Add option to make storage persistent to Offline page.

assets/javascripts/templates/pages/offline_tmpl.js

@@ -1,4 +1,4 @@
-app.templates.offlinePage = (docs) => `\
+app.templates.offlinePage = (docs, hasPersistence, isPersistent) => `\
 <h1 class="_lined-heading">Offline Documentation</h1>
 
 <div class="_docs-tools">
@@ -23,7 +23,9 @@ app.templates.offlinePage = (docs) => `\
     ${docs}
   </table>
 </div>
-<p class="_note"><strong>Note:</strong> your browser may delete DevDocs's offline data if your computer is running low on disk space and you haven't used the app in a while. Load this page before going offline to make sure the data is still there.
+<div id="_offline-persistence-note">
+  ${offlinePersistenceNote(hasPersistence, isPersistent)}
+</div>
 <h2 class="_block-heading">Questions & Answers</h2>
 <dl>
   <dt>How does this work?
@@ -42,6 +44,33 @@ app.templates.offlinePage = (docs) => `\
 </dl>\
 `;
 
+app.templates.persistenceError = function (exception) {
+  const reason = exception
+    ? `<code class="_label">${exception.name}: ${exception.message}</code>`
+    : "Bookmark this site and try again.";
+
+  return `<p class="_note _note-red"><strong>Persistent storage was denied by your browser.</strong> ${reason}`;
+};
+
+var offlinePersistenceNote = function (hasPersistence, isPersistent) {
+  if (isPersistent) {
+    return "";
+  }
+
+  let html =
+    "<p class=\"_note\"><strong>Note:</strong> your browser may delete DevDocs's offline data if your computer is running low on disk space and you haven't used the app in a while.";
+
+  if (hasPersistence) {
+    html +=
+      ' <button type="button" class="_btn-link _bold" data-enable-persistence>Enable persistent storage</button>.';
+  } else {
+    html +=
+      " Load this page before going offline to make sure the data is still there.";
+  }
+
+  return html;
+};
+
 var canICloseTheTab = function () {
   if (app.ServiceWorker.isEnabled()) {
     return ' Yes! Even offline, you can open a new tab, go to <a href="//devdocs.io">devdocs.io</a>, and everything will work as if you were online (provided you installed all the documentations you want to use beforehand). ';

assets/javascripts/views/content/offline_page.js

@@ -25,12 +25,19 @@ app.views.OfflinePage = class OfflinePage extends app.View {
       if (statuses === false) {
         this.html(this.tmpl("offlineError", app.db.reason, app.db.error));
       } else {
-        let html = "";
-        for (var doc of app.docs.all()) {
-          html += this.renderDoc(doc, statuses[doc.slug]);
-        }
-        this.html(this.tmpl("offlinePage", html));
-        this.refreshLinks();
+        this.checkPersistence((hasPersistence, isPersistent) => {
+          if (!this.activated) {
+            return;
+          }
+          let html = "";
+          for (var doc of app.docs.all()) {
+            html += this.renderDoc(doc, statuses[doc.slug]);
+          }
+          this.html(
+            this.tmpl("offlinePage", html, hasPersistence, isPersistent)
+          );
+          this.refreshLinks();
+        });
       }
     });
   }
@@ -93,6 +100,8 @@ app.views.OfflinePage = class OfflinePage extends app.View {
       for (el of Array.from(this.findAll(`[data-action='${action}']`))) {
         $.click(el);
       }
+    } else if (el.hasAttribute("data-enable-persistence")) {
+      this.requestPersistence();
     }
   }
 
@@ -142,4 +151,37 @@ app.views.OfflinePage = class OfflinePage extends app.View {
       app.settings.set("manualUpdate", !event.target.checked);
     }
   }
+
+  checkPersistence(callback) {
+    if (navigator.storage && navigator.storage.persisted) {
+      navigator.storage
+        .persisted()
+        .then((persisted) => callback(true, persisted))
+        .catch(() => callback(false, false));
+    } else {
+      callback(false, false);
+    }
+  }
+
+  requestPersistence() {
+    navigator.storage
+      .persist()
+      .then((success) => this.onPersistenceRequestCompleted(success))
+      .catch((exception) =>
+        this.onPersistenceRequestCompleted(false, exception)
+      );
+  }
+
+  onPersistenceRequestCompleted(success, exception) {
+    if (!this.activated) {
+      return;
+    }
+    const note = this.find("#_offline-persistence-note");
+    if (!note) {
+      return;
+    }
+    // Granting persistence retires the note, which is what a fresh render of
+    // the page would produce; the disappearing button is the confirmation.
+    note.innerHTML = success ? "" : this.tmpl("persistenceError", exception);
+  }
 };

assets/stylesheets/components/_content.scss

@@ -385,6 +385,7 @@
 ._bold { font-weight: var(--boldFontWeight); }
 ._note { @extend %note; }
 ._note-green { @extend %note-green; }
+._note-red { @extend %note-red; }
 ._label { @extend %label; }
 ._code { @extend %code; }
 ._highlight, ._highlight > td { background: var(--highlightBackground) !important; }