Question

I am trying to launch a background thread to retrieve XML data from a web service. I developed it synchronously – without threads, so I know that part works. Now I am ready to have a non-blocking service by spawning a thread to wait for the response and parse.

I created an NSAutoreleasePool inside the thread and release it at the end of the parsing. The code to spawn and the thread are as follows:

Spawn from main-loop code:

 .
 .
 [NSThread detachNewThreadSelector:@selector(spawnRequestThread:) 
                          toTarget:self withObject:url];
 .
 .

Thread (inside ‘self’):

-(void) spawnRequestThread: (NSURL*) url  {

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

 parser = [[NSXMLParser alloc] initWithContentsOfURL:url];

 [self parseContentsOfResponse];

 [parser release];
 [pool release];
}

The method parseContentsOfResponse fills an NSMutableDictionary with the parsed document contents. I would like to avoid moving the data around a lot and allocate it back in the main-loop that spawned the thread rather than making a copy. First, is that possible, and if not, can I simply pass in an allocated pointer from the main thread and allocate with ‘dictionaryWithDictionary’ method? That just seems so inefficient.

parseContentsOfResponse

-(void)parseContentsOfResponse {

    [parser setDelegate:self];
    [parser setShouldProcessNamespaces:YES];
    [parser setShouldReportNamespacePrefixes:YES];

    [parser parse];

    NSError *parseError = [parser parserError];
    if (parseError) {
        NSLog(@"%@", parseError);
        NSLog(@"publicID: %@", [parser publicID]);
        NSLog(@"systemID: %@", [parser systemID]);
        NSLog(@"line:%d column:%d", [parser lineNumber], [parser columnNumber]);
    }

    ready = YES;
}

First parse section

Each section creates element strings when its elementStart is signaled. The elementEnd will add the object to the dictionary and release the element. The remaining details are redundant and I think the point to note is that the allocations are not directed at an NSZone, therefore they should be residing in the thread’s memory pool.

- (void)parserDidStartDocument:(NSXMLParser *)parser {
    NSLog(@"%s", __FUNCTION__);
    currentChars    = [NSMutableString stringWithString:@""];
    elementQuestion = [NSMutableString stringWithString:@""];
    elementAnswer   = [NSMutableString stringWithString:@""];
    elementKeyword  = [NSMutableString stringWithString:@""];
}

Answer 1

The simplest thing to do would be to create the dictionary in the separate thread, then set it as a property on the main thread, like so:

- (void)spawnRequestThread: (NSURL*) url  {
    NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
    //do stuff with dict
    [self performSelectorOnMainThread:@selector(doneWithThread:) withObject:dict waitUntilDone:NO];
}

- (void)doneWithThread:(NSDictionary *)theDict {
    self.dict = theDict; //retaining property, can be an NSDictionary
}

Do you need to change the contents of the dictionary over time? If so, allocating on the main thread and changing the contents in the other thread is possible, but you have to worry about thread-safety issues–NSMutableDictionary isn’t thread-safe, so you’d have to use an atomic property and locks:

//.h
@property (retain) NSMutableDictionary *dict; //don't use "nonatomic" keyword
@property (retain) NSLock *dictLock;

//.m
- (id) init {
    //blah blah
    dict = [[NSMutableDictionary alloc] init];
    dictLock = [[NSLock alloc] init];
    return self;
}

- (void)spawnRequestThread: (NSURL*) url  {
    //whenever you have to update the dictionary
    [self.dictLock lock];
    [self.dict setValue:foo forKey:bar];
    [self.dictLock unlock];
}

Locking is quite expensive and inefficient in any case, so I’d tend to prefer the first method (I’m not sure which is more expensive, exactly, but the first is simpler and avoids thread-safety issues).

Also, looking at your code, it looks like your NSXMLParser is an ivar which you directly access. This is a bad idea, since NSXMLParser isn’t thread-safe–I would recommend implementing it as a local variable instead. If you do need it as an ivar, use an atomic property and locks and only access it through accessors.