Question

I have an NSObject that I create inside a block. As per the code below:

__block NSObject *myObject;

[self myMethod:^{
    myObject = [[NSObject alloc] init];
    ....
}];

if(myObject == nil){
    NSLog(@"Why is my object nil?!");
}

In the definition of myMethod I have the following:

  backgroundQueue = dispatch_queue_create("backgroundqueue", NULL);

            dispatch_async(backgroundQueue, 
                           ^{

                               dispatch_async(dispatch_get_main_queue(), 
                                              ^{
                                                  if(block){
                                                      block();//Never called.
                                                  }
                                              });

However the block is never called.

Answer 1

The problem here is that you never seem to execute the block in which you instantiate myObject. For illustration, run this little program:

#import <Foundation/Foundation.h>

typedef void(^MyTestBlock)(void);

@interface Foo:NSObject
- (id)initWithBlock:(MyTestBlock)aBlock;
- (void)someMethod;
@end

@implementation Foo {
    MyTestBlock _block;
}

- (id)initWithBlock:(MyTestBlock)aBlock {
    self = [super init];
    if( !self ) { return nil; }
    _block = aBlock;
    return self;
}

- (void)someMethod {
    _block();
}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        __block NSObject *myObject;

        Foo *myFoo = [[Foo alloc] initWithBlock:^{
            myObject = [[NSObject alloc] init];
        }];
        [myFoo someMethod];

        NSLog((myObject)?@"Your object was created":@"Why is my object nil?");
    }
}

This prints 2012-11-26 05:00:58.519 Untitled 2[23467:707] Your object was created to the console. The point is that blocks don’t execute themselves. In the code above, although we set the block as an ivar of the class, we don’t execute it until we call someMethod on our Foo.

EDIT:

An edit to your question states that the block is not executed in the context of an asynchronous dispatch block sent to the main queue. If this is a command line application, then you must call dispatch_main() at the end of main. See the man page for dispatch_get_main_queue(). Here is a full working command line application to illustrate this, as well as issues related to race conditions:

#import <Foundation/Foundation.h>

typedef void(^MyTestBlock)(void);

@interface Foo:NSObject
- (id)initWithBlock:(MyTestBlock)aBlock;
- (void)someMethod;
@end

@implementation Foo {
    MyTestBlock _block;
}

- (id)initWithBlock:(MyTestBlock)aBlock {
    self = [super init];
    if( !self ) { return nil; }
    _block = aBlock;
    return self;
}

- (void)someMethod {
    dispatch_queue_t backgroundQueue = dispatch_queue_create("backgroundqueue", NULL);
    dispatch_async(backgroundQueue, ^{
        dispatch_queue_t innerQueue = dispatch_get_main_queue();
        dispatch_async(innerQueue, ^{
            if( _block){
                NSLog(@"Will call block.");
                _block();
            }
        });
    });

}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        __block NSObject *myObject;

        Foo *myFoo = [[Foo alloc] initWithBlock:^{
            myObject = [[NSObject alloc] init];
        }];
        [myFoo someMethod];

        //  this log statement should show that myObject is nil because it will (probably)
        //  be executed before your block.
        NSLog((myObject)?@"Your object was created":@"Why is my object nil?");

        //  wait a little bit to resolve race condition (just for illustrative purposes)
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, 0.4f * NSEC_PER_SEC);
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            NSLog((myObject)?@"Your object was created":@"Why is my object nil?");
        });
    }
    //  this isn't a Cocoa app, so must call dispatch_main() at end of main
    dispatch_main();
}