Interfaces > Classes - A refactor story.


Recently I was tasked with making a sizeable change to my companies product. There were many parts to the change, but I will summarise the more important one.

The Problem


We have a chunk of code, let’s say a class called UserEvents, this class (not a class, but it makes more sense if I describe it as one) does many things, and a lot of business logic, but it is self contained, by this I mean that is it not aware of other parts of the code.

Importantly, this class has a method called apply, a function that takes in events. It is used by the class to use these user events and actually apply them to the UI.


The EventCapture is tasked with synching events from UserEvents and the outside. It has to do two things:

  • Take events from UserEvents and save them remotely.
  • Take remote events (from other clients for example), and sync them with the local UserEvents.

It is important to note, that every method that the UserEvents class has, were extended onto this class. I usually dislike inheritance, but in this case it actually made some sense. (However, it wasn’t without cost, I’ll discuss it the problems faced below).


The UserEvents class extended the UserEvents object, and took that important apply function, and wrapped it so it was able to capture all the events that would go into that function, like so:

const { apply } = userEvents;
userEvents.apply = (event) => {

I don’t love this implementation, as I think it obfuscates what the apply function actually is, and you become less confident in calling it. But I cannot deny that it did work well, for a long time without anyone needing to touch it.

The New Requirements

The new requirements made it so I would now need various UserEvents objects, and this number would be determined by the user. This breaks the relationship that the EventCapture has with UserEvents because it now needs to account for multiple of them.

Not only does it need to track all the events from different UserEvents it needs to be able to forward remote events to the correct one locally.

The Refactor

At this point I decided the best way forward was to create a new class called Controller (terrible name), which would intercept all the UserEvents classes locally, and forward them to EventCapture, meaning that EventCapture does not need to know about who sent the event, it just needs to track it, as all the events were massaged by my Controller class.

Remote events would be sent from EventCapture down into the Controller class which would then figure out which UserEvents it needs to forward events to.

And so I did, the new architecture looked like this:

                                                 /-- UserEvent
Remote <----> EventCapture <----> Controller <- ---- UserEvent
                                                 \-- UserEvent

This worked great architectural wise (there were some bugs, but that’s for another story).

The Problem

If you notice, I changed the EventCapture class to deal with Controller instead of UserEvent. This is not great because Controller does nothing by itself, it is a bridge between two other classes, so now I have tightly coupled EventCapture and any sub modules to deal with Controller classes, which are very different from UserEvent classes.

I had to change every test for EventCapture and many dependent modules, as well as a lot of other code which assumed a certain structure.

The Obvious Solution

Use. An. Interface.

For some reason this completely skipped my brain when implementing this. All I had to do was create an interface that mimics UserEvent, in this case the interface would have been as easy as:

interface EventEmitter {
  apply: (event: Event) => void;

If the Controller class implements this interface, I can change the EventCapture to deal with EventEmitter interfaces instead of concrete classes, meaning I can test it in a more isolated way, and remove this tight coupling which I created.

The Conclusion

Interfaces rule. To keep your code nice and decoupled try and use common interfaces between modules/classes so you don’t end up having to refactor entire modules. This also improves the test ability of code massively.

Isolated modules, that do one thing very well. Next time I know to think very hard, before adding another coupling between modules again.

John Costa

Software Engineer