Replicant - A Clojure(Script) DOM rendering library

Replicant turns hiccup into DOM. Over and over. Efficiently, without a single dependency.

(require '[replicant.dom :as d])

(def el (js/document.getElementById "app"))

;; Render to the DOM - creates all elements
(d/render el
  [:ul.cards
    [:li {:replicant/key 1} "Item #1"]
    [:li {:replicant/key 2} "Item #2"]
    [:li {:replicant/key 3} "Item #3"]
    [:li {:replicant/key 4} "Item #4"]])

;; This render call will only result in one DOM node being moved.
(d/render el
  [:ul.cards
    [:li {:replicant/key 1} "Item #1"]
    [:li {:replicant/key 3} "Item #3"]
    [:li {:replicant/key 2} "Item #2"]
    [:li {:replicant/key 4} "Item #4"]])

Status

Replicant is starting to shape up, but is still under development. Details are subject to change. The current focus is on finalizing APIs and hardening by porting some large UIs to it.

Features

Efficient hiccup to DOM renders and re-renders
Represent entire UIs with serializable data
Rich life-cycle hooks (mount, unmount, update attributes, move, etc)
Data-driven hooks and DOM event handlers
Stateless and component-free
Style/class/attribute overrides during mounting and unmounting for easy transitions
Small API surface: Two functions and a few keywords
Inline styles with Clojure maps
Class lists with Clojure collections
innerHTML support
No dependencies

Performance

Replicant performance is being tuned using https://github.com/krausest/js-framework-benchmark. See benchmarking instructions for how to run locally.

Data-driven hooks and events

As originally introduced in dumdom, and described in my talk about data-driven UIs, Replicant allows you to register a global function for handling events and life-cycles. This way your hiccup can be free of opaque functions, setting you up for good rendering performance and fully serializable UI data.

[:h1 {:on {:click [:whatever]}}
  "Click me"]

When Replicant encounters this hiccup, it will register a click handler that will pass the data ([:whatever]) to a global dispatcher. You register this by calling replicant.dom/set-dispatch!:

(require '[replicant.dom :as replicant])

(replicant/set-dispatch!
  (fn [replicant-data handler-data]
    (prn "Click!")))

(replicant/render
  (js/document.getElementById "app")
  [:h1 {:on {:click [:whatever]}} "Click me"])

In the above example, the first argument replicant-data is a map with information about the event from Replicant. Specifically, it contains the keys:

:replicant/trigger, which will have the value :replicant.trigger/dom-event,
:replicant/js-event, which will contain a reference to the DOM event object, and
:replicant/node, which will contain a reference to the DOM node the event occurred in.

The second argument handler-data is data from the hiccup element, e.g. [:whatever]. The same global dispatch will be called for all events that are expressed as data, and you would typically use the data to decide what to do.

You can of course also register a function event handler if you want. It will be called with just the DOM event object.

Life-cycle hooks

Replicant life-cycle hooks can also be expressed with data:

(require '[replicant.dom :as replicant])

(replicant/set-dispatch!
  (fn [replicant-data hook-data]
    (prn "DOM changed")))

(replicant/render
  (js/document.getElementById "app")
  [:h1 {:replicant/on-render ["Update data"]} "Hi!"])

replicant-data is the same map from before. For lifecycle hooks, the map contains:

:replicant/trigger, which will have the value :replicant.trigger/life-cycle,
:replicant/life-cycle, which describes what kind of event occurred, having one the following values:
- :replicant.life-cycle/mount
- :replicant.life-cycle/unmount
- :replicant.life-cycle/update
:replicant/node will contain a reference to the DOM node the event occurred in.

The second argument, hook-data is whatever data you set on :replicant/on-render.

:replicant/on-render can also take a function, in which case it will be called with a single argument - the hook details map described above.

Keys

Replicant uses keys to identify nodes when the overall structure changes. Set :replicant/key in the attributes map of any element that you do not want recreated unnecessarily. This key is local to the parent element (e.g. you may reuse the key at different levels). When it is set, Replicant will know to reuse the corresponding DOM element, even when it changes positions, etc. If you have CSS transitions on an element, you very likely want to give it a key.

Data-driven transitions

Sometimes it's nice when elements smoothly transition into and/or out of being. Replicant enables this by supporting overrides for inline styles, classes, and indeed any attribute during mount and/or unmount.

Mounting styles/classes/attributes

[:h1 {:style {:transition "left 0.25s"
              :position "absolute"
              :left 0}
      :replicant/mounting {:style {:left "-100%"}}}
 "Hello world"]

When this element is mounted to the DOM, it will slide in from the left. When initially rendered, it will have these styles:

{:transition "left 0.25s"
 :position "absolute"
 :left "-100%"}

Once mounted, it will be updated to:

{:transition "left 0.25s"
 :position "absolute"
 :left 0}

Which causes the CSS transition to trigger, and move the element in from the left.

Mounting styles are merged into your ordinary styles. Other attributes are completely overwritten. Classes are partly overwritten: the classes from the hiccup symbol will always be included, but :class will be overwritten:

[:h1.heading
  {:class [:mounted]
   :replicant/mounting {:class [:mounting]}}
 "Hello world"]

During mounting, this element will have the classes "heading mounting", and after mounting it will have the classes "heading mounted".

Unmounting styles/classes/attributes

Replicant supports changing an element's attributes, classes and styles to trigger a transition as the element leaves the DOM. When you do this, the node will not be removed from the DOM until its transitions have completed. The life-cycle hook will trigger after the element has transitioned and been removed from the DOM.

To expand on the mounting example, this component:

[:h1 {:style {:transition "left 0.25s"
              :position "absolute"
              :left 0}
      :replicant/mounting {:style {:left "-100%"}}
      :replicant/unmounting {:style {:left "-100%"}}}
 "Hello world"]

Would slide in from the left when mounted, and then slide out to the left again when unmounted. Only after the slide transition completes will it be removed from the DOM.

Class overrides

Given this CSS:

.pane {
  transition: left 0.25s;
  position: absolute;
  left: 0;
}

.mounting {
  left: -100%;
}

And this hiccup:

[:h1.pane {:replicant/mounting {:class :mounting}}
  "Hello world"]

The pane would slide in from the left, as it would be mounted with the two classes "pane mounting" and after mount it would have only "pane". Note that classes from the hiccup symbol are added to both mounting and mounted classes.

This feature was inspired by a similar feature in [snabbdom](https://github.com/snabbdom/snabbdom].

innerHTML

Sometimes all you have is a string of pre-rendered HTML. Replicant can render it for you via innerHTML:

[:div {:innerHTML "<h1>Oh, well</h1>"}]

When using :innerHTML any child elements will be ignored (without warning at the time being).

Differences from hiccup

Replicant has a more liberal understanding of hiccup data than the main hiccup library.

Styles

You may express :style as a map of styles, e.g.:

[:div {:style {:background "red", :width 320}}
  "A small red thing"]

Using maps for styles is suggested, as string values for the style attribute will be parsed to maps.

Classes

You can specify classes on the hiccup "selector", e.g. [:h1.heading "Hello"]. In addition, you can use :class, which takes a few different values:

A string
A keyword
A collection of string|keyword

The suggested value is keywords and collections of keywords. Strings will be split on space.

API Reference

`(replicant.dom/render el hiccup)`

Render the hiccup into the element el. Any pre-existing content not created by Replicant will be removed. Subsequent calls will efficiently update the rendered DOM elements by comparing the new and old hiccup.

`(replicant.dom/set-dispatch! f)`

Register the function to call when life cycle hooks and/or event handlers are data, not functions (see data-driven hooks). The function will be called with two arguments. The first is a map with details about the trigger, with the following keys:

:replicant/trigger Either :replicant.trigger/dom-event or :replicant.trigger/life-cycle
:replicant/node The triggering node
:replicant/js-event the JavaScript event object, when the trigger is :replicant.trigger/dom-event
:replicant/details A vector of keywords indicating what caused the life-cycle hook when the trigger is :replicant.trigger/life-cycle. One or more of:replicant/move-node, :replicant/updated-attrs, :replicant/updated-children.

The second argument is the data passed to the hiccup life-cycle/event handler attribute.

Keyword reference

Keywords in the attributes map:

:replicant/on-mount - A hook to be called when the element mounts. Either a function or arbitrary data, see data-driven hooks.
:replicant/on-unmount - A hook to be called when the element unmounts. Either a function or arbitrary data, see data-driven hooks.
:replicant/on-render - A hook to be called when the element renders (including when it mounts and unmounts). Either a function or arbitrary data, see data-driven hooks.
:replicant/mounting - Attribute (including class, styles) overrides to apply while node is mounting, see data-driven transitions.
:replicant/unmounting - Attribute (including class, styles) overrides to apply while node is unmounting, see data-driven transitions.
:replicant/on-update - A hook to be called when the element renders, excluding when it mounts and unmounts. Either a function or arbitrary data, see data-driven hooks.

Keywords used with hook and event handler dispatch:

:replicant/trigger
:replicant.trigger/dom-event
:replicant.trigger/life-cycle
:replicant/js-event
:replicant/node
:replicant.life-cycle/mount
:replicant.life-cycle/unmount
:replicant.life-cycle/update
:replicant/details
:replicant/move-node
:replicant/updated-attrs
:replicant/updated-children

Rationale

After 10+ years, React's original premise still stands out to me as the ideal approach to frontend development: Code as if every update is a full re-render. This gives you a delightfully straight-forward development model that does not have to be explicit about how a UI must change to go from one state to the next. Instead, you just describe the next state, and the rendering library figures it out for you.

This model has so many benefits that I recently gave a 40 minute talk about it.

React and other popular frontend technologies are not mere rendering libraries. Instead they have taken on the role of holistic UI frameworks: handling state changes, and packing on increasing amounts of features to do more and more things inside UI components.

Replicant is a rendering libary. That's it. There's no state management, there's no async rendering, there's no networking utilities. There's just a single function that renders and rerenders your hiccup to the DOM in an efficient manner.

What about components?

Replicant does not have components. Some common reasons to have components include:

Capture UI components in reusable artifacts
Use knowledge about domain data to short-circuit rendering for performance
Attach life-cycle hooks (used for animations, DOM manipulation, etc)
Wire in cross-cutting concerns (i18n, theming, etc)

Clojure provides the function as a mechanism for creating reusable pieces of logic. Components are just functions that return hiccup, e.g. something like (button {,,,}) which returns the appropriate hiccup for a button.

Short-circuiting rendering (e.g. something akin to React's original shouldComponentUpdate) is generally not necessary, as Replicant is already efficient enough. Should you have some heavy transformations from domain data to hiccup, you can use memoize or other more specialized tools. Since "components" are just functions that return hiccup, you don't need framework specific tooling to optimize your code.

Life-cycles are genuinely useful. That's why you can attach them directly to hiccup nodes, and Replicant will trigger them for you. You are not limited to placing hooks on "components" - any node in the hiccup tree can have them.

Replicant does not need to know about cross-cutting concerns like i18n, theming, etc. Since the entire UI can be represented as data, you can implement concerns like these with pure data transformations. However, there are speed gains to be had if you can postpone such transformations to just in time for rendering, which is why Replicant will eventually provide a hook for this. The hook will be a global one, and does not necessitate a bespoke component abstraction.

Contribute

Want to help make it fast? Awesome, please help in any way you can.

Tests

There are tests. Run them like so:

clojure -X:dev:test

...or start a REPL and evaluate at will.

Benchmarking

To run the benchmark, check out js-framework-benchmark and follow these steps:

git checkout https://github.com/cjohansen/js-framework-benchmark.git
cd js-framework-benchmark
npm ci
npm run install-server
npm start

Leave the server running.

Compiling the test runner

In another terminal:

cd webdriver-ts
npm ci
npm run compile

Build Replicant

Finally, build Replicant to run its benchmark:

cd frameworks/keyed/replicant
npm run build-prod

You should now be able to manually interact with Replicant on http://localhost:8080/frameworks/keyed/replicant/

Run the benchmark

With Replicant built, you can go to the root directory to run the benchmark:

npm run bench keyed/replicant

This will take about 5 minutes, and Chrome will open and close several times. When it's done, generate the report:

npm run results

And open http://localhost:8080/webdriver-ts-results/dist/index.html

It is also possible to run an all-in-one benchmark, validation, and report. This will run the benchmark headlessly:

npm run rebuild-ci keyed/replicant

Comparing frameworks

It can be useful to compare to some other frameworks. To do so, build them and run the benchmark for each one. Here are some suggestions:

cd frameworks/keyed/react
npm run build-prod
cd ../..
npm run bench keyed/react

cd frameworks/keyed/reagent
npm run build-prod
cd ../..
npm run bench keyed/reagent

cd frameworks/keyed/vanillajs
npm run build-prod
cd ../..
npm run bench keyed/vanillajs

Benchmarking and testing changes

If you want to try to optimize or tweak Replicant, you might want to run several benchmarks. For all I know, the directory name is used to key the frameworks in the report, so my current workflow consists of copying the frameworks/keyed/replicant directory, making changes to it, and running the benchmark, e.g.:

cp -r frameworks/keyed/replicant frameworks/keyed/replicant-20240103
cd frameworks/keyed/replicant-20240103
cp -r ~/projects/replicant/src/replicant src/.
cd ../../../
npm run bench keyed/replicant-20240103
npm run results

If you make changes to the code, you can start by running a faster test, to avoid running a long benchmark on broken code:

npm run isKeyed -- --headless true keyed/replicant-xyz

bpringe/replicant

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