User Guide

Simple and decomplexed UI library based on React >= 18 focused on performance.

Add to deps.edn:

funcool/rumext
{:git/tag "v2.17"
 :git/sha "f542785"
 :git/url "https://github.com/funcool/rumext.git"}

First Steps

Function components, as their name says, are defined using plain functions. Rumext exposes a lightweight macro over a fn that adds some additional facilities.

Let’s see an example of how to define a component:

(require '[rumext.v2 :as mf])

(mf/defc title*
  [{:keys [name] :as props}]
  [:div {:class "label"} name])

The received props are just plain JavaScript objects, so instead of destructuring, you can access props directly using an imperative approach (just a demonstrative example):

(mf/defc title*
  [props]
  (let [name (unchecked-get props "name")]
    [:div {:class "label"} name]))

And finally, we mount the component onto the DOM:

(ns myname.space
  (:require
   [goog.dom :as dom]
   [rumext.v2 :as mf]))

(def root (mf/create-root (dom/getElement "app")))
(mf/render! root (mf/element title* #js {:title "hello world"}))

NOTE: Important: the * in the name is a mandatory convention for proper visual distinction of React components and Clojure functions.

NOTE: It also enables the current defaults on how props are handled. If you don’t use the * suffix, the component will behave in legacy mode.

Props & Destructuring

The destructuring works very similar to the Clojure map destructuring with small differences and convenient enhancements for making working with React props and idioms easy.

(mf/defc title*
  [{:keys [name] :as props}]
  (assert (object? props) "expected object")
  (assert (string? name) "expected string")
  (assert (= (unchecked-get props "name")
             name)
          "expected string")

  [:label {:class "label"} name])

An additional idiom (specific to the Rumext component macro and not available in standard Clojure destructuring) is the ability to obtain an object with all non-destructured props:

(mf/defc title*
  [{:keys [name] :rest props}]
  (assert (object? props) "expected object")
  (assert (nil? (unchecked-get props "name")) "no name in props")

  ;; The `:>` will be explained later
  [:> :label props name])

This allows you to extract the props that the component has control of and leave the rest in an object that can be passed as-is to the next element.

JSX / Hiccup

You may already be familiar with Hiccup syntax (which is equivalent to the React JSX) for defining the React DOM. The intention of this section is to explain only the essential part of it and the peculiarities of Rumext.

Let’s start with simple generic elements like div:

[:div {:class "foobar"
       :style {:background-color "red"}
       :on-click some-on-click-fn}
  "Hello World"]

The props and the style are transformed at compile time into a JS object, transforming all keys from lisp-case to camelCase (and renaming :class to className); so the compilation results in something like this:

const h = React.createElement;

h("div", {className: "foobar",
          style: {"backgroundColor": "red"},
          onClick: someFn},
          "Hello World");

It should be noted that this transformation is only done to properties that are keyword types and that properties that begin with data- and aria- are left as-is without transforming, just like string keys. The properties can be passed directly using camelCase syntax (as React natively expects) if you want.

There are times when we’ll need the element name to be chosen dynamically or constructed at runtime; the props to be built dynamically or created as an element from a user-defined component.

For this purpose, Rumext exposes a special handler: :>, a general-purpose handler for passing dynamically defined props to DOM native elements or creating elements from user-defined components.

Let’s start with an example of how the element name can be defined dynamically:

(let [element (if something "div" "span")]
  [:> element {:class "foobar"
               :style {:background-color "red"}
               :on-click some-on-click-fn}
    "Hello World"])

The props also can be defined dynamically:

(let [props #js {:className "fooBar"
                 :style #js {:backgroundColor "red"}
                 :onClick some-on-click}]
  [:> "div" props "Hello World"])

Remember, if props are defined dynamically, they should be defined as plain JS objects respecting the React convention for props casing (this means we need to pass className instead of :class for example).

In the same way, you can create an element from a user-defined component:

(mf/defc my-label*
  [{:keys [name class on-click] :rest props}]
  (let [class (or class "my-label")
        props (mf/spread props {:class class})]
    [:span {:on-click on-click}
      [:> :label props name]]))

(mf/defc other-component
  []
  [:> my-label* {:name "foobar" :on-click some-fn}])

As you can observe, in destructuring, we use a Clojure convention for naming and casing of properties; it is the Rumext defc macro responsible for the automatic handling of all naming and casing transformations at compile time (for example: the on-click will match the onClick on received props, and class will match the className prop).

The mf/spread macro allows merging one or more JS object props into one, always respecting the casing and naming conventions of React for the props. Read more about it below.

Props Checking

The Rumext library comes with two approaches for checking props: simple and malli.

Let’s start with the simple, which consists of simple existence checks or plain predicate checking:

(mf/defc button*
  {::mf/expect #{:name :on-click}}
  [{:keys [name on-click]}]
  [:button {:on-click on-click} name])

The prop names obey the same rules as the destructuring so you should use the same names in destructuring. Sometimes a simple existence check is not enough; for those cases, you can pass a map where keys are props and values are predicates:

(mf/defc button*
  {::mf/expect {:name string?
                :on-click fn?}}
  [{:keys [name on-click]}]
  [:button {:on-click on-click} name])

If that is not enough, it also supports malli as a validation mechanism for props:

(def ^:private schema:props
  [:map {:title "button:props"}
   [:name string?]
   [:class {:optional true} string?]
   [:on-click fn?]])

(mf/defc button
  {::mf/props :obj
   ::mf/schema schema:props}
  [{:keys [name on-click]}]
  [:button {:on-click on-click} name])

NOTE: The props checking obeys the :elide-asserts compiler option and by default, they will be removed in production builds if the configuration value is not changed explicitly.

Higher-Order Components

This is the way you extend/add additional functionality to a function component. Rumext exposes one:

mf/memo: analogous to React.memo, adds memoization to the component based on props comparison.
mf/memo': identical to the React.memo

To use the higher-order components, you need to wrap the component manually or pass it as a special property in the metadata:

(mf/defc title*
  {::mf/wrap [mf/memo]}
  [{:keys [name]}]
  [:div {:class "label"} name])

By default, the identical? predicate is used to compare props; you can pass a custom comparator function as a second argument:

(mf/defc title*
  {::mf/wrap [#(mf/memo % =)]}
  [{:keys [name]}]
  [:div {:class "label"} name])

For convenience, Rumext has a special metadata ::mf/memo that facilitates the general case for component props memoization. If you pass true, it will behave the same way as ::mf/wrap [mf/memo] or React.memo(Component). You also can pass a set of fields; in this case, it will create a specific function for testing the equality of that set of props.

If you want to create your own higher-order component, you can use the mf/fnc macro:

(defn some-factory
  [component param]
  (mf/fnc my-high-order-component*
    [props]
    [:section
     [:> component props]]))

Hooks

The Rumext library exposes a few specific hooks and some wrappers over existing React hooks in addition to the hooks that React offers itself.

You can use both one and the other interchangeably, depending on which type of API you feel most comfortable with. The React hooks are exposed as they are in React, with the function name in camelCase, and the Rumext hooks use the lisp-case syntax.

Only a subset of available hooks is documented here; please refer to the API reference documentation for detailed information about available hooks.

use-state

This is analogous to the React.useState. It exposes the same functionality but uses the ClojureScript atom interface.

Calling mf/use-state returns an atom-like object that will deref to the current value, and you can call swap! and reset! on it to modify its state. The returned object always has a stable reference (no changes between rerenders).

Any mutation will schedule the component to be rerendered.

(require '[rumext.v2 as mf])

(mf/defc local-state*
  [props]
  (let [clicks (mf/use-state 0)]
    [:div {:on-click #(swap! clicks inc)}
      [:span "Clicks: " @clicks]]))

Alternatively, you can use the React hook directly:

(mf/defc local-state*
  [props]
  (let [[counter update-counter] (mf/useState 0)]
    [:div {:on-click (partial update-counter #(inc %))}
      [:span "Clicks: " counter]]))

use-var

In the same way as use-state returns an atom-like object. The unique difference is that updating the ref value does not schedule the component to rerender. Under the hood, it uses the useRef hook.

use-effect

Analogous to the React.useEffect hook with a minimal call convention change (the order of arguments is inverted).

This is a primitive that allows incorporating probably effectful code into a functional component:

(mf/defc local-timer*
  [props]
  (let [local (mf/use-state 0)]
    (mf/use-effect
      (fn []
        (let [sem (js/setInterval #(swap! local inc) 1000)]
          #(js/clearInterval sem))))
    [:div "Counter: " @local]))

The use-effect is a two-arity function. If you pass a single callback function, it acts as though there are no dependencies, so the callback will be executed once per component (analogous to didMount and willUnmount).

If you want to pass dependencies, you have two ways:

passing a JS array
using the rumext.v2/deps helper

(mf/use-effect
  (mf/deps x y)
  (fn [] (do-stuff x y)))

And finally, if you want to execute it on each render, pass nil as deps (much in the same way as raw useEffect works).

For convenience, there is an mf/with-effect macro that drops one level of indentation:

(mf/defc local-timer*
  [props]
  (let [local (mf/use-state 0)]
    (mf/with-effect []
      (let [sem (js/setInterval #(swap! local inc) 1000)]
        #(js/clearInterval sem)))
    [:div "Counter: " @local]))

Here, the deps must be passed as elements within the vector (the first argument).

Obviously, you can use the React hook directly via mf/useEffect.

use-memo

In the same line as the use-effect, this hook is analogous to the React useMemo hook with the order of arguments inverted.

The purpose of this hook is to return a memoized value.

Example:

(mf/defc sample-component*
  [{:keys [x]}]
  (let [v (mf/use-memo (mf/deps x) #(pow x 10))]
    [:span "Value is: " v]))

On each render, while x has the same value, the v only will be calculated once.

This also can be expressed with the rumext.v2/with-memo macro that removes a level of indentation:

(mf/defc sample-component*
  [{:keys [x]}]
  (let [v (mf/with-memo [x]
            (pow x 10))]
    [:span "Value is: " v]))

use-fn

Is a special case of use-memo. An alias for use-callback.

deref

A Rumext custom hook that adds reactivity to atom changes to the component:

Example:

(def clock (atom (.getTime (js/Date.))))
(js/setInterval #(reset! clock (.getTime (js/Date.))) 160)

(mf/defc timer*
  [props]
  (let [ts (mf/deref clock)]
    [:div "Timer (deref): "
     [:span ts]]))

Internally, it uses the react.useSyncExternalStore API together with the ability of atom to watch it.

Helpers

Working with props

The Rumext library comes with a small set of helpers that facilitate working with props JS objects. Let’s look at them:

`mf/spread`

A macro that allows performing a merge between two props data structures using the JS spread operator. Always preserving the React props casing and naming convention.

It is commonly used this way:

(mf/defc my-label*
  [{:keys [name class on-click] :rest props}]
  (let [class (or class "my-label")
        props (mf/spread props {:class class})]
    [:span {:on-click on-click}
      [:> :label props name]]))

The second argument should be a map literal in order to make the case and naming transformation work correctly. If you pass there a symbol, it should already be a correctly created props object.

`mf/props`

A helper macro that allows defining props objects from a map literal.

An example of how it can be used and combined with mf/spread:

(mf/defc my-label*
  [{:keys [name class on-click] :rest props}]
  (let [class (or class "my-label")
        new-props (mf/props {:class class})
        all-props (mf/spread props new-props)]
    [:span {:on-click on-click}
      [:> :label props name]]))

If you pass a symbol literal (props defined elsewhere using a Clojure data structure) to the mf/props macro, a dynamic transformation will be emitted for this expression.

(let [clj-props {:class "my-label"}
      props (mf/props clj-props)]
  [:> :label props name])

In this example, props binding will contain a plain props js object converted dinamically from clojure map at runtime. This should be avoided if performance is important because it adds the overhead of dynamic conversion on each render.

FAQ

Differences with RUM

This project is originated as a friendly fork of rum for a personal use but it is evolved to be a completly independent library that right now does not depend on it and probably no longer preserves any of the original code. In any case, many thanks to Tonksy for creating rum.

This is the list of the main differences:

use function based components instead of class based components.
a clojurescript friendly abstractions for React Hooks.
the component body is compiled statically (never interprets at runtime thanks to hicada).
performance focused, with a goal to offer almost 0 runtime overhead on top of React.

WARNING: it is mainly implemented to be used in penpot and released as separated project for conveniendce. Don’t expect compromise for backward compatibility beyond what the penpot project needs.

What is the legacy mode?

Components that name does not use * as a suffix behaves in legacy mode. It means parameter will be received as clojure map without any transformation (with some exceptions for className).

That components should use :& handler when creating JSX/Hiccup elements.

It is present for backward compatibility and should not be used.

License

Licensed under MPL-2.0 (see LICENSE file on the root of the repository)

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