Performing work in custom delegate blocks UI -- why?

I wrote a custom "JSON fetcher". From my view controller I call [loader start]; and when loader is done, it runs the delegate method [self.delegate doneLoading];.

This all works, however the UI is blocked -- can't move anything -- until the loader is done. I thought that if I did the loading over my own delegate method, this wouldn't happen. What am I missing here? Is there some extra method I need to call?

- (void)getBandArray
{
    if (![self localBandVersion] || [self remoteBandVersion] > [self localBandVersion] || !CACHING) {
        NSLog(@"Band array loaded remotely.");
    [self startLoadingBands];
}else{
    NSLog(@"Band array loaded from disk.");
    [self loadBandsFromDisk];

    }
}

startLoadingBands starts the NSURLConnection, loadBandsFromDisk looks like this:

- (void)loadBandsFromDisk
{
    NSData *dataFromDisk = [NSData dataWithContentsOfFile:[self fileStringForTag:JNKBandTag]];
if (dataFromDisk) {
    NSLog(@"Found band cache on disk...");

    NSString *strToParse = [[NSString alloc] initWithData:dataFromDisk encoding:NSUTF8StringEncoding];
    SBJsonParser *jsonObject = [[SBJsonParser alloc] init];
    NSError *jsonError;
    NSArray *parsedResult = [jsonObject objectWithString:strToParse error:&jsonError];
    [strToParse release];
    [jsonObject release];

    if (parsedResult && [parsedResult count] > 0) {
        NSLog(@"Parsed bands, handing over to delegate...");

        NSMutableArray *bandArray 开发者_如何学C= [NSMutableArray array];
        for (NSDictionary *bandDict in parsedResult) {
            [bandArray addObject:[JNKBand bandWithDictionary:bandDict]];
        } 

        if ([self.delegate respondsToSelector:@selector(bandArrayArrived:)]) {
            [self.delegate bandArrayArrived:bandArray];
        }
    }else{
        NSLog(@"Error parsing bands, calling delegate...");
        if ([self.delegate respondsToSelector:@selector(bandArrayFailed)]) {
            [self.delegate bandArrayFailed];
        }
    }        
}else{
    if ([self.delegate respondsToSelector:@selector(bandArrayFailed)]) {
        [self.delegate bandArrayFailed];
    }
}
}

The weird thing is that once the connection is finished it also eventually calls loadFileFromDisk but this works smoothley...

---

This is running on the main thread which is blocking everything else. You will have to do the loading in the background. Use performSelectorInBackground:withObject to do the loading in the background and if the delegate method is supposed to affect the UI, you should send the request back to the main thread.

---

Your approach is good, but you need to write code to put the work being done in [loader start] into a background thread. Right now the control flow just continues into [loader start] and executes whatever code you wrote there, calls your delegate, and returns to the next line of code after calling start.

Without knowing more about what you're doing I can't say exactly how to accomplish background threading, but taking a look at NSOperationQueue and the NSURLConnection asynchronous methods would be a good place to start.