Using a Background Page
Extending the example extension to use a background page.
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Extending the example extension to use a background page.
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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.
Actually defining a background HTML page, that uses script tags to load the JavaScript files.
Just defining the to-be-loaded JavaScript files and let Thunderbird create a background page on-the-fly.
We place the following background.html file into our hello-world project folder:
Let's also create an empty background.js script file in the hello-world project folder.
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:
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:
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 destructuring, 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. So, we create a modules subfolder into our hello-world project folder. Then, we place the messageTools.mjs file within. Here follows what it should contain.
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.
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:
Let's add the following code to our background script, which will add two menu entries and will react to them being clicked:
Using the menus API requires the menus permission, which needs to be added to the permissions key in our manifest.json file.
Using the notifications API requires the notifications permission, which needs to be added to the permissions key in our manifest.json file.
This is how our manifest.json should now look like:
Our background script should look as follows:
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.
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 , there are two ways to load background scripts:
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 . We therefore add the following section to our manifest.json file:
In order to listen for new messages, we have to add a listener for the event to our background script:
The callback function of the onNewMailReceived event receives two parameters: folder being a and messages being a . The defined event listener stores the folder and the message information of the new received mail in the extensions storage.
Since Thunderbird's WebExtension API potentially has to handle a lot of messages, the data type is paginated. Please check the tutorial for more information.
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 page.
The 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.
In order to do something when our menu is clicked, we add a listener for the 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 for each entry in line 20.
Let's double-check that we made the and have all the files in the right places:
As described in the , go to the Add-ons Manager to open the Debug Add-on Page and temporarily install the extension.
