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Managing State with Alerts @

Managing State with Alerts @

2023-05-17 04:45:53

After the previous post, I figured that one of the simplest ways to resolve on the route for Humble UI is to make an experiment.

And I did. I carried out a reactive/incremental computation engine (alerts) and wrote a easy TodoMVC in it. Following are my ideas on it.

The thought behind alerts could be very easy: you declare some mutable (!) information sources:

(s/defsignal *width

(s/defsignal *top

After which create a derived (computed) state that will depend on these:

(s/defsignal *space
  (println "Computing space of" @*width "x" @*top)
  (* @*width @*top))

Now, the primary time you dereference *space, it’s computed:

@*space => 144
;; Computing space of 16 x 9

After that, any subsequent learn is cached (discover the dearth of stdout):

If any of the sources change, it’s marked as soiled (however not instantly recomputed):

However in case you attempt to learn *space once more, it can recompute and cache its worth once more:

@*space => 180
;; Computing space of 20 x 9

@*space => 180
;; (no println)

You’ll be able to take a look at implementation here and a few utilization examples here. The implementation is a proof-of-concept, so perhaps don’t use it in manufacturing.

The attraction of alerts is that, when information adjustments, solely the mandatory minimal of computations occurs. That is, after all, cool, however not solely free — it comes at a value of some overhead for managing the dependencies.

I used to be notably serious about utilizing alerts for Humble UI as a result of they supply steady references. Let’s say you’ve got a tabbed interface that has a checkbox that allows a textual content discipline:

Now, our state may look considerably like this:

(s/defsignal *tab 

(s/defsignal *checked?

(s/defsignal *textual content

(s/defsignal *tab-content
    (ui/checkbox *checked?)
    (when @*checked?
      (ui/text-field *textual content))))

(s/defsignal *app
  (case @*tab
    :first  ...
    :second *tab-content
    :third  ...))

and the fantastic thing about it’s except the consumer switches a tab or performs with a checkbox, *tab-content can be cached and NOT recomputed as a result of its dependencies don’t change!

And that signifies that irrespective of what number of instances we dereference *tab-content e.g. for rendering or format, it can all the time return precisely the identical occasion of the checkbox and textual content discipline. As in, the identical object. Identical DOM node, if we have been within the browser.

Cool? Cool! No diffing wanted. No state monitoring and positional memoization both. We are able to put all inside state into objects as fields and never invent any particular “extra persistent” storage resolution in any respect!

This was my fundamental motivation to look into incremental computations. I don’t actually care about optimum efficiency, as a result of—how a lot is there to compute in UI anyhow?

And likewise—it’s not apparent to me that in case you make each + and concat incremental it’ll be a internet win due to overhead. However steady objects in a dynamic sufficient UI? Lack of diffing and VDOM? That is one thing I can use.

One of many examples the place VDOM mannequin doesn’t shine is props drilling. Think about an app like this:

  {:font-ui ...}
            (ui/label "Whats up")))))))

The precise particulars don’t matter, however the level is: there’s a default theme on the very high of your app and a label someplace deep down.

Should you cross font-ui as an argument to each part, it can create a false dependency for each intermediate container that it passes by. When the time for the replace comes, the entire UI can be re-created:

In an ideal world, although, font-ui change ought to solely have an effect on parts that truly use that font. E.g. it shouldn’t have an effect on paddings, backgrounds, or scrolls, however ought to have an effect on labels and paragraphs.

Nicely, incremental computation solves this drawback fantastically! Should you make your default font a sign, then solely parts that truly learn it will subscribe to its adjustments:

However, how typically do you alter fonts in your complete app? Ought to it actually be optimized? The query of whether or not this use case is necessary stays open.

Now let’s dig into implementation particulars slightly bit. What now we have to date is, I consider, known as reactive, however not incremental. To be known as incremental we should in some way reuse, not simply re-run, earlier computations.

A easy instance. Think about now we have a listing of todos and a perform to render them. Then we are able to outline our UI like this:

(s/defsignal *todos

(s/defsignal *column
    (map render-todo @*todos)))

This could work advantageous the primary time, but when we add a brand new to-do, the entire checklist can be re-rendered. *todos adjustments, *column physique will get re-executed, render-todo is utilized to each todo once more by map.

To unravel simply this drawback, we may introduce incremental s/map that solely re-computes the mappings that weren’t computed earlier than:

(def column
    (s/map render-todo *todos)))

Underneath the hood, s/map caches earlier computation and its outcome and, when re-evaluated, tries to reuse it as a lot as potential. Which means, if it already noticed the identical todo earlier than, it can return a cached model of (render-todo todo) as an alternative of calculating it anew.

Two necessary issues to notice right here. First, if we care about object identities, we have to make use of incremental map. In any other case including new todo to the top of the checklist will reset the interior part state of each different one. Not good!

Second, though “incremental map” sounds fancy and sensible, below the hood it does the identical factor that React does: diffing. It diffs new assortment towards earlier assortment and tries to seek out matches.

It’s (most likely) a perf win total, however, extra importantly, diff nonetheless does occur. That’s the rationale why all incremental frameworks have their very own variations of for/map:

{#every arr as el}

I can think about it could possibly be higher when a diff occurs on the info layer as an alternative of on the ultimate UI layer as a result of generated UI is often a lot bigger than the supply information. Both means, at the least you may select the place it occurs.

The dangerous information is, it’s important to assume about it, whereas in React mannequin you often don’t trouble with such minute particulars in any respect.

One can think about that we’ll want incremental variations of filter, concat, cut back and many others, and our customers will have to study them and use them in the event that they need to preserve steady identities. And we’ll have to supply sufficient incremental variations of base core capabilities to maintain everybody completely satisfied, and doubtlessly educate them to write down their very own. Sounds harsh.

One necessary function we’re lacking in our incremental framework is results.

We implement a combined push/pull mannequin: recalculating values is lazy (not carried out till explicitly requested), however marking as soiled is raring (fast dependencies are marked as :soiled and their transitive deps are marked with :verify, which suggests may or may not be soiled):

(s/defsignal *a

(s/defsignal *b
  (+ 10 @*a))

(s/defsignal *c
  (+ 100 @*b))

@*a ; => 1
@*b ; => 11
@*c ; => 111

(:state *b) ; => :clear
(:worth *b) ; => 11

(s/reset! *a 2)

(:state *b) ; => :soiled
(:worth *b) ; => 11
(:state *c) ; => :verify
(:worth *c) ; => 111

@*b ; => 12

(:state *c) ; => :soiled
(:worth *c) ; => 111

@*c ; => 112

Or for us visible thinkers:

For particulars, see Reactively algorithm description.

An impact is a sign that watches when it will get marked :verify (one thing down the deps tree has modified) and forces its dependencies to see if any of them are literally :soiled. If any of them are, it evaluates its physique:

(s/defsignal *a

(s/defsignal *b
  (mod @*a 3))

(s/impact [*b]
  (println @*a "mod 3 =" @*b))

(s/reset! *a 2) ; => "2 mod 3 = 2"
(s/reset! *a 3) ; => "3 mod 3 = 0"
(s/reset! *a 6) ; => (no stdout: *b didn’t change)

That is precisely what we have to schedule re-renders. We put an impact as a downstream dependency on each sign that was learn over the past draw. Which means we’ll create an specific dependency for the whole lot that affected the ultimate image a technique or one other.

Then, when any of the sources change and the redraw impact is definitely a downstream dependency on it, we’ll set off a brand new redraw.

What I did have issues with is useful resource administration. First, let’s contemplate one thing like this:

(s/defsignal *object

(def label
  (s/sign (str "Whats up, " @*object "!")))

Now think about we lose a reference to the label. Irresponsible, I do know, however issues occur, particularly in end-user code. The best instance: we’re in REPL and we re-evaluate (def label ...) once more. What is going to occur?

Nicely, because of the nature of alerts, they really preserve references to each upstream (for re-calculation) and downstream (for invalidation) dependencies. Which means, the earlier model of the sign will nonetheless be referenced from *object together with the brand new one:

We are able to introduce dispose technique that could possibly be known as to unregister itself from upstream, however no person can assure that customers will name that in time. It’s really easy to unintentionally lose a reference in a garbage-collected language!

And that is what I’m combating. The sign community has to be dynamic. Which means, new dependencies will come and go. However de-registering one thing doesn’t actually really feel pure in Clojure and even Java code, and there’s no approach to implement that each useful resource that’s not wanted can be correctly disposed of.

A standard resolution is to make downstream references weak. Which means, if we misplaced all references to the dependant sign (label on the image beneath), it can ultimately be rubbish collected.

What I don’t like about that resolution (that we use anyhow within the prototype) is that till GC is named, these pointless dependencies nonetheless grasp round and take sources e.g. throughout downstream invalidation.

One thought is to eliminate alerts explicitly when their part unmounts. It really works for some alerts, however not basically. Contemplate this:

(s/defsignal *textual content
  "Whats up")

(ui/label *textual content)

*textual content sign is created outdoors of the label and shouldn’t be disposed of by it. On the similar time,

  (s/sign (str @*textual content ", world!")))

On this case, the sign is created particularly for the label, thus ought to be disposed of similtaneously the label. However specific that?

Understand that we most likely need each use instances on the similar time:

  (ui/header *textual content)
    (s/sign (str @*textual content ", world!")))
  (ui/label *textual content))

Ultimately, unused alerts can be cleaned up by GC, so we are able to depend on that. I’m simply unsure what kinds of issues it’d trigger in apply.

Identical drawback I’ve with alerts I even have with parts. As a result of all parts are values, nothing stops me from saving them in a var, utilizing them a number of instances, and many others. Contemplate this UI:

(s/defsignal *cond

(def the-label
  (ui/label "Whats up"))

(def *ui
    (if @*cond
      (ui/label "Not whats up"))))

If we toggle *cond on and off, the-label will seem and disappear from our UI, calling on-mount and on-unmount a number of instances. So if we do some useful resource cleanup in on-unmount, we must always in some way restore it in on-mount? Feels unusual, however why not?

(core/deftype+ Label [*paint *text ^:mut *line]
  (-on-mount-impl [_]
    (set! *line
          (str (s/maybe-read *textual content))
  (-on-unmount-impl [_]
    (s/dispose! *line)
    (set! *line nil)))

This manner, if a part wants some heavy sources for rendering (textures, pre-rendered strains, or different native sources) it could actually clear it up and restore solely when it’s truly on the display screen. Or depend on GC as soon as once more (not advisable).

One other factor that I used to take without any consideration for the reason that world switched to React: lifecycle callbacks. Plenty of stuff comes down to those callbacks. Enabling/disabling alerts. Liberating costly sources held by parts. Customers’ use instances, like setting a timer or making a fetch request. It’s so handy to have the ability to tie some costly useful resource’s lifetime to the lifetime of a part. We actually need these!

How does React do it? Nicely, it takes mount/unmount API away from you and takes management over it, so it could actually assure to name you again on the proper time.

The answer I got here up with could be very easy: the part is mounted if it was drawn throughout render, and never mounted in any other case. On the very high stage, I’m retaining observe of the whole lot that was rendered final body and what’s rendered this body. For brand spanking new stuff, -on-mount is named, for stuff that’s not seen, -on-unmount. The gotcha right here is, as I stated above, that some parts may “come again” after being unmounted. I suppose it’s okay?

Working with an incremental framework breaks each crucial and practical instinct. It’s an entire different factor. I made numerous errors and had to consider stuff I often don’t have to consider. Listed below are a couple of gotchas:

Dependency too huge

Think about we need to render a TODO from quite simple EDN information:

We’d write one thing like this:

(defn render-todo [*todo]
  (let [*text (s/signal
                (str (:id @*todo)))]
    (ui/label *textual content)))

This render perform returns a label object that has a sign as its textual content. To date so good.

The issue is, we over-depend right here: we solely use :id from *todo however we rely on your complete factor. A greater resolution could be:

(defn render-todo [*todo]
  (let [*id   (s/signal (:id @*todo))
        *text (s/signal (str *id))]
    (ui/label *textual content)))

which appears a bit too tedious to write down. It most likely doesn’t matter all that a lot on this explicit case, however what if computations are dearer?

My level is: it’s too simple to make this error.

Ambrose Bonnaire-Sergeant has identified that Reagent and CljFX resolve this by offering an specific API:

Dependency on the improper time

Think about you’ve got a UI like this:

You’ve got a sign that is attached to your textual content discipline and a button that converts it right into a label:

(s/defsignal *textual content
  "Your identify")

(s/defsignal *checklist

(def app
    (s/mapv ui/label *checklist)

    (ui/text-field {:placeholder "Kind right here"}
      *textual content)

      #(s/swap! *checklist conj *textual content)
      (ui/label "Add"))))

Do you see it? We truly retailer the unique sign in *checklist as an alternative of creating a replica. This manner, once we edit textual content, each ingredient in our checklist will even be edited!

We’d repair it like so:

#(s/swap! *checklist conj (s/sign @*textual content))

however it’s no good both.

Sure, we create a brand new sign, however it will depend on the outdated one 🙂 That is an API drawback, and I feel perhaps I ought to have separate capabilities for supply alerts and derived alerts. Proper now the correct approach to write it could be:

#(let [text @*text]
   (s/swap! *checklist conj (s/sign textual content)))

which is nearly equivalent! however the outcome could be very completely different.

It jogs my memory so much about Clojure laziness puzzles, which is each okay (all of us realized to take care of them) and never a lot (one of the simplest ways to take care of laziness is to not use it).

Recomputing an excessive amount of

There’s one other gotcha within the earlier instance. column takes a group or a sign that accommodates a group, so now we have to fulfill that:

      (mapv ui/label @*checklist)
      [(ui/text-field ...)
       (ui/button ...)]))))

However now our s/sign will re-create a text-field and a button every time *checklist adjustments. The answer is likely to be:

(let [text-field (ui/text-field ...)
      button     (ui/button ...)]
        (mapv ui/label @*checklist)

which, once more, form of breaks referential transparency. Relying on the place we allocate our parts, an app behaves in another way. Doesn’t matter for the button, because it doesn’t have an inside state, however does matter for the textual content discipline.

Alternatively, we’d introduce a model of concat that accepts each alerts wrapping sequences in addition to sequence values. Then argument analysis will lock the text-field worth for us:

      (s/mapv ui/label *checklist)
      [(ui/text-field ...)
       (ui/button ...)])))


It was not all the time clear to me which components of the state ought to be alerts and which ought to be values. Proper now, for instance, a listing of todos is sign containing alerts that time to todos:

(defn random-todo []
  {:id       (rand-int 1000)
   :checked? (rand-nth [true false])})

(s/defsignal *todos
  [(s/signal (random-todo))
   (s/signal (random-todo))
   (s/signal (random-todo))

This manner checklist of todos could possibly be decoupled from the todos themselves. Once we add new todo, we have to change the checklist and generate a brand new part. However when a person todo is e.g. toggled, it’s dealt with solely inside and shouldn’t have an effect on the checklist.

I suppose this resolution is okay, though double-nested mutable constructions do give me pause.

Might the identical be carried out “single atom”-style? In all probability, with some form of keyed map operator and lenses?

(s/defsignal *todos

(def *todo-0
    {:learn  (nth @*todos 0)
     :write #(s/replace *todos assoc 0 %)}))

The identical ambiguity drawback occurs right here:

See Also

(s/defsignal *textual content
  "Whats up")
(ui/label *textual content)


  (ui/label @*textual content))

Ought to I take advantage of a label that accommodates a sign or a sign that accommodates a label? Each are viable.

This isn’t essentially an issue, simply an statement. I suppose I desire Python’s “There ought to be one—and ideally just one—apparent approach to do it” to Perl’s “There’s multiple approach to do it”.

Repeating computations

I’ve a couple of constants outlined in my app, together with *scale (UI scale, e.g. 2.0 on Retina) and *padding (in logical pixels, e.g. 10).

However precise rendering requires display screen pixels, not UI pixels. For that, I used to be utilizing the derived sign calculated contained in the padding constructor:

(defn padding [*amount]
    {:quantity (s/sign (* @*scale @*quantity))}))

The issue? I used to be utilizing default *padding in every single place:

(padding *padding ...)
(padding *padding ...)
(padding *padding ...)

This manner I ended up with dozens of equal alerts (completely different identities, similar worth, dependencies, and performance) that multiply the identical numbers to get the identical outcome.

Is it dangerous? On this case, most likely not. It simply doesn’t really feel as clear, contemplating that the remainder of the app makes use of absolutely the required minimal of computations and the dependency graph is rigorously constructed.

However I don’t see a approach to merge equivalent alerts collectively, both. I suppose we’ll should stay with this imperfection.

I began this experiment impressed by Svelte, Strong, and Electrical Clojure. All of them have compilation steps that I wished to keep away from.

Probably the most non-obvious outcome I get from that is that it seems such as you want pre-compilation for higher ergonomics and useful resource administration. Each of those issues go away if we don’t let customers work together with our incremental engine instantly, however as an alternative, do it for them.

We are able to substitute calls to if/map/concat with their incremental variations transparently, observe dependencies reliably, and add dispose calls the place wanted—principally, all these items you may’t belief people to get proper.

I’m additionally getting reviews that Reagent (that has an identical factor, r/observe) is difficult to make use of appropriately at scale. Can anybody verify?

Perhaps it’s price working one other experiment to see if I can get pre-compilation working and the way a lot it helps.

Some preliminary outcomes from the experiment:

It really works

After some massaging, I used to be capable of construct incremental TodoMVC that retains the state of its parts that don’t instantly change.

Right here’s a video:

The magenta define signifies that the part was simply created and is rendered for the primary time.

Discover how once I add new TODO solely its row is highlighted. That’s as a result of the remaining reuses the identical parts that have been created earlier than.

If you swap between tabs, it causes a few of the rows to be filtered out. If you return to “All”, solely those that weren’t seen are recreated.

Additionally, discover the identical impact on tabs: while you swap e.g. from “All” to “Lively”, “All” turns into a button however “Lively” turns into only a label, so that they each should be recreated. However “Accomplished” stays a button, so it doesn’t get recreated.

And the very last thing: once I toggle TODOs, nothing will get highlighted. It is because I made labels settle for alerts as textual content:

(s/defsignal *textual content
  "Whats up")

(ui/label *textual content)

So the label may keep the identical whereas the textual content it shows adjustments. Not vital, however feels neat, truly. One other approach to do it could’ve been:

  (ui/label @*textual content))

Then it could be highlighted on the toggle:

It feels very satisfying

…understanding no computation is wasted on diffs and solely the mandatory minimal of UI is recreated.

Props drilling works

I made UI scale, padding, and button fill coloration alerts and once I change them vital components of UI are up to date:

This feels very satisfying, too: understanding that you simply made the dependency very specific and really exact, not the hacky “let’s simply reset the whole lot simply in case” means. And it requires no particular setup, it “simply works”.

No VDOM wanted

I don’t should implement VDOM and diffing! And I don’t want each heavy- and light-weight variations of every part. I don’t want to trace the state individually from the parts. That’s an enormous burden off my shoulders.

We want incremental algorithms

I do want to supply a set of incremental algorithms. Incremental map, incremental filter, concat and many others. for macro, too.

Ideally, we wish customers to have the ability to write their very own.

It breaks instinct

Working with incremental computations could possibly be difficult. Making a mistake is straightforward, and double-checking your self is difficult, so it’s onerous to know if you’re doing the fitting factor.

However it appears that evidently the stakes will not be that prime: the worst that would occur is you re-create an excessive amount of and your efficiency suffers. I’d say it’s a ~related deal you get with React.

Is there a deeper purpose?

There’s most likely an excellent purpose React received and FRP/incremental stay marginal applied sciences which were tried dozens of instances. I perceive the attraction, however I additionally see the way it’s not everyone’s cup of tea.

OTOH, Reagent appears to be doing advantageous in Clojure land, though many individuals desire to pair it with re-frame.

Supply code

In case you are curious, the code is on Github. The run script is at scripts/

Let me know what you assume! And I’m going to strive VDOM method subsequent. After which I suppose I’ll should make a decision matrix.


I’m Nikita. Right here I write about programming and UI design Subscribe

I additionally create open-source stuff: Fira Code, AnyBar, DataScript and Rum. Should you like what I do and need to get early entry to my articles (together with different advantages), you must support me on Patreon.

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