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SubZero

Build web components in Clojure/Script (the easy way).

Highlights

Depends only on the Clojure/Script runtime and browser APIs
Small, simple, easy
Hot reload friendly
Focus-aware virtual DOM reconciliation
'Good enough' performance with progressive optimization tools

Project Status

Experimental. Major breaking changes unlikely, but possible.

Example: Custom Element

(ns increment-counter
  (:require
   [subzero.core :as sz]
   [subzero.plugins.component-registry :as reg]
   [subzero.plugins.web-components :as wc]))

(defn on-click
  [event]
  (let [increment-button (.-host (.-currentTarget event))
        clicks           (js/parseInt (.-clicks increment-button))]
    (set! (.-clicks increment-button) (inc clicks))))

(defn button-view
  [{:keys [clicks]}]
  [:root> {:#on {:click on-click}}
   [:button (str "Clicked " clicks " times")]])


(defonce !db
 (doto (sz/create-db)
  (reg/install!)
  (wc/install! js/document js/customElements)))

(reg/reg-component !db :incrementing-button
  :view button-view
  :props #{:clicks})

<incrementing-button clicks="0"></incrementing-button>

Example: SSR (With Declarative Shadow DOM)

(ns identity-card
  (:require
   [subzero.core :as sz]
   [subzero.plugins.component-registry :as reg]
   [subzero.plugins.html :as html]))

(defn identity-card-view
  [{:keys [name age sex]}]
  [:root>
   [:table
    [:tbody
     [:tr
      [:th "Name"]
      [:td name]]
     [:tr
      [:th "Age"]
      [:td age]]
     [:tr
      [:th "Sex"]
      [:td sex]]]]])

(defonce !db
 (doto (sz/create-db)
  (reg/install!)
  (html/install!)))

(reg/reg-component !db :identity-card
  :view identity-card-view
  :props #{:name :age :sex})

(print
 (html/html !db {:doctype "html"}
  [:identity-card
   :name "John Doe"
   :age 46
   :sex "Male"]))

<!DOCTYPE html>
<html>

<body>
    <identity-card name="John Doe" age="46" sex="Male">
        <template shadowrootmode="open">
            <table>
                <tbody>
                    <tr>
                        <th>Name</th>
                        <td>John Doe</td>
                    </tr>
                    <tr>
                        <th>Age</th>
                        <td>46</td>
                    </tr>
                    <tr>
                        <th>Sex</th>
                        <td>Male</td>
                    </tr>
                </tbody>
            </table>
        </template>
    </identity-card>
</body>

</html>

Component Registration Options

`:view`

A function which takes a map of prop values, and yields the markup to be used in rendering the component.

`:props`

Either a set or a map specifying the props to be provided to the :view function, and how the library should source them.

If given as a set of keywords, these are taken as the prop names; with a value of :default being implied for each. For simple components, this is the most concise, and often the correct, way to specify the component's props. See below for the behavior of :default props.

If given as a map, the key for each entry is taken as the prop name. This is the key that'll be used in the map of prop values passed to the :view function. The value of this map determines where the prop value comes from, and can be one of the following:

:field - SubZero will generate a matching JavaScript property for this prop. The name of this property will be a cammelCase version of the prop name. The value of the property reflects that of the prop, updating the property updates the prop.
:attr - The prop value will be sourced from element attributes with the same name as the prop. Attribute writers and readers can be registered to customize how the library serializes values as attributes, or parses attribute strings back into useful values.
:default - A combination of :field and :attr. The current prop value will be the last updated of either the JavaScript property, or the element attribute matching the prop name.
A function - Equivalent to {:state-factory the-function}. See below.
An IWatchable - Equivalent to {:state-factory (constantly <the-watchable>)}. See below.
{:state-factory factory-fn :state-cleanup ?cleanup-fn :field ?field-name} - The factory-fn will be called to produce an IWatchable, which SubZero will watch for new prop values. If the returned value also satisfies IDeref, it'll be deref'd for an initial prop value. An optional cleanup-fn can be provided, which will be called to perform any cleanup when a component instances is disconnected from the DOM. The optional :field option can provide a name for a read-only JavaScript property whose value will reflect that of this prop.
{:attr ?attr-name :field ?field-name} - Similar to :field, :attr, and :default, except the field and attribute names are given explicitly.

`:focus`

This determines how the component should handle focus. It can be specified as either :self or :delegate. If not specified, the component will not be focusable.

The :self option indicates that the component itself serves as some kind of control, and should thus be focusable. SubZero will implicitly set tabIndex = 0 for these, if the tab index isn't otherwise given.

The :delegate option indicates that the component wraps some kind of control. This causes the component's first focusable child to be focused in place of the component itself. See delegatesFocus for details. Warning: if changed in a hot reload, the new value won't apply to component instances that existing prior to the change.

`:inherit-doc-css?`

If truthy, SubZero will check the top-level document for stylesheet <link> elements, and import linked stylesheets into this component. Note that this wraps the stylesheets with CSSStyleSheet, which ignores imports.

`:form-associated?`

If truthy, the component produced will be form associated. Allowing it to report a current form value, errors, etc via the special :#internals prop on :root> (see below).

Markup

SubZero uses a markup notation similar to that of Hiccup. This is the notation that should be produced by component :view functions, or passed into the HTML rendering functions.

In brief, most values are stringified and treated as text. The following are the exceptions.

Vectors represent elements. They should have a keyword (representing the element tag) as the first value. Following that, either a prop map; or a keyword-value sequence or props can be given. Anything that follows makes up the element body.

[:div]
;; -> <div></div>

[:div :id "my-div" :class "foo" "The " [:b "body"]]
;; -> <div id="my-div" class="foo">The <b>body</b></div>

[:div {:id "my-div" :class "foo"} "The " [:b "body"]]
;; -> <div id="my-div" class="foo">The <b>body</b></div>

Sequences are flattened and expanded inline.

[:ul (map (fn [x] [:li x]) ["fee" "fi" "fo" "fum"])]
;; -> <ul><li>fee</li><li>fi</li><li>fo</li><li>fum</li></ul>

This means nil isn't rendered at all.

Functions are called (passed the prop map) and their returned markup rendered as normal. When combined with tags (see below), this is a powerful tool for optimization.

Special Props

SubZero recognizes some special keys that can be given in an element's prop map, which have special behavior. The following special props apply to all elements.

:#style - Sugar for the regular :style prop. Renders a map of style properties (e.g {:display :none :color :red}).
:#class - Sugar for the regular :class prop. Accepts a string, keyword, or symbol; or a collection of the same. Flattens, stringifies, and joins the values together into a class list.
:#on - Registers a map of event listeners (e.g {:click my-click-fn :focus my-focus-fn}). Multiple listeners for the same event can be specified by namespacing the keywords (e.g :0/click first-click-fn :1/click second-click-fn).
:#bind - Creates reactive bindings between regular props (i.e no # prefix) and IWatchable things. When the watchable thing updates, SubZero will update the bound prop in response. If the watchable thing is also derefable then it'll be deref'd for an initial value. (e.g [:input :#bind {:value !my-atom} :#on {:input #(reset! !my-atom (-> % .-target .-value))}]).
:#key - Similar to React keys. Creates a consistent mapping between this vdom node and a particular DOM element instance.
:#tag - Like an ETag for vdom nodes. Used to help optimize rendering. If a node's tag is the same across renders then SubZero won't need reconcile it. This is a powerful tool for progressive optimizations.
:#opaque? - Indicates that the contents (body) of this node are rendered by some other means, so SubZero shouldn't touch it. (e.g [:div :innerHTML "<b>foo</b>" :#opaque? true]).

Regular Props

Regular props are rendered by either setting a matching JavaScript property (if one is found on the prototype of the element being rendered), or as attributes. SubZero looks for properties that either match the given prop name exactly, or match the cammelCase'd form of the prop name. So for example the innerHTML JavaScript property can be set either as :innerHTML or :inner-html.

The `:root>`

A component :view function can return a special [:root> ...] form as its top level value. This form is similar to element nodes, except its only handles special props; and these apply to the component instance itself rather than any child elements.

The :root> node shares the :#on, #:style, :#tag, and :#opaque? props with regular vnodes, with the following caveats:

:#on - The listeners are applied to the component's ShadowRoot, on which SubZero dispatches custom lifecycle events: connect, render, update, disconnect.
:#style - Applies the given style properties to the component instance as defaults, which can be overidden externally.

Some additional special props can also be set on this node:

:#css - A string, URL (js/URL or java.net.URL), js/CSSStyleSheet, or a collection of the same. If the string starts with http then it's treated as a URL. The contents are fetched and wrapped in a js/CSSStyleSheet. Otherwise it's treated as CSS content and wrapped directly. After coersion, the stylesheets are adopted by the component's ShadowRoot. When rendering to HTML as a declarative shadow DOM, produces <script> elements instead.
:#internals - A map of fields to set on the component's ElementInternals. Also supports special sugar keys:
- :#states - Sets ElementInternals#states from a collection of keywords
- :#value - Sets the form value (only for form associated components)
- :#validity - Should be a map of {:message ? :anchor ? :report? ?}. Calls ElementInternals#setValidity with the given message and anchor. If :report? is truthy, also calls ElementInternals#reportValidity.
:#on-host - Like :#on, but registers the listeners on the element itself, rather than its ShadowRoot.

Performance Optimization

Use :#tag in combination with laziness and function substitution to optimize rendering performance progressively as bottlenecks are found. A node whose tag is the same across renders has the following performance advantages:

Its props don't need to be compared with the previous version, they're assumed to be the same. Children also don't need to be reconciled.
Since children don't need to be reconciled, any lazy seqs or functions found in the body don't need to be realized.

When rendering lists of vnodes with the same tag. If new items can be added to the list, or exsiting items re-arranged, then make sure to give each item a unique :#key.

Attribute Readers/Writers

You can customize how attributes are serialized and parsed (in both HTML and custom elements) by registering handlers via reg/reg-attribute-writers and reg/reg-attribute-readers respectively. These take keyval seqs, with the key for each entry being one of: 1) a component name, 2) :default, 3) a wildcard pattern like :ns-to-match/*.

(defn json-reader
  [attribute-string attribute-name component-name]
  (js/JSON.parse attribute-string))

(defn json-writer
  [attribute-value attribute-name component-name]
  (js/JSON.stringify attribute-value))

(reg/reg-attribute-readers :my-app/* json-reader :other-component json-reader)
(reg/reg-attribute-writers :my-app/* json-writer :other-component json-writer)

Questions, Feedback, etc.

Join the #zero-lib Clojurians channel or open an issue.

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