objective c Thread in Thread variables life time

Question

I have an NSOperation where inside its -main method I use [NSThread detachNewThreadSelector:@selector(aMethod:) toTarget:self withObject:anArgument];

aObject (instance variable of my NSOperation subclass) is a weak reference to an object of an autoreleased array returned inside the -main method...

-(void)main {

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

    NSArray *clients = [group produceClients]; // clients array is an autorelease instance
    self->aObject = [clients objectAtIndex:3]; // a weak reference, Lets say at index three!

    [NSThread detachNewThreadSelector:@selector(aMethod:) 
                             toTarget:self 
                           withObject:@"I use this for another thing"];

    // Do other things here that may take some time

    [pool release];
}

-(void)aMethod:(NSString*)aStr {

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

    // aStr object is reserved for another functionality
    // I know that I could pass a NSDictionary with a number of entries, which would be
    // retained by `detachNewThreadSelector` but then ... 
    // I wouldn't have to ask this question ! :)

    // Do several things with aObject, which is a weak reference as described above.
    NSLog(@"%@", [self->aObject.id]);
    // Is it safe ?

    [pool release];
}

I know that NSThread's detachNewThreadSelector method retains self and withObject:anArgument, but what happens to aObject ?? Is it sure that will exist during the execution of the detached thread (aMethod:) ? Self is retained by detachNewThreadSelector, does this mean that the pool of the -main thread will be delayed released since it is retained and thus the clients will exist and thus the aObject will exist ?
Or the -main (NSOperation) thread will finish execution and released before -aMethod (NSThread) finishes so it's unsafe to use aObject there ?

The real question is: When calling [NSThread detachNewThreadSelector:@selector(aMethod:) ...toTarget:self ...] from inside a thread, does the last thread being retained in a way that its autoreleased instances (clients array) are safe to be used in aMethod (self->aObject) (lets say via weak references) ?

score 0 · Answer 1 · answered Jun 04 '11 at 01:00

Your approach seems highly unstable, but I'm not an expert on multithreading so I could be wrong. Your clients array is in the main autorelease pool, which you cannot be assured will wait till your aMethod thread is completed to drain. What about this:

-(void)main {

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

    [NSThread detachNewThreadSelector:@selector(aMethod:) 
                             toTarget:self 
                           withObject:@"I use this for another thing"];

    [pool release];
}

-(void)aMethod:(NSString*)aStr {

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

    NSArray *clients = [group produceClients]; // clients array is an autorelease instance
    self->aObject = [clients objectAtIndex:3]; // a weak reference, Lets say at index three!

    [pool release];
}

With this approach, your clients array is in the thread's autorelease pool.

Thanks for your respond. I maybe miss to mention that many things and processing happens before and after the `self->aObject` is set in the `-main` thread. So I want these two threads to be executed in parallel. I wouldn't use two threads otherwise, would I ? — Vassilis, Jun 04 '11 at 01:18

objective c Thread in Thread variables life time

1 Answers1