The more you fight Cocoa, which I believe this stuff does, the bigger the mess you'll create for yourself once you view hierarchy becomes large and filled with custom views that don't really fit what 99% of these tutorials cover.
Furthermore, trying emulate "dom" diffing by replacing views, instead of updating them, as they seem to be illustrating towards the end of the article (could be reading this wrong though) is pretty inefficient way of doing things in Cocoa.
1. Apps are more than views.
2. ReactiveCocoa and RxSwift are just asynchronous data over time, represented as Just Another Data Structure (cough monad). Even if you just use them instead of callback blocks, it's an improvement because they can be composed, stored in data structures, and used in ways that methods taking a callback function and returning Void cannot. Cancellation is then implicit on deallocation, instead of needing to reference a "cancellation token" or other icky state like that.
3. Complex UIs are exactly where you want this type of system, because it provides type safety and compiler verification, instead of hoping that your target/selectors and KVO work and never break when you edit something and the compiler doesn't complain.
I think that a popular JavaScript UI library being named "React" has damaged the perception of FRP-like systems. Generic FRP-like systems are not related to UI, other than that they can be used for it. People are confusing "DOM diffing" for having anything to do with signals/streams.
In UIs, FRP-like systems allow you to take some evil imperative state (a slider was moved!), lift it into a happy pure-functional world, process the inputs, and drop out of evil imperative state at the end ("update the color"). For example, an RGB slider (using a bit of shorthand in places):
combineLatest(redSlider.value, greenSlider.value, blueSlider.value)
.throttle(0.5, onScheduler: UIScheduler())
.map({ r, g, b in UIColor(r, g, b) })
.startWithNext({ color in colorView.color = color })
combineLatest(redSlider.value, greenSlider.value, blueSlider.value)
.sampleOn(button.tapped)
.map({ r, g, b in UIColor(r, g, b) })
.startWithNext({ color in colorView.color = color })
However, now that I do understand it, I agree with parent. I don't want to use this for complex UIs. Point by point:
> Cancellation is then implicit on deallocation
In practice, you're going to have retain cycles, no? I mean I don't know where these four lines "live", but if we write them in a closure and ship them off to a sync/diff/runloop engine, unless we are quite careful, that sync engine is going to hold a great many strong references. Unless you intend this to be an IBAction definition, in which case...
> because it provides type safety and compiler verification, instead of hoping that your target/selectors and KVO work and never break when you edit something and the compiler doesn't complain.
As long as the underlying Cocoa uses target-action, true compiler verification is impossible. Compiler verification checks something (these four lines) but it doesn't check that these lines actually run when the slider is moved in any way.
> FRP-like systems allow you to take some evil imperative state (a slider was moved!), lift it into a happy pure-functional world,
It's not immediately clear to me how e.g. throttle is implemented, but it must accumulate state inside it somehow in order to replay the event after the timer.
> Complex UIs are exactly where you want this type of system, because it provides type safety and compiler verification, instead of hoping that your target/selectors and KVO work and never break when you edit something and the compiler doesn't complain.
Compiler verification is good; tests are better. And I do not understand how you would even begin to write unit tests for this.
Now we get to:
1. Stopping in the debugger and trying to reason about these signal chains is complicated, because what we have here is a datastructure in memory, not lines of code I can step through
2. This example does not account for threading, and in any nontrivial example you want to move between background and foreground a few times. It also does not deal with "splitting/merging" (multiple observers, etc.) and I suspect the intersection of those two features is a sharp edge.
3. Finally, let's compare against a slightly more traditional syntax:
@IBAction func valueChanged() {
dispatch_throttle(0.5, onQueue: dispatch_get_main_queue()) {
colorView.color = colorToValue(red: sliderA.value, green: sliderB.value, blue: sliderC.value)
}
}
To evolve from your first example to your second example we would just change
slider.addTarget(self, action: "valueChanged", forControlEvents: .ValueChanged)
button.addTarget(self, action: "valueChanged", forControlEvents: .TouchUpInside)
It's usually "we take a model, apply a transformation, creating a new model, and display it - no side effects, we're heroes, now everyone agree to never use imperative languages ever again!" but then forget that they've now created a copy of their entire system, which may cause havoc with anything from memory budgets to publish-subscribe setups or asynchronous update/display loops.
At least by not sidestepping this thorny part of the problem the authors are showing a sensible solution which doesn't try to pretend a UI composed from NSViews is a stateless system. Instead they show a reasonable way to maintain its state through diffing it against an idealised model, localising the changes (additions, deletions, updates) to the minimum necessary.
With persistent data structures you don't have a whole copy of the entire system. The two views of your system share memory where information didn't change. Sort of like how Git works.
Sorry for the marketing shots.
This is an excellent synthesis and transfer of related technology. Nice work! Some of the Swift- and NS-isms are tricky to follow without knowing Mac dev, and I'm still wrapping my head around how it all wires together, but especially the update loop is very clear and small and I think a unique use of Rx to blend into React/virtual-dom/Elm ideas.
My perspective is I am trying to bring the Elm Architecture to F# but started out porting virtual-dom.js on the bottom. It's early and ugly and just at the point of having to deal with patching children nodes, so parts of this are quite relevant. One avenue I intend to check out is performing set operations on a flat node list and using something like protocol methods on parent views to add and remove children, so that part of the code is definitely going to trigger some thoughts.
The dictionary of observables indexed by node threw me at first. AFAICT the virtual views, action types, and updates all compose, and then once it's time to do anything with real views (the patch operation in diff/patch terms) just that part is handled independently per node? (EDIT: Ah, I see the context dispatch is composed as in Elm, and only pushing down model updates is separate.) It's dead easy compared to the work virtual-dom.js has to do to index patches, index DOM nodes, and walk the DOM applying updates. This seems to handle all that with much less code, with a couple caveats:
1) The ability to reorder children is missing, right?
2) Any plan to support laziness to curb rerendering sections of the virtual tree? It's probably not needed for many apps, but for some reason the feature gets air time in discussions around React and Elm. (Looks like all observables get called unconditionally once per update, too?)
There's not much to say on the F# front yet except that the goal is to get a taste of functional rendering from an app state atom for Windows GUIs and F# and the Elm Architecture seem like the most approachable way to get there for me. So far the work consists of porting the node, thunk, and widget designs from virtual-dom.js to F# types and functions and using .NET reflection to create and patch live views. Work continues on the patch function, but seeing your code is definitely creating an itch to move on to the architecture.
On the other hand, this project, though interesting, isn't something that the process of learning Swift and Cocoa would really benefit from. If it seems kind of interesting, starting with Elm as a separate project is probably a better way to get context for evaluating using the Elm architecture in Swift.