Make relative-time a self-maintaining custom element (#8134)

Fixes #8124
Replaces #8130

Use a custom element for relative-time. Thanks to @Beowulf for suggesting this approach.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8134
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Beowulf <beowulf@beocode.eu>
Co-authored-by: Benedikt Straub <benedikt-straub@web.de>
Co-committed-by: Benedikt Straub <benedikt-straub@web.de>

web_src/js/webcomponents/relative-time.js

@@ -130,21 +130,42 @@ export function DoUpdateRelativeTime(object, now) {
   return HALF_MINUTE;
 }
 
-/** Update the displayed text of one relative-time DOM element with its human-readable, localized relative time string. */
-function UpdateRelativeTime(object) {
-  const next = DoUpdateRelativeTime(object);
-  if (next !== null) setTimeout(() => { UpdateRelativeTime(object) }, next);
-}
+window.customElements.define('relative-time', class extends HTMLElement {
+  static observedAttributes = ['datetime'];
 
-/** Update the displayed text of all relative-time DOM elements with their respective human-readable, localized relative time string. */
-function UpdateAllRelativeTimes() {
-  for (const object of document.querySelectorAll('relative-time')) UpdateRelativeTime(object);
-}
+  alive = false;
+  contentSpan = null;
 
-document.addEventListener('DOMContentLoaded', () => {
-  UpdateAllRelativeTimes();
-  // Also update relative-time DOM elements after htmx swap events.
-  document.body.addEventListener('htmx:afterSwap', () => {
-    for (const object of document.querySelectorAll('relative-time')) DoUpdateRelativeTime(object);
-  });
+  update = (recurring) => {
+    if (!this.alive) return;
+
+    if (!this.shadowRoot) {
+      this.attachShadow({mode: 'open'});
+      this.contentSpan = document.createElement('span');
+      this.shadowRoot.append(this.contentSpan);
+    }
+
+    const next = DoUpdateRelativeTime(this);
+    if (recurring && next !== null) setTimeout(() => { this.update(true) }, next);
+  };
+
+  connectedCallback() {
+    this.alive = true;
+    this.update(true);
+  }
+
+  disconnectedCallback() {
+    this.alive = false;
+  }
+
+  attributeChangedCallback(name, oldValue, newValue) {
+    if (name === 'datetime' && oldValue !== newValue) this.update(false);
+  }
+
+  set textContent(value) {
+    if (this.contentSpan) this.contentSpan.textContent = value;
+  }
+  get textContent() {
+    return this.contentSpan?.textContent;
+  }
 });

web_src/js/webcomponents/relative-time.test.js

@@ -23,6 +23,7 @@ test('CalculateRelativeTimes', () => {
     'relativetime.years': ['%d year ago', '%d years ago'],
   };
   const mock = document.createElement('relative-time');
+  document.body.append(mock);
 
   const now = Date.parse('2024-10-27T04:05:30+01:00');  // One hour after DST switchover, CET.