. But when I try to init all view at once (at the same time), than it seems, that the XML-parser-instances are disturbing each other. Somehow data from one XML-Feed are “broadcasted” into other xml-parser instances, where they do not belong. Example: there is a “teammember” item, with “This is my name”. When this bug occurs, there is a string from another xml-feed added, i.e. resulting in: “This is my name58”, where 58 is the chart-position of something from the other view. “58” seems to miss then on the other instance.

It looks to me, that this bug occurs because of the NSXMLParser-delegate method:

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {
    if (!currentStringValue) {
        currentStringValue = [[NSMutableString alloc] initWithCapacity:50];
    }
    [currentStringValue appendString:string];   
}

In this case “by coincidence” bytes are appended to strings, where they do not belong to.

The strange thing is, that every instance of NSXMLParser is unique, got its own unique delegates, that are attached to their own ViewController. Every parsing-requests spawns it own background-task, with its own (also also unique named) Autorelease-pool.

I am calling the NSXMLParser like this in the ViewController:

// prepare XML saving and parsing
currentStringValue = [[[NSMutableString alloc] initWithCapacity:50] retain];    
charts = [[NSMutableArray alloc] init];

NSURL *url = [[NSURL alloc] initWithString:@"http://(SOME XML URL)"];
xmlParser = [[[NSXMLParser alloc] initWithContentsOfURL:url] retain];

//Set delegate
[xmlParser setDelegate:self];

//loading indicator 
progressWheel = [[[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(150.0,170.0,20.0,20.0)] autorelease];
progressWheel.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray;

[self.view addSubview:progressWheel];
[progressWheel startAnimating];

// start loading and parsing the xml-feed in the background
//[self performSelectorInBackground:@selector(parse:) withObject:xmlParser]; -> I also tried this
[NSThread detachNewThreadSelector:@selector(parse:) toTarget:self withObject:xmlParser];

And this is one of the background-tasks, parsing the feed:

-(void)parse:(NSXMLParser*)myParser {

NSAutoreleasePool *schedulePool = [[NSAutoreleasePool alloc] init];

BOOL success = [myParser parse];

if(success) {
    NSLog(@"No Errors. xmlParser got: %@", myParser);

    (POST-PROCESSING DETAILS OF THE DATA RETURNED)

    [self.tableView reloadData];            

} else {    
    NSLog(@"Couldn't initalize XMLparser");
}

[progressWheel stopAnimating];

[schedulePool drain];

[myParser release]; 

}

What could cause this issue? Am I calling the background-task in the right way? Why is this bug approaching, since every XML-Parser got its own, unique instance?

Answer 1

You should not be updating UI elements (like progressWheel) from inside a background thread. UI updates should be done on the main thread.

Use -performSelectorOnMainThread:withObject:waitUntilDone: to update UI elements from within a background thread.