--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/background.js	Wed Oct 03 23:40:57 2018 +0200
@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2018 Guido Berhoerster <guido+feed-preview@berhoerster.name>
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+'use strict';
+
+var tabsFeeds = new Map();
+
+// until content handlers become available to webextensions
+// (https://bugzilla.mozilla.org/show_bug.cgi?id=1457500) intercept all
+// responses and change the content type from application/atom+xml or
+// application/rss+xml to application/xml which will then be handled by a
+// content script
+browser.webRequest.onHeadersReceived.addListener(details => {
+    if (details.statusCode != 200 ||
+            typeof details.responseHeaders === 'undefined') {
+        return;
+    }
+
+    let contentTypeHeader = details.responseHeaders.find(element => {
+        return element.name.toLowerCase() === 'content-type';
+    });
+    if (typeof contentTypeHeader !== 'undefined') {
+        let contentType = contentTypeHeader.value.split(';');
+        let mediaType = contentType[0].trim().toLowerCase();
+        if (mediaType === 'application/atom+xml' ||
+                mediaType === 'application/rss+xml') {
+            contentType[0] = 'application/xml';
+            contentTypeHeader.value = contentType.join(';');
+        }
+    }
+
+    return {responseHeaders: details.responseHeaders};
+}, {urls: ['http://*/*', 'https://*/*'], types: ['main_frame']},
+['blocking', 'responseHeaders']);
+
+browser.runtime.onMessage.addListener((request, sender, sendResponse) => {
+    let tab = sender.tab;
+    if (typeof tab !== 'undefined') {
+        // content script sending feeds
+        tabsFeeds.set(tab.id, request);
+        browser.pageAction.show(tab.id);
+    } else {
+        let response = tabsFeeds.get(request);
+        // popup querying feeds
+        sendResponse(tabsFeeds.get(request));
+    }
+});
+
+browser.tabs.onUpdated.addListener((id, changeInfo, tab) => {
+    if (typeof changeInfo.url === 'undefined') {
+        // filter out updates which do not change the URL
+        return;
+    }
+
+    // hide the page action when the URL changes since it is no longer valid,
+    // it will be shown again if the content script detects a feed
+    browser.pageAction.hide(tab.id);
+});
+
+browser.tabs.onRemoved.addListener((tabId, removeInfo) => {
+    tabsFeeds.delete(tabId);
+});