Question

I’m building an iPhone app that aggregates data from several different data sources and displays them together in a single table. For each data source, I’ve created a class (WeatherProvider, TwitterProvider) that handles connecting to the datasource, download the data and storing it in an object.

I have another class ConnectionManager, that instantiates and calls each of the two provider classes, and merges the results into a single NSArray which is displayed in the table.

When I run the program calling just the WeatherProvider or just the TwitterProvider, it works perfectly. I can call each of those objects again and again to keep them up-to-date without a problem. I can also call TwitterProvider and then WeatherProvider without a problem.

However, if I call WeatherProvider then TwitterProvider, the TwitterProvider hangs just as I call: [[NSURLConnection alloc] initWithRequest:request delegate:self];.

Other points:
– it doesn’t seem to matter how long between when I call WeatherProvider and TwitterProvider, TwitterProvider still hangs.
– in WeatherProvider, I’m using NSXMLParser with NSAutoreleasePool to parse the output from the WeatherProvider.
– ConnectionManager creates an instance of WeatherProvider and TwitterProvider when the application is started, and reuses those instances when the user requests a data refresh.
– I ran the app with the activity monitor connected and it verifies that the app is basically just hanging. No CPU usage, or additional memory allocations or network activity seems to be happening.

There’s a bunch of code spread across a couple files, so I’ve tried to include the relevant bits (as far as I can tell!) here. I very much appreciate any help you can provide, even if it is just additional approaches to debugging.

WeatherProvider

-(void)getCurrentWeather: (NSString*)lat lon:(NSString*)lon lastUpdate:(double)lastUpdate
{
double now = [[NSDate date] timeIntervalSince1970];
NSString *noaaApiUrl;

// don't update if current forecast is < than 1 hour old
if(now - lastUpdate < 3600)
{
    [[self delegate] weatherUpdaterComplete:1];
    return;
}

// if we already have a forecast, delete and refill it. 
if(forecast)
{
    [forecast release];
    forecast = [[WeatherForecast alloc] init];
}

forecast.clickThroughUrl = [NSString stringWithFormat:@"http://forecast.weather.gov/MapClick.php?lat=%@&lon=%@",
                            lat, lon];  

noaaApiUrl = [NSString stringWithFormat:@"http://www.weather.gov/forecasts/xml/sample_products/browser_interface/ndfdBrowserClientByDay.php?lat=%@&lon=%@&format=24+hourly", 
             lat, lon];

NSURLRequest *noaaUrlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:noaaApiUrl]];
[[NSURLConnection alloc] initWithRequest:noaaUrlRequest delegate:self]; 
}

#pragma mark -
#pragma mark NSURLConnection delegate methods

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response 
{
    self.noaaData = [NSMutableData data];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data 
{
    [noaaData appendData:data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error 
{
    self.noaaConnection = nil;
    [[self delegate] weatherUpdaterError:error];
    [connection release];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection 
{
    // Spawn a thread to fetch the data so UI isn't blocked while we parse
    //  the data. IMPORTANT - don't access UIKit objects on 2ndary threads
    [NSThread detachNewThreadSelector:@selector(parseNoaaData:) toTarget:self withObject:noaaData];

    // the noaaData will be retailed by the thread until parseNoaaData: has finished executing
    //  so, we'll no longer need a reference to it in the main thread.
    self.noaaData = nil;
    [connection release];
}

#pragma mark -
#pragma mark NSXMLParser delegate methods

- (void)parseNoaaData:(NSData *)data 
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSXMLParser *parser = [[NSXMLParser alloc] initWithData:data];
    [parser setDelegate:self];
    [parser parse];

    [parser release];        
    [pool release];
}

TwitterProvider

-(void)getLocationTimeline:(NSString*)lat lon:(NSString*)lon lastUpdate:(double)lastUpdate refreshUrl:(NSString*)newUrl
{
    NSString *updateURL;

    if(tweets.count > 1)
        [tweets removeAllObjects];

    updateURL = [NSString stringWithFormat:@"http://search.twitter.com/search.json?geocode=%@,%@,1mi&rpp=%@&page=1", lat, lon, TWITTER_LOCAL_UPDATES_URL_RESULTS];      

    _serviceResponse = [[NSMutableData data] retain];
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:updateURL]];  
    [[NSURLConnection alloc] initWithRequest:request delegate:self];
}

#pragma mark -
#pragma mark NSURLConnection delegate methods

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{   
    NSString *responseString = [[NSString alloc] initWithData:_serviceResponse encoding:NSUTF8StringEncoding];

        // parse tweets

    [responseString release];
    [_serviceResponse release]; 
    [connection release];

    // tell our delegate that we're done!
    [[self delegate] twitterUpdaterComplete:tweets.count];  
}

- (void)connection:(NSURLConnection *)connection 
didReceiveResponse:(NSURLResponse *)response
{
    [_serviceResponse setLength:0];
}

- (void)connection:(NSURLConnection *)connection 
    didReceiveData:(NSData *)data
{
    [_serviceResponse appendData:data];
}

- (void)connection:(NSURLConnection *)connection 
  didFailWithError:(NSError *)error
{
    [connection release];

    [[self delegate] twitterUpdaterError:error];
}

Answer 1

I was able to fix this by removing the threading in WeatherUpdater that was being used for the NSXMLParser. I had ‘barrowed’ that code from Apple’s SiesmicXML sample app.