Publish and Subscribe — Decoupling Deep View Hierarchies from Event Handlers

Imagine a complex view with many sub components. This is more common in Mac apps where a window contains multiple panes, lists, graphs, whatever. How do you react to interactions 5 levels deep? Let’s say you avoid massive view controllers which do everything on their own and want to encapsulate event handling outside the view hierarchy – what should you do?

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Events as Declarative State

Teaser image

I think I found something better than closures to handle events. Instead of dispatching closures that are themselves the changes that should be performed, simply declare the change.

enum Action {
    case ResizeBanana(BananaId, Size)
}

Now you need some kind of action handler, which Benjamin Encz calls “reducer” in his presentation “Unidirectional Data Flow in Swift”. Think of a list of reducers as a Chain of Responsibility only without the final consummation of the action. It is passed throughout the whole responder chain. Why reducer? Because they operate similar to the reduce function. The end result will be the combined result of all reducer’s mutations.

This is cool, because no component needs to know what such an action entails – none except the appropriate reducers.

If closures were great because where you type them you can see what is going on (more like “how” in my opinion), a simple action enum is even simpler, telling what should happen.

Works well with CQRS, I imagine. Will have to write a demo app using CQRS sometime to validate.

The current project is called ReSwift and code is available on GitHub.