This is a post in a series on Advanced Coordinators. If you haven’t read the original post or the longer follow-up, make sure to check those out first. The series will cover a few advanced coordinator techniques, gotchas, FAQs, and other trivia.

When splitting up the responsibilities of a view controller, I do a curious thing. While I leave reading data (for example, a GET request, or reading from a database or cache) in the view controller, I move writing data (such as POST requests, or writing to a database) up to the coordinator. In this post, I’ll explore why I separate these two tasks.

Coordinators are primarily in charge of one thing: flow. Why sully a beautiful single responsibility object with a second responsibility?

I make this distinction because I think flow is the wrong way to think about this object’s responsibility. The correct responsibility is “handle the user’s action”. The reason to draw this distinction is so that the knowledge of when to “do a thing” (mutate the model) and when to “initiate a flow step” can be removed from the view controller. I don’t want a view controller to know what happens when it passes the user’s action up to a coordinator.

You can imagine a change to your app’s requirements that would make this distinction clear. For example, let’s say you have an app with an authentication flow. The old way the app worked was that the user typed their username and password into one screen, and then the signup request could be fired. Now, the product team wants the user to be able to fill out the profile on the next screen, before firing off the signup request. If you keep model mutation in the view controller and the flow code in the coordinator, you’ll have to make a change to both the view controller and the coordinator to make this work.

It gets even worse if you’re A/B testing this change, or slowly rolling it out. The view controller would need an additional component to tell it how to behave (not just how to present its data), which means either a delegate method back up to the coordinator or another object entirely, which would help it decide if it should call inform the coordinator to present the next screen or if it should just post the signup call itself.

If you keep model mutation and flow actions together, the view controller doesn’t have to change at all. The view controller gets to mostly act like it’s in the view layer, and the coordinator, with its fullness of knowledge, gets to make the decision about how to proceed.

Another example: imagine your app has a modal form for posting a message. If the “Close” button is tapped, it should dismiss the modal and delete the draft from the database (which, let’s say, is saved for crash protection). If your designer decides that they want an alert view that asks “Are you sure?” before deleting the draft, your flow and your database mutation are again intertwined. Showing the dialog is presenting a view controller, which is a flow change, and deleting an item from the database is a model mutation. Keeping these responsibilities in the same place will ease your pain when you have to make changes to your app.

One additional, slightly related note: the coordinator’s mutative effect on the model should happen via a collaborator. In other words, your coordinator shouldn’t touch URLSession directly, nor any database handle, like an NSManagedObjectContext. If you like thinking about view models, you might consider a separation between read-only view models (which you could call a Presenter) and write-only view models (which you could call an Interactor or a Gateway). Read-only view models can go down into the view controller, and write-only view models stay at the coordinator level.

The line between model mutation and flow step is thinner than you’d expect. By treating those two responsibilities as one (responding to user action), you can make your app easier to change.