Blocks

// Synchronous
@implementation A
{
   B *b;
}

- (void)doSomething {
   /* ... */
   BOOL x = [b performOne];
   if (x) [b performTwo];
}
@end

@interface B : NSObject
- (BOOL)performOne;
- (void)performTwo;
@end

// Making this asynchronous
@interface AsyncThread : NSObject
- (void)executeAsync(Block block); // Execute in the thread associated with this object
@end

// Block style #1:
@interface B : AsyncThread
- (BOOL)performOne;
- (void)peformTwo;
@end

@implementation A
{
   B *b;
}

- (void)doSomething {
   /* ... */
   [b executeAsync:^{
      BOOL x = [b performOne];
      if (x) [b performTwo];
   }];
}

// Block style #2
@interface A : AsyncThread
@interface B : AsyncThread
- (void)performOneMaybeTwo; // performOne performTwo no longer exposed externally.
@end

@implementation A
{
   B *b;
}

- (void)doSomething {
   /* ... */
   [b executeAsync:^{
      [b performOneMaybeTwo];
   }];
}
@end

@implementation B
- (void)performOneMaybeTwo {
   BOOL x = [self performOne];
   if (x) [self performTwo];
}
@end


// With messages instead:
@interface AsyncThread2
- (void)executeAsync:(SEL)selector withObject:(id)arg; // Execute on object thread
@end

@interface A : AsyncThread2

@interface B : AsyncThread2
- (void)performOneMaybeTwo;
@end

@implementation A
{
   B *b;
}
- (void)doSomething {
   /* ... */
   [b executeAsync:@selector(performOneMaybeTwo) withObject:nil];
}
@end