Using a Background Page
Extending the example extension to use a background page.
In the third part of the Hello World Extension Tutorial, we will introduce the concept of the WebExtension background page.
We will keep track of incoming mails, add a menu entry to the tools menu and also a context menu entry to our button in Thunderbird's main toolbar and a click on both will open notifications with the collected information from the last 24 hours.
In the first two parts of the Hello World Extension Tutorial, we used well-defined UI hooks to load HTML pages when the user opened one of our popups. In contrast, the background page - if defined - is automatically loaded when the add-on is enabled during Thunderbird start or after the add-on has been manually enabled or installed. It is automatically destroyed when the add-on is shutting down.
The background page is a standard HTML page, supporting the same technologies as ordinary HTML pages, but it is never shown to the user. Its main purpose is to load one or more JavaScript files into the background. Those background scripts can be used to listen for events or to initialize and properly set up the add-on. As described in the MailExtension guide, there are two ways to load background scripts:
- 1.Actually defining a background HTML page, that uses
scripttags to load the JavaScript files. - 2.Just defining the to-be-loaded JavaScript files and let Thunderbird create a background page on-the-fly.
The author of this example prefers the first option. It allows to use the
await keyword in file scope code and it allows to load other ES6 modules. We therefore add the following section to our manifest.json file:"background": {
"page": "background.html"
},
We place the following
background.html file into our hello-world project folder:background.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<script type="module" src="background.js"></script>
</head>
</html>
Let's also create an empty
background.js script file in the hello-world project folder.In order to listen for new messages, we have to add a listener for the
onNewMessageReceived event to our background script:background.js
1
// Import all functions defined in the messageTools module.
2
import * as messageTools from '/modules/messageTools.mjs';
3
4
messenger.messages.onNewMailReceived.addListener(async (folder, messages) => {
5
let { messageLog } = await messenger.storage.local.get({ messageLog: [] });
6
7
for await (let message of messageTools.iterateMessagePages(messages)) {
8
messageLog.push({
9
folder: folder.name,
10
time: Date.now(),
11
message: message
12
})
13
}
14
15
await messenger.storage.local.set({ messageLog });
16
})
The above code is using an inline arrow function to define the callback function for the event listener (which is called for each
onNewMailReceived event). This is identical to the following implicit function definition:
async function onNewMailReceivedCallback(folder, messages) {
...
}
messenger.messages.onNewMailReceived.addListener(
onNewMailReceivedCallback
);The callback function of the
onNewMailReceived event receives two parameters: folder being a MailFolder and messages being a MessageList. The defined event listener stores the folder and the message information of the new received mail in the extensions storage.The
onNewMessageReceived event requires the accountsRead permission, which needs to be added to the permissions key in our manifest.json file.In line
5 of the shown background script, we request the current messageLog entry from the WebExtensions local storage. The used syntax allows to define a default value of [](an empty Array), if there is currently no messageLog entry stored.We could also request multiple values from the local storage:
let rv = await messenger.storage.local.get({
messageLog: [],
aBoolValue: true,
aStringValue: "none"
});
console.log(rv);
The call to
storage.local.get() returns a Promise for a single object with the requested entries, for example the above console.log(rv) could produce the following output:{
messageLog: [],
aBoolValue: false,
aStringValue: "The sky is the limit."
}
To access the content of the
messageLog member, one would have to use rv.messageLog. That is sometimes not the desired behaviour, and instead we store the requested value directly in a variable. This is called object destructering, and it maps the content of the messageLog member of the returned object to the messageLog variable. Any other non-matching returned member is ignored.Access to the local storage requires the
storage permission, which needs to be added to the permissions key in our manifest.json file.The listener is using a helper function to be able to loop over the received messages. The
iterateMessagePages function is defined in an ES6 module, which is loaded in line 2 of the background script shown above.messageTools.mjs
// A wrapper function returning an async iterator for a MessageList. Derived from
// https://webextension-api.thunderbird.net/en/91/how-to/messageLists.html
async function* iterateMessagePages(page) {
for (let message of page.messages) {
yield message;
}
while (page.id) {
page = await messenger.messages.continueList(page.id);
for (let message of page.messages) {
yield message;
}
}
}
To identify the script file as an ES6 module, which does not include file scope code, but only defines functions, we use the
*.mjs file extension.Since Thunderbird's WebExtension API potentially has to handle a lot of messages, the
MessageList data type is paginated. Please check the Working with Message Lists tutorial for more information.The provided
iterateMessagePages() wrapper function is doing most of the heavy lifting and allows to asynchronously loop over the returned messages in line 7 of the shown background script. For each received message, we subsequently push a new entry into the messageLog Array.In line
15 we store the updated messageLog Array back into the local storage. We use the object shorthand notation, which allows leaving out the actual value definition, if the value is stored in a variable with the same name as the object's member name. If the shorthand notation is unwanted, one could instead write the following:await messenger.storage.local.set({ messageLog: messageLog });
Let's add the following code to our background script, which will add two menu entries and will react to them being clicked:
1
// Create the menu entries.
2
let menu_id = await messenger.menus.create({
3
title: "Show received email",
4
contexts: [
5
"browser_action",
6
"tools_menu"
7
],
8
});
9
10
// Register a listener for the menus.onClicked event.
11
messenger.menus.onClicked.addListener(async (info, tab) => {
12
if (info.menuItemId == menu_id) {
13
// Our menu entry was clicked
14
let { messageLog } = await messenger.storage.local.get({
15
messageLog: []
16
});
17
18
let now = Date.now();
19
let last24h = messageLog.filter(e => (now - e.time) < 24 * 60 * 1000);
20
21
for (let entry of last24h) {
22
messenger.notifications.create({
23
"type": "basic",
24
"iconUrl": "images/internet.png",
25
"title": `${entry.folder}: ${entry.message.author}`,
26
"message": entry.message.subject
27
});
28
}
29
}
30
});
In line
2 we create a new menu entry. We use the title Show received email and we add it to the browser_action context and to the tools_menu context. A list of other available contexts can be found on the Supported UI Elements page.The menus.create() function returns the
id of the new menu, which we can use to identify our menu, or - for example - add submenus by using the id as the parentId for other menu entries.Using the
menus API requires the menus permission, which needs to be added to the permissions key in our manifest.json file.In order to do something when our menu is clicked, we add a listener for the onClicked event. We check the
id of the clicked menu to see which of our menus was clicked (we only added one, but checking here anyhow).After we have retrieved the current
messageLog from the local storage, we loop over all entries and create a notification for each entry in line 20.Using the
notifications API requires the notifications permission, which needs to be added to the permissions key in our manifest.json file.hello-world/
├── background.html
├── background.js
├── manifest.json
├── images/
├── internet.png
├── internet-16px.png
└── internet-32px.png
├── mainPopup/
├── popup.css
├── popup.html
└── popup.js
├── messagePopup/
├── popup.css
├── popup.html
└── popup.js
└── modules/
└── messageTools.mjs
This is how our
manifest.json should now look like:manifest.json
{
"manifest_version": 2,
"name": "Hello World Example",
"description": "A basic Hello World example extension!",
"version": "3.0",
"author": "Thunderbird Team",
"browser_specific_settings": {
"gecko": {
"id": "[email protected]",
"strict_min_version": "78.0"
}
},
"browser_action": {
"default_popup": "mainPopup/popup.html",
"default_title": "Hello World",
"default_icon": "images/internet-32px.png"
},
"message_display_action": {
"default_popup": "messagePopup/popup.html",
"default_title": "Details",
"default_icon": "images/internet-32px.png"
},
"permissions": [
"messagesRead",
"accountsRead",
"storage",
"menus",
"notifications"
],
"background": {
"page": "background.html"
},
"icons": {
"64": "images/internet.png",
"32": "images/internet-32px.png",
"16": "images/internet-16px.png"
}
}
Our background script should look as follows:
background.js
// Import all functions defined in the messageTools module.
import * as messageTools from '/modules/messageTools.mjs';
// Add a listener for the onNewMailReceived events.
messenger.messages.onNewMailReceived.addListener(async (folder, messages) => {
let { messageLog } = await messenger.storage.local.get({ messageLog: [] });
for await (let message of messageTools.iterateMessagePages(messages)) {
messageLog.push({
folder: folder.name,
time: Date.now(),
message: message
})
}
await messenger.storage.local.set({ messageLog });
})
// Create the menu entries.
let menu_id = await messenger.menus.create({
title: "Show received email",
contexts: [
"browser_action",
"tools_menu"
],
});
// Register a listener for the menus.onClicked event.
await messenger.menus.onClicked.addListener(async (info, tab) => {
if (info.menuItemId == menu_id) {
// Our menu entry was clicked
let { messageLog } = await messenger.storage.local.get({ messageLog: [] });
let now = Date.now();
let last24h = messageLog.filter(e => (now - e.time) < 24 * 60 * 1000);
for (let entry of last24h) {
messenger.notifications.create({
"type": "basic",
"iconUrl": "images/internet.png",
"title": `${entry.folder}: ${entry.message.author}`,
"message": entry.message.subject
});
}
}
});
As described in the first part of the Hello World Extension Tutorial, go to the Add-ons Manager to open the Debug Add-on Page and temporarily install the extension.
After you have received one or more new messages, while the add-on has been active, open the context menu of our
browser_action button in Thunderbird's main toolbar and click on "Show received emails". For each received message, you should see a notification.
Last modified 4mo ago