How do I do an Asychronous NSURLConnection inside an NSOperation?

Question

I want to do an Asynchrous NSURLConnection inside of an NSOperation on a background thread.

(it is because I'm doing some very expensive operations on the data as they come back that want to be done as the data comes in and in background)

Here is my first attempt:

IN my AppDelegate:

// create the opperation and add it to the queue:
self.sharedOperationQueue = [[[NSOperationQueue alloc] init] autorelease];
LibXMLOperation *op = [[[LibXMLOperation alloc] init] autorelease];
[self.sharedOperationQueue addOperation:op];

Here is my operation:

@interface EbirdLibXMLOperation : NSOperation {
@private
  NSURLConnection *urlConnection;
 // Overall state of the parser, used to exit the run loop.
 BOOL done;
 // properties to maintain the NSOperation
 BOOL finished;
 BOOL executing;  
}
- (void)downloadAndParse:(NSURL *)url;
- (void)start;
- (BOOL)isConcurrent;
- (BOOL)isFinished;
- (BOOL)isExecuting;

@property BOOL done;
@property (nonatomic, retain) NSURLConnection *ebirdConnection;
// The autorelease pool property is assign because autorelease pools cannot be retained.
@property (nonatomic, assign) NSAutoreleasePool *downloadAndParsePool;

@end


@implementation LibXMLOperation
@synthesize urlConnection, done;

- (void)start{
  if (![self isCancelled]) {
    [self willChangeValueForKey:@"isExecuting"];
    executing = YES;
    //set up the thread and kick it off...
    [[NSURLCache sharedURLCache] removeAllCachedResponses];
    NSURL *url = [NSURL URLWithString:@"http://google.com"];
    [NSThread detachNewThreadSelector:@selector(downloadAndParse:) toTarget:self withObject:url];
    [self didChangeValueForKey:@"isExecuting"];
  } else {
    // If it's already been cancelled, mark the operation as finished.
    [self willChangeValueForKey:@"isFinished"];
    finished = YES;
    [self didChangeValueForKey:@"isFinished"];
  }
}

- (BOOL)isConcurrent {
  return YES;
}

- (BOOL)isExecuting {
  return executing;
}

- (BOOL)isFinished {
  return finished;
}


- (void)downloadAndParse:(NSURL *)url {
  self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];
  done = NO;
  self.characterBuffer = [NSMutableData data];
  [[NSURLCache sharedURLCache] removeAllCachedResponses];
  NSURLRequest *theRequest = [NSURLRequest requestWithURL:url];
  urlConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
  if (urlConnection != nil) {
    do {
      [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    } while (!done);
  }
  [self willChangeValueForKey:@"isFinished"];
  [self willChangeValueForKey:@"isExecuting"];
  finished = YES;
  executing = NO;
  // Clean up.
  self.urlConnection = nil;
  [downloadAndParsePool release];
  NSLog(@"download and parse cleaning up");
  self.downloadAndParsePool = nil;
  [self didChangeValueForKey:@"isExecuting"];
  [self didChangeValueForKey:@"isFinished"];  
}


#pragma mark NSURLConnection Delegate methods

// Disable caching so that each time we run this app we are starting with a clean slate.

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse {
  return nil;
}

// Forward errors to the delegate.
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
  done = YES;
}

// Called when a chunk of data has been downloaded.
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
  // Process the downloaded chunk of data.
  NSLog(@"Did received %i bytes", [data length]);
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
  // Set the condition which ends the run loop.
  done = YES; 
}    

@end

When this runs, I see the following message in my log:

2009-08-20 15:18:48.858 App[1001:3e03]*** _NSAutoreleaseNoPool(): Object 0x1126a20 of class NSCFArray autoreleased with no pool in place - just leaking Stack: (0x305a2e6f 0x30504682 0x3057deba 0x305ced09 0x30577ddf 0x3056b43e 0x3050764a 0x58fc3 0x3050a79d 0x3050a338 0x94568155 0x94568012)

This event happens at the very last [self didChangeValueForKey:@"isFinished"]; which suggests to me that I'm setting up the NSOperation wrong.

Any suggestions are appreciated, thank you.

Answer 1

Move the lines:

[downloadAndParsePool release];
self.downloadAndParsePool = nil;

to the end of the -downloadAndParse: method.

Answer 2

I am looking to do something similar. How are you handling returning the data from this operation: KVO, delegate, performSelectorOnMainThread?

Answer 3

I'm curious about a design decision in the above code. The start method of a thread is used to start an asynchronous operation. The fetching of HTTP content is already asychronous, and yet the above code also launches a new thread manually. What is the point in that? Couldn't you just do away with the 'done' boolean, remove the polling loop in start, and handle the connection finished stuff by calling a helper method you write from connectionDidFinishLoading and didFailWithError?