How can I disable a button while NSXMLParser runs, then enable it when it completes?

Question

I have a viewController which imports XMLParser.h as the class xmlParser

I'm passing an NSURL object in my viewController to the xmlParser class with the getXML method below

goButton is the button I tap to call the getXML method below. I disable the button which I tapped to trigger the getXML method, but I'm not sure where to put the code to enable it again once the xmlParser has finished parsing the returned XML.

- (IBAction) getXML {
    goButton.enabled = NO;

    // allocate and initialize the xmlParser 
    xmlParser = [[XMLParser alloc] init];

// then generate the URL we are going to pass to it and call the fetchXML method passing the URL.
    NSURL *xmlurl = [[NSURL alloc] initWithString:@"http://www.mysite.com/myfile.xml"];
    [xmlParser fetchXMLFromURL:xmlurl];

    // release objects
    [xmlurl release];
    [xmlParser release];

}

---

As per @Squeegy recommendation, I modified my code.

- (IBAction) getXML {
    goButton.enabled = NO;

    xmlParser = [[XMLParser alloc] init];

    [self performSelectorInBackground:@selector(parseInBackground:) withObject:xmlParser];

}

- (void)parseInBackground:(XMLParser*)parser {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSURL *xmlurl = [[NSURL alloc] initWithString:@"http://www.mysite.com/myfile.xml"];

    [parser fetchXMLFromURL:xmlurl];
    [self performSelectorOnMainThread:@selector(didFinishXMLParsing:) withObject:parser];

    [xmlurl release];
    [pool drain];

}

- (void)didFinishXMLParsing:(NSXMLParser*)parser {
    goButton.enabled = YES;
}

Looks to be working until it gets to the line

[self performSelectorOnMainThread:@selector(didFinishXMLParsing:) withObject:parser];

The compiler complains as follows:

2010-02-17 00:22:20.574 XMLApp[2443:521b] *** -[viewController performSelectorOnMainThread:withObject:]: unrecognized selector sent to instance 0x1285a0
2010-02-17 00:22:20.578 XMLApp[2443:521b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[viewController performSelectorOnMainThread:withObject:]: unrecognized selector sent to instance 0x1285a0'
2010-02-17 00:22:20.583 XMLApp[2443:521b] Stack: (
    861696817,
    860329709,
    861700631,
    861203093,
    861166272,
    18715,
    846004025,
    845672609,
    848189713
)

Answer 1

When the parser finishes parsing, it will call it's delegate's:

- (void)parserDidEndDocument:(NSXMLParser *)parser

In that method, you can re-enable the button. You should probably do so with a performSelectorInMainThread call, since it involves changing a view.

Answer 2

- (IBAction)getXML {
    goButton.enabled = NO;

    xmlParser = [[XMLParser alloc] init];
    NSURL *xmlurl = [[NSURL alloc] initWithString:@"http://www.mysite.com/myfile.xml"];
    [xmlParser fetchXMLFromURL:xmlurl];

    [self performSelectorInBackground:@selector(parseInBackground) withObject:xmlParser];

    [xmlurl release];
    [xmlParser release];
}

- (void)parseInBackground:(NSXMLParser*)parser {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    [parser parse];
    [self performSelectorOnMainThread:@selector(didFinishXMLParsing:)
                           withObject:parser
                        waitUntilDone:NO];

    [pool drain];
}

- (void)didFinishXMLParsing:(NSXMLParser*)parser {
    goButton.enabled = YES;
}

The trick is to do the processing on a background thread, which allows the UI to do stuff. When parsing is done, you have to make any UI changes back on the main thread.