OH JUSTIN.m

RACCommand *example = [[RACCommand alloc] initWithSignalBlock:^(id input) { 
  return [RACSignal createWithBlock:^(id<RACSubscriber> subscriber){ 
    [subscriber sendCompleted]; 
    return nil; 
  }];
];

// Now, how will I know when each task has completed?
// I certainly cannot tell from `example.executionSignals.flatten`.

RACSignal *completions = [example.executionSignals flattenMap:^(RACSignal *transaction) {
  return [transaction then:^{
    [RACSignal return:RACUnit.defaultUnit]
  }];
}];

RACCommand *resetCommand = [[RACCommand alloc] initWithSignalBlock:^(id input) {
  return [RACSignal return:[RACUnit defaultUnit]];
}];

RACSignal *incrementing = [completions mapReplace:^(NSNumber *count) {
  return @(count.integerValue + 1);
}];

RACSignal *resetting = [resetCommand.executionSignals.flatten mapReplace:^(NSNumber *count) {
  return @0;
}];

RACSignal *transformers = [RACSignal merge:@[ incrementing, resetting ]];
RACSignal *runningCount = [transformers scanWithStart:@0 reduce:^(NSNumber *count, NSNumber *(^transformer)(NSNumber *)) {
  return transformer(count);
}];