DEV Community

Cover image for RxJS: Advanced challenge to build a reactive split-flap display

Posted on

RxJS: Advanced challenge to build a reactive split-flap display


In this article, I'm defining an exercise, challenging you to it, and giving my own answer to it.

I'll explain in details my thinking and little spoiler alert, we'll be using operators that are not used very often!

Time to shut down your imperative programming mind and turn on your reactive one. It'll be an interesting ride.


If you've been following for me while, you may know by now that I'm a huge RxJS fan. And if you're not yet following me, it's a really good time to chill a bit and click that follow button before we start melting our brains ๐Ÿง ๐Ÿ”ฅ!

So far on, I've written 3 articles that are either heavily using or completely dedicated to RxJS:

This one is by far the most challenging of them all.

Recently, we bought at work a Split flap display, also known as a "Solari board" or "Solari departure board".

Split-flap counter - Fully 3d printable - YouTube



The one we got is connected one where we can from an app set the text and it'll update IRL. It's funny to display all kind of important messages or statistics.

I'm sure you know how mesmerizing it can be when all the letters are spinning and suddenly, they all stop, one after another, to display the final sentence. Wouldn't it be a cool challenge to build our own with RxJS?

Rules of the challenge

Before we get started on this, lets define the expected result:

  • Reactive, reactive, reactive. No imperative programming. Everything should be managed through streams and not rely on external state. Of course it's fine for example to define the allowed letters in a global variable as a constant. But all the internal state of the app should be self contained in the streams
  • 20 letters in total on the display (easy to change anyway)
  • Can display all the letters from A to Z, can have spaces between letters/words, and numbers from 0 to 9
  • The page should output the new state of the board whenever one letter is updated
  • If the text changes at any point in time, whether there's one ongoing already or if it's currently stopped, it shouldn't start from scratch. Just like in reality, it should continue from where it is currently and roll the letters to the new position

Example of an expected output:

'Split-flap example'

Challenging you!

Yes you! I think it'd be a lot of fun if people reading this were to give it a go on their own and share how far they got.

In order to make this easier and not dwell on painful setup, I've created a Stackblitz template ready to go!

It contains some utility functions in utils.ts to manipulate the letters and the DOM, so that we remain focused on the streams.

If you decide to give this a go, fork the original Stackblitz or start from scratch if you prefer to. Feel free to modify anything from the template I've made. It's just here as a base to help, not here to hold you back.

Don't forget to save your Stackblitz and share it as a comment. Even if you didn't manage to go all the way. Let's discuss the results!

My solution

In programming, it's likely that there is more than one solution to achieve the same result. So keep in mind that this is my own approach and someone could come up with a different or even better solution.

From now on, be aware that if you want to work on your own solution first, the following will is one possible answer, so it's really a big spoiler โš ๏ธ.

Without further ado, let's jump straight in.

'A roller coaster'

"Here we gooooooooooooo".

The first thing I try doing when I have to build a feature that relies on observables, is picture in my head how that should all interact. Outter streams, inner streams. While it may be hard to represent visually, I visualise what I'm trying to build as a mix of marble diagrams and pipes that are all connected. I'll do my best to illustrate this but bear with me as this is no easy task.

'Streams representation'

This is as close as I can represent how things work in a visual way. Now let's dig into the code.

As a reminder, I'm not going to dive into the utils file I've created to manage letters and DOM manipulation. Feel free to take a look here for the whole implementation.

Now, if we focus on the stream itself and put aside the update of the DOM for now, here's the core of our code:

'Core of our RxJS code'

Before we dig into the specifics, let's contemplate for a second the beauty and the power of RxJS.

'Contemplating our code'

Keep in mind that the 14 lines of code above are capable of displaying a string just like a split-flap board, with animations (delay applied separately on each letter), keep the current state and if the text changes, start changing the letters from exactly where we are to the new position. It also manages correctly if we try to apply a text and change to a different one while the first one is still running, without resetting from scratch and picking up exactly where it is ๐Ÿ”ฅ. RxJS is truly a thing of beauty.

The first thing we use is our input. It's a simple Subject that we can next into whenever we want. This could be bound to an input text for example.

Then, we map that string to an input that'd match the board letters. Meaning, we want it to be an array of chars. One char for each entry of our display. For example, assuming the board supports 10 chars display for simplicity, if we pass 'HELLO' we'd get back ['H', 'E', 'L', 'L', 'O', '', '', '', '', ''].

Then ๐Ÿฅ... We use a switchScan, which in all honnestly is the first time I could find a use case for it ๐ŸŽ‰. As the name suggests, it's a mix of switch and scan:

  • switch means that we'll stop the inner stream if it's still running when the parent stream emits again
  • scan is like a reduce. Meaning it take an accumulator, a current value, and the value you return from it becomes the new accumulator. Except that with reduce, it'll wait for the stream to complete before emitting and with scan it'll emit all the intermediate results. Here, switchScan means that our inner stream will be subscribed to, it'll be able to make multiple emissions that will become the new accumulator, and if the parent streams emits again (our input$$ subject), we stop changing the letters and start changing again to reach the new string just provided. Here, this operator is doing a fantastic job, it's the key to being able to stop an animation in the middle of it and continue from where the board is currently without resetting

We then have a combineLatest. In the diagram I drew above, the combineLatest is here to assemble all the streams (1 per letter) that are representend as vertical green arrows.

The build the array of streams that is passed to the combineLatest, we loop on the current letters we just received and for each, we compute all the letters in between what we currently have and the target letter for this tile. We then emit each of these intermediate letters that we'll need to go through by using from.

Finally, with the from emitting all the intermediate letters, we use concatMap with a delay to make sure we emit the intermediate letters in the same order, with a delay in between each for simulate the animation.

Here's the final version live in Stackblitz:


This was a lot to take in, but was an interesting ride. Wasn't it?

Whenever I get the chance to write RxJS code like this, it really reminds me why I love coding. When I found out about reactive programming about 6 years ago, it really blew my mind and I wanted to play with it till I'd be confortable enough to express myself with it, just like most developers are confortable expressing themselves with imperative code that we tend to learn first. Being able to write this now makes me really happy and I try to share that passion for reactive programming whenever possible.

RxJS is a powerful tool and it takes time to learn. But once you manage to wrap your head around it, it's possible to handle really complex use case, in a relatively easy or at least readable way.

I hope you enjoyed this article, if you did let me know with a reaction and eventually drop a comment. It's always nice to hear back from people who took the time to read a post ๐Ÿ˜„! If you gave a go to the challenge yourself, share a link to your Stackblitz or let us know how far you went too!

If you're interested in more articles about Angular, RxJS, open source, self hosting, data privacy, feel free to hit the follow button for more. Thanks for reading!

Found a typo?

If you've found a typo, a sentence that could be improved or anything else that should be updated on this blog post, you can access it through a git repository and make a pull request. Instead of posting a comment, please go directly to and open a new pull request with your changes. If you're interested how I manage my posts through git and CI, read more here.

Follow me

ย  ย  ย  ย  ย  ย 
Dev Github Twitter Reddit Linkedin Stackoverflow

You may also enjoy reading

Top comments (5)

dariomannu profile image
Dario Mannu • Edited

Love this, and the switchScan operator. Had never come across it before! :)

I joined the challenge and posted a switchMap + async generator based one (were async generators allowed, btw?)


maxime1992 profile image

Love this, and the switchScan operator. Had never come across it before! :)

Thanks! Yeah I remember seeing it in the doc and wondering when the hell switchScan could be useful. When I thought of that challenge, somehow it clicked and instantly thought that it may be the one to go with. It's a pretty funny operator! :D

I joined the challenge


switchMap + async generator

That's a very good looking solution, nice one! (just read the code, I'll read the article now and reply there too).

Can you spot it?

I'm quite tired this evening and after reading it 3 times, I still can't think of any bug. Tell me :)

Congrats again for the challenge!

Sloan, the sloth mascot
Comment deleted
maxime1992 profile image

That's not the case? The concatMap is part of the switchScan, which will cancel the whole inner stream stream if there's a new emission upstream. Meaning as soon as the input changes, it cancels everything within the switchScan first, then start again.

I've forked my stackblitz and just slowed down the emission rate instead of 150ms I've put 1000ms for demo.

I've set the default value of the string to display to be "5" as it starts with numeric chars. Before it reaches 5, type for example 3. You'll see that it'll only go 1, 2, 3 and not go through the whole alphabet.

Thread Thread
dariomannu profile image
Dario Mannu

actually, you're right, I thought I did reproduce it earlier Today, but I can't anymore, so I must have done something wrong myself. There's no bug :)