9 files changed, 84 insertions, 4 deletions

diff --git a/README.md b/README.md
index f3f5a72..e1b24ff 100644
--- a/README.md
+++ b/README.md
@@ -185,6 +185,10 @@ disableKinds = ['taxonomy', 'taxonomyTerm']
   # Portable links meant to work with text editors and let you write markdown without {{< relref >}} shortcode
   # Theme will print warning if page referenced in markdown does not exists.
   BookPortableLinks = true
+
+  # /!\ This is an experimental feature, might be removed or changed at any time
+  # (Optional, experimental, default false) Enables service worker that caches visited pages and resources for offline use.
+  BookServiceWorker = true
 ```
 
 ### Multi-Language Support
diff --git a/assets/search.js b/assets/search.js
index d6ae18b..0393d2c 100644
--- a/assets/search.js
+++ b/assets/search.js
@@ -12,20 +12,27 @@
 
   document.addEventListener('keypress', focusSearchFieldOnKeyPress);
 
-  function focusSearchFieldOnKeyPress(e) {
+  /**
+   * @param {Event} event
+   */
+  function focusSearchFieldOnKeyPress(event) {
     if (input === document.activeElement) {
       return;
     }
 
-    const characterPressed = String.fromCharCode(e.charCode);
+    const characterPressed = String.fromCharCode(event.charCode);
     if (!isHotkey(characterPressed)) {
       return;
     }
 
     input.focus();
-    e.preventDefault();
+    event.preventDefault();
   }
 
+  /**
+   * @param {String} character
+   * @returns {Boolean} 
+   */
   function isHotkey(character) {
     const dataHotkeys = input.getAttribute('data-hotkeys') || '';
     return dataHotkeys.indexOf(character) >= 0;
@@ -63,6 +70,10 @@
     });
   }
 
+  /**
+   * @param {String} src 
+   * @param {Function} callback 
+   */
   function loadScript(src, callback) {
     const script = document.createElement('script');
     script.defer = true;
diff --git a/assets/sw-register.js b/assets/sw-register.js
new file mode 100644
index 0000000..583532c
--- /dev/null
+++ b/assets/sw-register.js
@@ -0,0 +1,6 @@
+if (navigator.serviceWorker) {
+  navigator.serviceWorker.register(
+    "{{ "/sw.js" | relURL }}", 
+    { scope: "{{ "/" | relURL }}" }
+  );
+}
diff --git a/exampleSite/config.toml b/exampleSite/config.toml
index edb0690..a44dc39 100644
--- a/exampleSite/config.toml
+++ b/exampleSite/config.toml
@@ -97,3 +97,7 @@ enableGitInfo = true
   # Portable links meant to work with text editors and let you write markdown without {{< relref >}} shortcode
   # Theme will print warning if page referenced in markdown does not exists.
   BookPortableLinks = true
+
+  # /!\ This is an experimental feature, might be removed or changed at any time
+  # (Optional, experimental, default false) Enables service worker that caches visited pages and resources for offline use.
+  BookServiceWorker = true
diff --git a/exampleSite/config.yaml b/exampleSite/config.yaml
index 6a26b13..d7038ba 100644
--- a/exampleSite/config.yaml
+++ b/exampleSite/config.yaml
@@ -93,3 +93,7 @@ params:
   # Portable links meant to work with text editors and let you write markdown without {{< relref >}} shortcode
   # Theme will print warning if page referenced in markdown does not exists.
   BookPortableLinks: true
+
+  # /!\ This is an experimental feature, might be removed or changed at any time
+  # (Optional, experimental, default false) Enables service worker that caches visited pages and resources for offline use.
+  BookServiceWorker: true
diff --git a/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.content b/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.content
new file mode 100644
index 0000000..33ab914
--- /dev/null
+++ b/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.content
@@ -0,0 +1 @@
+@charset "UTF-8";/*!normalize.css v8.0.1 | MIT License | github.com/necolas/normalize.css*/html{line-height:1.15;-webkit-text-size-adjust:100%}body{margin:0}main{display:block}h1{font-size:2em;margin:.67em 0}hr{box-sizing:content-box;height:0;overflow:visible}pre{font-family:monospace,monospace;font-size:1em}a{background-color:transparent}abbr[title]{border-bottom:none;text-decoration:underline;text-decoration:underline dotted}b,strong{font-weight:bolder}code,kbd,samp{font-family:monospace,monospace;font-size:1em}small{font-size:80%}sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}sub{bottom:-.25em}sup{top:-.5em}img{border-style:none}button,input,optgroup,select,textarea{font-family:inherit;font-size:100%;line-height:1.15;margin:0}button,input{overflow:visible}button,select{text-transform:none}button,[type=button],[type=reset],[type=submit]{-webkit-appearance:button}button::-moz-focus-inner,[type=button]::-moz-focus-inner,[type=reset]::-moz-focus-inner,[type=submit]::-moz-focus-inner{border-style:none;padding:0}button:-moz-focusring,[type=button]:-moz-focusring,[type=reset]:-moz-focusring,[type=submit]:-moz-focusring{outline:1px dotted ButtonText}fieldset{padding:.35em .75em .625em}legend{box-sizing:border-box;color:inherit;display:table;max-width:100%;padding:0;white-space:normal}progress{vertical-align:baseline}textarea{overflow:auto}[type=checkbox],[type=radio]{box-sizing:border-box;padding:0}[type=number]::-webkit-inner-spin-button,[type=number]::-webkit-outer-spin-button{height:auto}[type=search]{-webkit-appearance:textfield;outline-offset:-2px}[type=search]::-webkit-search-decoration{-webkit-appearance:none}::-webkit-file-upload-button{-webkit-appearance:button;font:inherit}details{display:block}summary{display:list-item}template{display:none}[hidden]{display:none}.flex{display:flex}.flex-auto{flex:1 1 auto}.flex-even{flex:1 1}.flex-wrap{flex-wrap:wrap}.justify-start{justify-content:flex-start}.justify-end{justify-content:flex-end}.justify-center{justify-content:center}.justify-between{justify-content:space-between}.align-center{align-items:center}.mx-auto{margin:0 auto}.text-center{text-align:center}.hidden{display:none}.clearfix::after{content:"";display:table;clear:both}html{font-size:16px;letter-spacing:.33px;scroll-behavior:smooth;touch-action:manipulation}html,body{min-width:20rem;overflow-x:hidden}body{color:#000;background:#fff;font-weight:400;text-rendering:optimizeLegibility;-webkit-font-smoothing:antialiased;-moz-osx-font-smoothing:grayscale;box-sizing:border-box}body *{box-sizing:inherit}h1,h2,h3,h4,h5{font-weight:400}a{text-decoration:none;color:#05b}img{vertical-align:baseline}aside nav ul{padding:0;margin:0;list-style:none}aside nav ul li{margin:1em 0;position:relative}aside nav ul a{display:block}aside nav ul a:hover{opacity:.5}aside nav ul ul{padding-left:1rem}ul.pagination{display:flex;justify-content:center;list-style-type:none}ul.pagination .page-item a{padding:1rem}.container{max-width:80rem;margin:0 auto}.book-icon{filter:none}.book-brand{margin-top:0}.book-brand img{height:1.5em;width:auto;vertical-align:middle;margin-right:.5rem}.book-menu{flex:0 0 16rem;font-size:.875rem}.book-menu nav{width:16rem;padding:1rem;position:fixed;top:0;bottom:0;overflow-x:hidden;overflow-y:auto}.book-menu a{color:inherit;word-wrap:break-word}.book-menu a.active{color:#05b}.book-menu a.collapsed::after{content:"▸";position:absolute;right:0}.book-section-flat{margin-bottom:2rem}.book-section-flat:not(:first-child){margin-top:2rem}.book-section-flat>a,.book-section-flat>span{font-weight:bolder}.book-section-flat>ul{padding-left:0}.book-page{min-width:20rem;flex-grow:1;padding:1rem}.book-post{margin-bottom:3rem}.book-header{display:none;margin-bottom:1rem}.book-header label{line-height:0}.book-search{position:relative;margin:1rem 0;border-bottom:1px solid transparent}.book-search::after{display:block;content:"";clear:both}.book-search input{width:100%;padding:.5rem;border:0;border-radius:.25rem;background:#f8f9fa;color:#000}.book-search input:required+.book-search-spinner{display:block}.book-search .book-search-spinner{position:absolute;margin:.5rem;right:0;top:0;width:1rem;height:1rem;border:1px solid transparent;border-top-color:#000;border-radius:50%;animation:spin 1s ease infinite}@keyframes spin{100%{transform:rotate(360deg)}}.book-toc{flex:0 0 16rem;font-size:.75rem}.book-toc nav{width:16rem;padding:1rem;position:fixed;top:0;bottom:0;overflow-x:hidden;overflow-y:auto}.book-toc img{height:1em}.book-toc nav>ul>li:first-child{margin-top:0}.book-footer{padding-top:1rem;font-size:.875rem}.book-footer img{height:1em;margin-right:.5rem}.book-comments{margin-top:1rem}.book-languages{position:relative;overflow:visible;padding:1rem;margin:-1rem}.book-languages ul{margin:0;padding:0;list-style:none}.book-languages ul li{white-space:nowrap;cursor:pointer}.book-languages:hover .book-languages-list,.book-languages:focus .book-languages-list,.book-languages:focus-within .book-languages-list{display:block}.book-languages .book-languages-list{display:none;position:absolute;bottom:100%;left:0;padding:.5rem 0;background:#fff;box-shadow:0 0 .25rem rgba(0,0,0,.1)}.book-languages .book-languages-list li img{opacity:.25}.book-languages .book-languages-list li.active img,.book-languages .book-languages-list li:hover img{opacity:1}.book-languages .book-languages-list a{color:inherit;padding:.5rem 1rem}.book-home{padding:1rem}aside nav,.book-page,.book-header aside,.markdown{transition:.2s ease-in-out;transition-property:transform,margin,opacity;will-change:transform,margin}@media screen and (max-width:56rem){.book-menu{margin-left:-16rem;font-size:16px}.book-toc{display:none}.book-header{display:block}#menu-control:checked+main .book-menu nav,#menu-control:checked+main .book-page{transform:translateX(16rem)}#menu-control:checked+main .book-header aside,#menu-control:checked+main .markdown{opacity:.25}#menu-control:checked+main .book-menu-overlay{display:block;position:absolute;top:0;bottom:0;left:0;right:0}#toc-control:checked+aside{display:block}}@media screen and (min-width:80rem){.book-page,.book-menu nav,.book-toc nav{padding:2rem 1rem}}@font-face{font-family:roboto;font-style:italic;font-weight:300;font-display:swap;src:local("Roboto Light Italic"),local("Roboto-LightItalic"),url(fonts/roboto-v19-latin-300italic.woff2)format("woff2"),url(fonts/roboto-v19-latin-300italic.woff)format("woff")}@font-face{font-family:roboto;font-style:normal;font-weight:400;font-display:swap;src:local("Roboto"),local("Roboto-Regular"),url(fonts/roboto-v19-latin-regular.woff2)format("woff2"),url(fonts/roboto-v19-latin-regular.woff)format("woff")}@font-face{font-family:roboto;font-style:normal;font-weight:700;font-display:swap;src:local("Roboto Bold"),local("Roboto-Bold"),url(fonts/roboto-v19-latin-700.woff2)format("woff2"),url(fonts/roboto-v19-latin-700.woff)format("woff")}@font-face{font-family:roboto mono;font-style:normal;font-weight:400;font-display:swap;src:local("Roboto Mono"),local("RobotoMono-Regular"),url(fonts/roboto-mono-v6-latin-regular.woff2)format("woff2"),url(fonts/roboto-mono-v6-latin-regular.woff)format("woff")}body{font-family:roboto,sans-serif}code{font-family:roboto mono,monospace}@media print{.book-menu,.book-footer,.book-toc{display:none}.book-header,.book-header aside{display:block}main{display:block!important}}.markdown{line-height:1.6em}.markdown>:first-child{margin-top:0}.markdown h1,.markdown h2,.markdown h3,.markdown h4,.markdown h5,.markdown h6{font-weight:400;line-height:1em;margin-top:1.5em;margin-bottom:1rem}.markdown h4,.markdown h5,.markdown h6{font-weight:bolder}.markdown h5{font-size:.875em}.markdown h6{font-size:.75em}.markdown b,.markdown optgroup,.markdown strong{font-weight:bolder}.markdown a{text-decoration:none}.markdown a:hover{text-decoration:underline}.markdown a:visited{color:#8440f1}.markdown img{max-width:100%}.markdown code{padding:0 .25rem;background:#e9ecef;border-radius:.25rem;font-size:.875em}.markdown pre{padding:1rem;background:#f8f9fa;border-radius:.25rem;overflow-x:auto}.markdown pre code{padding:0;background:0 0}.markdown blockquote{margin:1rem 0;padding:.5rem 1rem .5rem .75rem;border-left:.25rem solid #e9ecef;border-radius:.25rem}.markdown blockquote :first-child{margin-top:0}.markdown blockquote :last-child{margin-bottom:0}.markdown table{overflow:auto;display:block;border-spacing:0;border-collapse:collapse;margin-top:1rem;margin-bottom:1rem}.markdown table tr th,.markdown table tr td{padding:.5rem 1rem;border:1px solid #e9ecef}.markdown table tr:nth-child(2n){background:#f8f9fa}.markdown hr{height:1px;border:none;background:#e9ecef}.markdown ul,.markdown ol{padding-left:2rem}.markdown dl dt{font-weight:bolder;margin-top:1rem}.markdown dl dd{margin-left:2rem}.markdown .highlight table tr td:nth-child(1) pre{margin:0;padding-right:0}.markdown .highlight table tr td:nth-child(2) pre{margin:0;padding-left:0}.markdown details{padding:1rem;border:1px solid #e9ecef;border-radius:.25rem}.markdown details summary{line-height:1;cursor:pointer}.markdown-inner>:first-child{margin-top:0}.markdown-inner>:last-child{margin-bottom:0}.markdown .book-expand{margin-top:1rem;margin-bottom:1rem;border:1px solid #e9ecef;border-radius:.25rem;overflow:hidden}.markdown .book-expand .book-expand-head{background:#f8f9fa;padding:.5rem 1rem;cursor:pointer}.markdown .book-expand .book-expand-content{display:none;padding:1rem}.markdown .book-expand input[type=checkbox]:checked+.book-expand-content{display:block}.markdown .book-tabs{margin-top:1rem;margin-bottom:1rem;border:1px solid #e9ecef;border-radius:.25rem;overflow:hidden;display:flex;flex-wrap:wrap}.markdown .book-tabs label{display:inline-block;padding:.5rem 1rem;border-bottom:1px transparent;cursor:pointer}.markdown .book-tabs .book-tabs-content{order:999;width:100%;border-top:1px solid #f8f9fa;padding:1rem;display:none}.markdown .book-tabs input[type=radio]:checked+label{border-bottom:1px solid #05b}.markdown .book-tabs input[type=radio]:checked+label+.book-tabs-content{display:block}.markdown .book-columns{margin-left:-1rem;margin-right:-1rem}.markdown .book-columns>div{margin:1rem 0;min-width:10rem;padding:0 1rem}.markdown a.book-btn{display:inline-block;font-size:.875rem;color:#05b;line-height:2rem;padding:0 1rem;border:1px solid #05b;border-radius:.25rem;cursor:pointer}.markdown a.book-btn:hover{text-decoration:none}.markdown .book-hint.info{border-left-color:#6bf;background-color:rgba(102,187,255,.1)}.markdown .book-hint.warning{border-left-color:#fd6;background-color:rgba(255,221,102,.1)}.markdown .book-hint.danger{border-left-color:#f66;background-color:rgba(255,102,102,.1)}
\ No newline at end of file
diff --git a/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.json b/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.json
new file mode 100644
index 0000000..293c3d6
--- /dev/null
+++ b/exampleSite/resources/_gen/assets/scss/example/book.scss_50fc8c04e12a2f59027287995557ceff.json
@@ -0,0 +1 @@
+{"Target":"book.min.284c8fc21ced13c579d9027a9d14893c56b243c6045001180391cebb4cc36ab8.css","MediaType":"text/css","Data":{"Integrity":"sha256-KEyPwhztE8V52QJ6nRSJPFayQ8YEUAEYA5HOu0zDarg="}}
\ No newline at end of file
diff --git a/layouts/partials/docs/html-head.html b/layouts/partials/docs/html-head.html
index eb013d2..4b0c62f 100644
--- a/layouts/partials/docs/html-head.html
+++ b/layouts/partials/docs/html-head.html
@@ -16,11 +16,16 @@
 <link rel="stylesheet" href="{{ $styles.RelPermalink }}" integrity="{{ $styles.Data.Integrity }}">
 
 {{- if default true .Site.Params.BookSearch }}
-{{- $searchJSFile := printf "%s.search.js" .Language.Lang -}}
+{{- $searchJSFile := printf "%s.search.js" .Language.Lang }}
 {{- $searchJS := resources.Get "search.js" | resources.ExecuteAsTemplate $searchJSFile . | resources.Minify | resources.Fingerprint }}
 <script defer src="{{ $searchJS.RelPermalink }}" integrity="{{ $searchJS.Data.Integrity }}"></script>
 {{ end -}}
 
+{{- if .Site.Params.BookServiceWorker }}
+{{- $swJS := resources.Get "sw-register.js" | resources.ExecuteAsTemplate "sw.js" . | resources.Minify | resources.Fingerprint }}
+<script defer src="{{ $swJS.RelPermalink }}" integrity="{{ $swJS.Data.Integrity }}"></script>
+{{ end -}}
+
 {{- template "_internal/google_analytics_async.html" . -}}
 
 <!-- RSS -->
diff --git a/static/sw.js b/static/sw.js
new file mode 100644
index 0000000..d42b04c
--- /dev/null
+++ b/static/sw.js
@@ -0,0 +1,44 @@
+self.addEventListener("install", function (event) {
+  self.skipWaiting();
+});
+
+self.addEventListener("fetch", (event) => {
+  const cacheName = self.location.pathname
+  const request = event.request;
+
+  if (request.method !== "GET") {
+    return;
+  }
+
+  /**
+   * @param {Response} response
+   * @returns {Promise<Response>}
+   */
+  function saveToCache(response) {
+    if (cacheable(response)) {
+      return caches
+        .open(cacheName)
+        .then((cache) => cache.put(request, response.clone()))
+        .then(() => response);
+    } else {
+      return response;
+    }
+  }
+
+  /**
+   * @param {Error} error
+   */
+  function serveFromCache(error) {
+    return caches.open(cacheName).then((cache) => cache.match(request.url));
+  }
+
+  /**
+   * @param {Response} response
+   * @returns {Boolean}
+   */
+  function cacheable(response) {
+    return response.type === "basic" && response.ok && !response.headers.has("Content-Disposition")
+  }
+
+  event.respondWith(fetch(request).then(saveToCache).catch(serveFromCache));
+});