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Objective C send message to creator
@implementation SomeInterface

-(void)DoSomething
{
     MyObj * mo = [MyObj new];
     [mo doJob];
}
end

// create a typedef for our type of completion handler, to make
// syntax cleaner elsewhere
typedef void (^MyObjDoJobCompletionHandler)(void);

@interface MyObj

- (void)doJobWithCompletionHandler:(MyObjDoJobCompletionHandler)completionHandler;

@end

- (void)doJobWithCompletionHandler:(MyObjDoJobCompletionHandler)completionHandler
{
    /* do job here ... */

    // we're done, so give the completion handler a shout.
    // We call it exactly like a C function:
    completionHandler();

    /* alternatives would have been to let GCD dispatch it,
    but that would lead to discussion of GCD and a bunch of
    thread safety issues that aren't really relevant */
}

-(void)DoSomething
{
     MyObj * mo = [MyObj new];
     [mo doJobWithCompletionHandler:
         ^() // you can omit the brackets if there are no parameters, but
             // that's a special case and I don't want to complicate things...
         {
             NSLog(@"whoop whoop, job was done");

             // do a bunch more stuff here
         }
     ];
}