# Chapter 10 comment - page 217

**URL:** <https://forums.kodeco.com/t/chapter-10-comment-page-217/90624>\
**Category:** RxSwift\
**Created:** [December 6, 2019, 9:15pm UTC](https://forums.kodeco.com/t/chapter-10-comment-page-217/90624 "2019-12-06T21:15:55Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![konic](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/konic/32/81494_2.png) [@konic](https://forums.kodeco.com/u/konic)\
**Post date:** [December 6, 2019, 9:15pm UTC](https://forums.kodeco.com/t/chapter-10-comment-page-217/90624/1 "2019-12-06T21:15:55Z")

</div>

The explanation of **reason** to use _concat(\_ : )_ operator in the following code:

> eoCategories  
> .concat(updatedCategories)  
> .bind(to: categories)

was a bit unclear for me (as a beginner).  
Sure, this works fine and as explained, but the following worked fine as well:

> updatedCategories  
> .bind(to: categories)

i.e. **without** concat()!?  
To make the explanation somewhat clear I replaced

> let downloadedEvents = EONET.events(forLast: 360)

with following (i.e. _poor connection_):

> ```
> let downloadedEvents = Observable<[EOEvent]>.create { observer in
> return Disposables.create()
> }
> 
> ```

In this case **updatedCategories** emits nothing and we will not see even categories.  
BTW, it is mentioned in the note on the page **194** that **combineLatest** emits something only in case when **both** sources emit at least one non-stop event.  
Like if in the above code to add

> observer.onNext()

categories will appear, although with count (0).

---

<div class="post-metadata">

**Author:** ![shogunkaramazov](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/shogunkaramazov/32/108826_2.png) [@shogunkaramazov](https://forums.kodeco.com/u/shogunkaramazov)\
**Post date:** [January 13, 2020, 5:16am UTC](https://forums.kodeco.com/t/chapter-10-comment-page-217/90624/2 "2020-01-13T05:16:06Z")

</div>

@fpillet Can you please help with this when you get a chance? Thank you - much appreciated! :]

---

<div class="post-metadata">

**Author:** ![konic](https://assets.chunter.kodeco.com/user_avatar/forums.kodeco.com/konic/32/81494_2.png) [@konic](https://forums.kodeco.com/u/konic)\
**Post date:** [January 13, 2020, 5:19pm UTC](https://forums.kodeco.com/t/chapter-10-comment-page-217/90624/3 "2020-01-13T17:19:15Z")

</div>

I believe, after the first piece of code there should be added some explanation why do we need to _concat_ **eoCategories** and **updatedCategories**.  
Maybe with reference to that page # **194** I mentioned at the end.

---

<div class="post-metadata">

**Author:** ![system](https://s3.amazonaws.com/cdn.raywenderlich.com/community/forum-assets/noun-game-ghost-878668.png) [@system](https://forums.kodeco.com/u/system)\
**Post date:** [May 21, 2020, 1:15pm UTC](https://forums.kodeco.com/t/chapter-10-comment-page-217/90624/4 "2020-05-21T13:15:56Z")

</div>

This topic was automatically closed after 166 days. New replies are no longer allowed.