>>> [!note] Migrated issue <!-- Drupal.org comment --> <!-- Migrated from issue #3611154. --> Reported by: [andileco](https://www.drupal.org/user/2054544) Related to !1 >>> <h3 id="summary-problem-motivation">Problem/Motivation</h3> <p>ePub Generator can only produce a file to download. Users have to leave the site and open the file in an external reader, and every request regenerates the ePub &mdash; rendering the entity and assembling the ZIP &mdash; even for identical, repeated requests.</p> <h4 id="summary-steps-reproduce">Steps to reproduce</h4> <ol> <li>Enable ePub Generator and enable ePub output for a content type.</li> <li>Visit a node and use the "Download ePub" tab.</li> <li>Note that the only option is a file download, and that reloading regenerates the ePub from scratch each time.</li> </ol> <h3 id="summary-proposed-resolution">Proposed resolution</h3> <p>Add an <code>epub_generator_viewer</code> submodule providing an in-browser reader built on epub.js, plus a response cache for generated ePubs.</p> <p>The reader is a normal render array; epub.js fetches the book client-side via same-origin XHR from the <em>existing</em> download routes, so the session cookie applies and the current access checks ("generate epub" permission, entity view access, bundle gating) are reused unchanged. No new file-serving surface.</p> <p>Caching is a kernel event subscriber rather than a service decorator, because the expensive work happens in the download controllers before the generator service is called. On RESPONSE it captures the <code>BinaryFileResponse</code> bytes into a dedicated swappable bin; on REQUEST, at a priority after routing and access enforcement, a hit replays the bytes, so a cached copy is never served more widely than a generated one. Downloads benefit alongside the viewer.</p> <p>Keys vary by route, raw route parameters, query string, content language, and (configurable, default on) role fingerprint. Invalidation is tag-driven: entity tags, both settings config tags, the <code>epub</code>/<code>full</code> view display config tags, and a custom <code>epub_generator_viewer:book:{bid}</code> tag fired from node insert/update/delete so editing any child page flushes the assembled book. ETag/304 revalidation for the viewer's repeat fetches; size cap (default 8MB) protects database cache backends.</p> <p>Known limitation, to be documented: per-role variance is supported, per-user variance is not &mdash; disable caching in that case.</p> <p>Install epub.js and JSZip via pinned npm tarballs as <code>package</code> repositories typed <code>drupal-library</code>, which needs no extra plugin and resolves to <code>libraries/epubjs/dist/epub.min.js</code> and <code>libraries/jszip/dist/jszip.min.js</code>. asset-packagist is <strong>not</strong> recommended: it serves the unbuilt source tree with no <code>dist/</code> directory, and pulls the full transitive npm dependency tree into the libraries directory.</p> <h3 id="summary-remaining-tasks">Remaining tasks</h3> <ul> <li>Review the submodule.</li> <li>Run the unit tests for <code>EpubCachePolicy</code>.</li> <li>Add kernel/functional test coverage for the cache subscriber (hit/miss, access enforcement on hit, book tag invalidation).</li> <li>Verify the reader against fixed-layout and RTL ePubs.</li> <li>Decide whether the generic uploaded-ePub field formatter belongs here long-term or in its own project.</li> </ul> <h3 id="summary-ui-changes">User interface changes</h3> <ul> <li>New "Read online" local task on nodes, next to "Download ePub" (weight 99). For nodes in a Book outline it opens the whole assembled book.</li> <li>New reader page with a toolbar: table of contents, title, progress, optional download link, fullscreen toggle. Keyboard navigation and position resume via localStorage.</li> <li>New "ePub viewer (epub.js)" field formatter for file fields; non-ePub files fall back to a plain file link.</li> <li>The generator settings page becomes tabbed: "Generator" and a new "Viewer" tab at <code>/admin/config/content/epub-generator/viewer</code> with reader defaults, cache controls, and a "Flush cached ePubs" button.</li> <li>Status report warning when epub.js/JSZip are missing.</li> </ul> <h3 id="summary-api-changes">API changes</h3> <p>None to existing APIs. New in the submodule: routes <code>epub_generator_viewer.node_view</code>, <code>.view</code>, <code>.settings</code>; services <code>epub_generator_viewer.cache_policy</code>, <code>.response_cache_subscriber</code>, and the <code>cache.epub_generator_viewer</code> bin; theme hook <code>epub_viewer</code>; field formatter plugin <code>epub_viewer</code>; cache tag <code>epub_generator_viewer:book:{bid}</code>.</p> <h3 id="summary-data-model-changes">Data model changes</h3> <p>None. New config object <code>epub_generator_viewer.settings</code> (with schema) and <code>field.formatter.settings.epub_viewer</code>. No entity, field, or schema changes; the cache bin is a standard swappable bin.</p>