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Announcement Bar `<mv-announcement-bar>`

网站顶部的细条，用于发布上线、促销、维护通知和功能预告，带有 Dismiss 按钮，关闭状态按 id-key 记录在 localStorage 中（新的活动 id 会再次显示），可选的 until 日期会让它自动下线，提供 info、accent 和 warning 三种色调，并为插槽中的倒计时预留位置。

工作原理

网站顶部的细条，用于发布上线、促销、维护通知和功能预告，带有 Dismiss 按钮，关闭状态按 id-key 记录在 localStorage 中（新的活动 id 会再次显示），可选的 until 日期会让它自动下线，提供 info、accent 和 warning 三种色调，并为插槽中的倒计时预留位置。其他实现容易出错的细节：它从不造成布局偏移（标签在 JavaScript 运行前就已应用样式并预留按钮空间，一行 head 脚本确保之前访问时已关闭的通知条根本不会绘制）；它是带标签的 region，而不是 live region，因此屏幕阅读器不会在每次页面加载时被打断；关闭后焦点交给下一个可聚焦元素（否则为 <main>），而不是丢到 <body> 上；连续的多个通知条会合并为一条。

分类	反馈
类型	Web Component（`<mv-announcement-bar>`）
状态	稳定版
Keywords	exclusive, light, announcement, banner, top-bar, promo, launch, maintenance, dismissible, cls

When to use

A launch, sale or new feature needs a thin site-wide notice above the header that visitors can close for good
A maintenance window or policy change must stay visible until a set date and then disappear without a deploy
A promo should count down to its end inside the top bar and stop showing once the campaign is over
Several notices (a launch and a maintenance warning) may need to stack at the top of the page

Avoid when

The message is about a section or a form and belongs next to that content, with a title and actions → use Alert instead
The feedback follows a user action and should disappear on its own after a few seconds → use Toast instead
A dismissed banner should leave a small badge the user can reopen later → use Grin instead

安装

node scripts/add.mjs announcement-bar --out ./src/marvelous

使用 Marvelous UI MCP 服务器的 AI 智能体：install_components({ slugs: ["announcement-bar"], target_dir: "<absolute path>/src/marvelous", framework: "react" })。

复制的文件（含依赖）：tokens/tokens.css, core/base.css, core/element.js, core/focus.js, core/motion.js, components/announcement-bar/announcement-bar.js, components/announcement-bar/announcement-bar.css。

用法

快速开始，最小可运行标记：

<mv-announcement-bar id-key="launch-2026-10">Marvelous UI launches Oct 7 · <a href="/waitlist">Join the waitlist</a></mv-announcement-bar>
<!-- optional, in <head>, so a bar dismissed earlier never paints: -->
<script>try{var k=JSON.parse(localStorage.getItem("mv-announcement-bar")||"[]"),s=document.createElement("style");s.textContent=k.map(function(i){return 'mv-announcement-bar[id-key="'+CSS.escape(i)+'"]:not([data-state])'}).join()+"{display:none}";k.length&&document.head.append(s)}catch(e){}</script>

标准标记，可在此基础上通过属性、data-* 和 CSS 变量进行定制：

<div id="mv-abar-demo">
  <style>
    #mv-abar-demo { display: grid; gap: 1.25rem; width: min(100%, 52rem); margin-inline: auto; }
    #mv-abar-demo .page { overflow: hidden; border: 1px solid var(--mv-border); border-radius: var(--mv-radius-xl); background: var(--mv-bg); box-shadow: var(--mv-shadow-md); }
    #mv-abar-demo .site { display: flex; align-items: center; gap: 1rem; padding: .75rem 1.25rem; border-bottom: 1px solid var(--mv-border); }
    #mv-abar-demo .brand { display: flex; align-items: center; gap: .5rem; font-weight: 650; letter-spacing: -.01em; color: var(--mv-fg); text-decoration: none; border-radius: var(--mv-radius-sm); }
    #mv-abar-demo .brand span { width: 1.25rem; height: 1.25rem; border-radius: .35rem; background: linear-gradient(135deg, var(--mv-accent), var(--mv-accent-fg)); }
    #mv-abar-demo .site nav { display: flex; gap: .15rem; margin-inline-start: auto; }
    #mv-abar-demo .site nav a { padding: .35rem .6rem; border-radius: var(--mv-radius-md); font-size: .875rem; color: var(--mv-fg-muted); text-decoration: none; }
    #mv-abar-demo .site a:focus-visible { outline: 2px solid var(--mv-ring); outline-offset: 1px; }
    #mv-abar-demo main { padding: 1.75rem 1.5rem 2rem; }
    #mv-abar-demo main:focus-visible { outline: 2px solid var(--mv-ring); outline-offset: -2px; }
    #mv-abar-demo h2 { margin: 0 0 .4rem; font-size: 1.4rem; letter-spacing: -.02em; }
    #mv-abar-demo main p { margin: 0; max-width: 34rem; color: var(--mv-fg-muted); font-size: .92rem; line-height: 1.6; }
    #mv-abar-demo .variants { display: grid; gap: .75rem; }
    #mv-abar-demo .variants mv-announcement-bar { border-radius: var(--mv-radius-lg); }
    #mv-abar-demo .foot { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: .75rem; font-size: .8rem; color: var(--mv-fg-subtle); }
    @media (max-width: 36rem) { #mv-abar-demo .site nav a:not(:last-child) { display: none; } }
  </style>

  <div class="page">
    <mv-announcement-bar id-key="launch-2026-10" tone="accent">
      Marvelous UI launches Oct 7 · <a href="#">Join the waitlist</a>
    </mv-announcement-bar>
    <mv-announcement-bar id-key="maintenance-2026-10-04" tone="warning" permanent id="mv-abar-demo-maint">
      Scheduled maintenance Sunday, Oct 4, 2:00-3:00 AM UTC. Checkout may be briefly unavailable.
    </mv-announcement-bar>
    <header class="site">
      <a class="brand" href="#"><span></span>Tidewell Studio</a>
      <nav aria-label="Main">
        <a href="#">Pricing</a>
        <a href="#">Docs</a>
        <a href="#">Sign in</a>
      </nav>
    </header>
    <main>
      <h2>Ship your storefront in 12 languages</h2>
      <p>Two bars stacked above the header: the launch notice can be dismissed and stays gone on reload; the maintenance notice has no close button and retires itself when its date passes.</p>
    </main>
  </div>

  <div class="variants">
    <mv-announcement-bar id-key="summer-sale-2026">
      Summer sale: 30% off annual plans · ends in <mv-countdown data-variant="compact" data-offset="187200" to="2026-10-01T18:00:00-04:00" label="Summer sale">Oct 1, 6:00 PM EDT<template slot="ended">a moment</template></mv-countdown> · <a href="#">Upgrade for $84/year</a>
    </mv-announcement-bar>
    <mv-announcement-bar id-key="dark-dashboards" tone="info">
      New: dark mode for dashboards, shared by Aiko Tanaka’s team · <a href="#">See what’s new</a>
    </mv-announcement-bar>
  </div>

  <div class="foot">
    <span>Dismissals are remembered per <code>id-key</code>; a new campaign id shows again.</span>
    <button class="mv-button" data-variant="ghost" data-size="sm" id="mv-abar-demo-reset">Show dismissed bars again</button>
  </div>
</div>

<script type="module">
  // Demo only: keep the dates ahead of today so every bar and timer stays live.
  const root = document.getElementById("mv-abar-demo");
  root.querySelectorAll("mv-countdown[data-offset]").forEach((el) => { el.to = new Date(Date.now() + Number(el.dataset.offset) * 1000).toISOString(); });
  document.getElementById("mv-abar-demo-maint").until = new Date(Date.now() + 3 * 86400000).toISOString();
  document.getElementById("mv-abar-demo-reset").addEventListener("click", () => {
    root.querySelectorAll("mv-announcement-bar").forEach((bar) => bar.reset());
  });
</script>

API

Attributes

Name	类型	Default	Description
`id-key`	string		Campaign id the dismissal is remembered under (localStorage key mv-announcement-bar, a JSON list of the last 50 dismissed ids). Change it for a new campaign and the bar shows again. Without it, a hash of the bar’s own text is used (the head script then cannot hide it before paint).
`until`	ISO 8601 date-time		Moment after which the bar hides itself, on load or live while the page is open (e.g. 2026-10-14T23:59:00+02:00). Invalid or missing: no end.
`tone`	info \| accent \| warning	`(neutral ink)`	Color: neutral dark ink, accent, tinted info or tinted warning. Works in light and dark themes.
`permanent`	boolean		No Dismiss button and stored dismissals are ignored (a maintenance notice that must stay until `until`).
`data-state`	open \| dismissed		Set by the component on upgrade (for styling); the head script only hides bars that do not have it yet.

Properties

Name	类型	Description
`idKey / until / permanent`	string / string / boolean	Mirror the attributes.
`strings`	object	Override default texts: label (region name, “Announcement”) and dismiss (button name, “Dismiss announcement”). An aria-label or aria-labelledby you set on the bar is kept.
`dismissKey`	string	Key the dismissal is stored under: id-key, else auto-`<hash of the text>` (read-only).
`dismissed`	boolean	True while the bar is hidden because it was dismissed, remembered or expired (read-only).
`MvAnnouncementBar.earlyScript`	string (static)	The `<head>` snippet as a string, to inline from a server template or framework layout. Also exported as EARLY_SCRIPT.

Methods

Name	Description
`dismiss()`	Hides the bar (height collapse), remembers its key and moves focus on if it was inside; returns false if vetoed or already hidden.
`reset()`	Forgets the dismissal for this key and shows the bar again (unless until has passed).

Events

Name	Description
`mv-dismiss`	detail = { key, reason: "user" \| "expired" }. Cancelable when reason is user: preventDefault() keeps the bar open and nothing is stored. Also fired (not cancelable) when until passes while the bar is visible.

Content structure

Name	Description
`(content)`	The message: text, links, a `<mv-countdown data-variant="compact">` or any inline content. Nested custom elements are ignored when hashing the text for the automatic key.

CSS classes

Name	Description
`mv-announcement-bar-dismiss`	Generated Dismiss button (end side, aligned to the first line).

CSS variables

Name	Default	Description
`--mv-announcement-bar-bg`	`per tone`	Background color.
`--mv-announcement-bar-fg`	`per tone`	Text, link and icon color.
`--mv-announcement-bar-border`	`per tone`	Bottom hairline color (transparent for the neutral and accent tones).
`--mv-announcement-bar-height`	`2.5rem`	Minimum height, reserved before JavaScript runs.

Accessibility

The bar gets role=region with the accessible name “Announcement” (strings.label, or your own aria-label), so it is listed with landmarks and can be skipped, but it is not a live region: nothing is read out on page load or navigation. Put it first in <body>, before the header, so it comes first in reading and tab order. The Dismiss button is a real <button> named “Dismiss announcement” (strings.dismiss), the × icon is aria-hidden, the glyph stays small while the hit area grows to about 42 px, focus is a visible 2 px outline in the bar’s text color on every tone. When the bar is dismissed (or expires) while focus is inside it, focus moves to the next focusable element after it (the next stacked bar, the logo, the nav), else to <main> (made focusable with tabindex=-1 for that moment), never to <body>; the bar is inert while it collapses. The collapse is a short height and opacity animation, instant under prefers-reduced-motion or data-motion=reduce. No layout shift: the tag is styled and the button space reserved by CSS alone, so the upgrade changes nothing; for returning visitors, the optional <head> snippet (quickstart, or MvAnnouncementBar.earlyScript) hides dismissed id-keys before first paint. Forced colors: system border and button colors. Hidden when printing. Dismissals sync across open tabs. SSR-safe: nothing touches window or document at import, and the content is plain HTML readable without JavaScript. Known limits: when localStorage is blocked (some private modes, strict storage policies), a dismissal lasts only for the current page view; the optional head snippet has to be pasted by hand in the page <head>, the component cannot inject it early enough itself.