Make the site installable and add an Install app button

docusaurus.config.ts

@@ -46,6 +46,18 @@ const config: Config = {
     },
   ],
 
+  // Makes the site installable ("Add to Home screen" / "Install app"): the manifest names
+  // the app and its icons. There is deliberately no service worker, so the installed app
+  // needs a connection, like the site itself.
+  headTags: [
+    { tagName: 'link', attributes: { rel: 'manifest', href: '/manifest.webmanifest' } },
+    { tagName: 'meta', attributes: { name: 'theme-color', content: '#1b1b1d' } },
+    { tagName: 'link', attributes: { rel: 'apple-touch-icon', href: '/img/app/icon-192.png' } },
+  ],
+
+  // Catches the browser's install offer for the homepage's "Install app" button.
+  clientModules: ['./src/clientModules/installPrompt.ts'],
+
   presets: [
     [
       'classic',

e2e/install.spec.ts

@@ -0,0 +1,37 @@
+import { test, expect } from '@playwright/test';
+
+test('links a web app manifest with installable icons', async ({ page, request }) => {
+  await page.goto('/');
+
+  await expect(page.locator('link[rel="manifest"]')).toHaveAttribute('href', '/manifest.webmanifest');
+
+  const manifest = await (await request.get('/manifest.webmanifest')).json();
+  expect(manifest.display).toBe('standalone');
+  for (const size of ['192x192', '512x512']) {
+    const icon = manifest.icons.find((candidate: { sizes: string }) => candidate.sizes === size);
+    expect((await request.get(icon.src)).status()).toBe(200);
+  }
+});
+
+test('shows the install button only while the browser offers to install', async ({ page }) => {
+  await page.goto('/');
+  const button = page.getByRole('button', { name: 'Install app' });
+  await expect(button).toHaveCount(0);
+
+  // Browsers only fire the real event for a site they consider installable, so stand in for it.
+  await page.evaluate(() => {
+    const offer = new Event('beforeinstallprompt', { cancelable: true });
+    Object.assign(offer, {
+      prompt: () => {
+        document.body.dataset.installPrompted = 'yes';
+        return Promise.resolve();
+      },
+    });
+    window.dispatchEvent(offer);
+  });
+  await expect(button).toBeVisible();
+
+  await button.click();
+  await expect(page.locator('body')).toHaveAttribute('data-install-prompted', 'yes');
+  await expect(button).toHaveCount(0);
+});

src/clientModules/installPrompt.ts

@@ -0,0 +1,53 @@
+import ExecutionEnvironment from '@docusaurus/ExecutionEnvironment';
+
+/**
+ * Chromium browsers announce that the site can be installed through a single
+ * `beforeinstallprompt` event, usually right after the page loads and before React has
+ * hydrated. This client module runs as soon as the bundle does, keeps the event, and lets
+ * the InstallButton component ask for it later, on any page and after any navigation.
+ * Safari and Firefox never fire the event, so nothing is ever offered there.
+ */
+export interface BeforeInstallPromptEvent extends Event {
+  prompt(): Promise<void>;
+}
+
+type Listener = (available: boolean) => void;
+
+let deferred: BeforeInstallPromptEvent | null = null;
+const listeners = new Set<Listener>();
+
+function notify(): void {
+  listeners.forEach((listener) => listener(deferred !== null));
+}
+
+/** Calls `listener` now and whenever the install offer appears or goes away. */
+export function onInstallAvailable(listener: Listener): () => void {
+  listeners.add(listener);
+  listener(deferred !== null);
+  return () => {
+    listeners.delete(listener);
+  };
+}
+
+/** Raises the browser's install dialog. An offer can be used once. */
+export function promptInstall(): void {
+  const prompt = deferred;
+  deferred = null;
+  notify();
+  prompt?.prompt().catch(() => {
+    // Already used or blocked: the browser's own menu still offers installing.
+  });
+}
+
+if (ExecutionEnvironment.canUseDOM) {
+  window.addEventListener('beforeinstallprompt', (event) => {
+    // The button replaces the browser's own mini-infobar.
+    event.preventDefault();
+    deferred = event as BeforeInstallPromptEvent;
+    notify();
+  });
+  window.addEventListener('appinstalled', () => {
+    deferred = null;
+    notify();
+  });
+}

src/components/InstallButton/index.tsx

@@ -0,0 +1,21 @@
+import { useEffect, useState } from "react";
+import { onInstallAvailable, promptInstall } from "../../clientModules/installPrompt";
+
+/**
+ * "Install app": rendered only while the browser offers to install the site (see
+ * clientModules/installPrompt.ts), so it never shows in Safari or Firefox, or once the
+ * site is installed.
+ */
+export default function InstallButton(): JSX.Element | null {
+  const [available, setAvailable] = useState(false);
+  useEffect(() => onInstallAvailable(setAvailable), []);
+
+  if (!available) {
+    return null;
+  }
+  return (
+    <button type="button" className="button button--secondary button--lg" onClick={promptInstall}>
+      📲 Install app
+    </button>
+  );
+}

src/pages/index.tsx

@@ -3,6 +3,7 @@ import Link from "@docusaurus/Link";
 import useDocusaurusContext from "@docusaurus/useDocusaurusContext";
 import Layout from "@theme/Layout";
 import HomepageFeatures from "../components/HomepageFeatures";
+import InstallButton from "../components/InstallButton";
 import Heading from "@theme/Heading";
 
 import styles from "./index.module.css";
@@ -17,6 +18,7 @@ function HomepageHeader() {
         </Heading>
         <p className="hero__subtitle">{siteConfig.tagline}</p>
         <div className={styles.buttons}>
+          <InstallButton />
           {/* <Link
             className="button button--secondary button--lg"
             to="/docs/intro">

static/manifest.webmanifest

@@ -0,0 +1,15 @@
+{
+  "name": "codeuctivity",
+  "short_name": "codeuctivity",
+  "description": "Turning Coffee into Code: A Journey of Joyful Creation",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "theme_color": "#1b1b1d",
+  "background_color": "#1b1b1d",
+  "icons": [
+    { "src": "/img/app/icon-192.png", "type": "image/png", "sizes": "192x192", "purpose": "any" },
+    { "src": "/img/app/icon-512.png", "type": "image/png", "sizes": "512x512", "purpose": "any" },
+    { "src": "/img/app/icon-maskable-512.png", "type": "image/png", "sizes": "512x512", "purpose": "maskable" }
+  ]
+}