RxSwift MVVM API Manual 📃

Ok, we have all these tools, we’re aware of all the dangers. Let’s think now how to use it best to create a clear and safe API. Let’s use MVVM as the context.

There are many approaches to writing an RxSwift API. My approach is: I use RxSwift as a very cool observer pattern, for managing async tasks and make use of it’s awesome operators. I do not strive to go 100% clean, declarative RxSwfit. I’ve tried that and lost many valuable hours trying to connect the declarative and imperative world.

Here are some tips I’ve learned over the time I’m using RxSwift.

Be Consistent

It’s the most important thing in any API! If you choose one approach: stick with it!

Having that said, let’s start with the basic: input vs output.

class FilterViewModel {
 let filterButtonsEvents : BehaviorSubject<Int> 
 let filterSelectionEvent : PublishSubject<Int>
 let currentFilter : Observable<Int>
 let shouldShowFilter : Observable<Bool>

There are a couple of combinations of how you can declare input/output but this is my favorite one.


By having input as Subjects you can easily make use of many useful RxSwift operators. I think I use throttle, the most often.

.throttle(0.5, scheduler: MainScheduler.instance)
.subscribe(onNext: { [weak self] (counter) in

You can also accept input as Observable in the constructor.


Remember to have ouput declared always as an Observable. Even if it’s a Subject. Otherwise any external class could use it as input and we don’t want that - it breaks encapsulation. The way I do it is just force cast when I want to emit a new element. Please let me know if you know a better way, because this is not pretty 😱

 (observable as! PublishSubject)


Remember the lecture about RxSwift Safety? It’s easy to remember all the rules in a simple example. It’s much harder in a complex app with hundreds of observers. Observable type is a very broad type and doesn’t say if the stream is hot, or cold, or if it runs on the main queue.

That’s one of the reasons Driver was created! You’re encouraged to create your own units and make the API more explicit.

Driver is your hot, main scheduler unit. We’re missing a cold one, right? It’s quite easy, to create! I just called it Template, because a cold observable is a template really, and you can use it and run it using subscribe.

class Template<Element> {
    let observable : Observable<Element>
    init(_ subscribe: @escaping (AnyObserver<Element>) -> Disposable) {
        observable = Observable.create(subscribe)

So a Template is guaranteed to be cold and run on main schedule!

class MyViewModel {
 //Cold ❄️
 let createRequest : Template<Int>
 //Hot 🌶
 let shouldShowelement : Driver<Bool>

Now your API is clear and explicit!

MVVM State Machine

Just a little bonus. Something that has been on my mind lately. Complex view controllers very often get very messy when it comes to managing state. Consider using a state machine in your view model to tell your view controller what it should do. Here’s a great article that could get you started.

• It could profoundly reduce the number of Observables in your MVVM API, merging it into one enum state, instead of many granular states.

• It’ll also force you to move move of your logic to your view model

• It’ll make your code more declarative!


Michal Ciurus

A passionate iOS dev always trying to get to the bottom of stuff.