How to stop BFTask Chain ？

[Fykec]

Hi Guys

I have a question "how to stop the BFTask chain, break from the chain execution"

See Commnets

 BFTask *task = [_model getX1Task];

    [[task continueWithBlock:^id(BFTask *task) {
        if (task.error || task.exception || task.isCancelled) {
            
            return nil;
        } else {
            NSDictionary *dic = (NSDictionary *)task.result;
            if ([dic isKindOfClass:[NSDictionary class]])
            {
                if ([self checkX1OK])
                {
                
                    return [_model getX2Task];
                    
                }
                else
                {
                    //here I want break 
                    //so i return nil, but I found the continueWithBlock will still return a new task have a nil result, so the chain automaticall go to below 
                    return nil
                }
            }
        }
    }] continueWithBlock:^id(BFTask *task) {
    //but the code come here, even the before task is return nil 
        if (task.error || task.exception || task.isCancelled) {
            
            return nil;
        }
        else {
            if ([self checkX2OK])
            {
               
                return [_model getX3Task];
            }
            else
            {
                //here want break 
                //so i return nil
                return nil
            }
        }
    }] ;

I found why the nil case return default task in BFTask's method
continueWithExecutor:withBlock:

- (BFTask *)continueWithExecutor:(BFExecutor *)executor
                       withBlock:(BFContinuationBlock)block {
    BFTaskCompletionSource *tcs = [BFTaskCompletionSource taskCompletionSource];
    ............................
    //because the result is nil, so it's not a BFTask class
            if ([result isKindOfClass:[BFTask class]]) {
                [(BFTask *)result continueWithBlock:^id(BFTask *task) {
                    ...........
                                        return nil;
                }];
            } else {
                tcs.result = result; //so nil result will go here
            }
        }];
    };
    
    ......
    
    return tcs.task; 
    //so even a nil result will return a default task which is create by BFTaskCompletionSource
}

I have thought a way like this to add one stop:BOOL in the block

typedef id(^BFContinuationWithStopBlock)(BFTask *task, BOOL *stop);
so I can mark &stop = YES, when I want to stop the execution, what are your opinions？

[grantland]

The best way I would recommend doing this is to use a nested Task:

[task continueWithBlock:^id(BFTask *task) {
  bool continue = // should continue?

  if (continue) {
    return [task continueWithBlock:^id(BFTask *task) {
      // do other things
    }];
  }

  return nil;
}];