Today’s post comes from Michel Gutierrez (mig), the developer of Video DownloadHelper, among other add-ons. He shares his story about the process of modernizing his XUL add-on to make it compatible with multiprocess Firefox (e10s).

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Video DownloadHelper (VDH) is an add-on that extracts videos and image files from the Internet and saves them to your hard drive. As you surf the Web, VDH will show you a menu of download options when it detects something it can save for you.

It was first released in July 2006, when Firefox was on version 1.5. At the time, both the main add-on code and DOM window content were running in the same process. This was helpful because video URLs could easily be extracted from the window content by the add-on. The Smart Naming feature was also able to extract video names from the Web page.

When multiprocess Firefox architecture was first discussed, it was immediately clear that VDH needed a full rewrite with a brand new architecture. In multiprocess Firefox, DOM content for webpages run in a separate process, which means required asynchronous communication with the add-on code would increase significantly. It wasn’t possible to simply make adaptations to the existing code and architecture because it would make the code hard to read and unmaintainable.

The Migration

After some consideration, we decided to update the add-on using SDK APIs. Here were our requirements:

Code running in the content process needed to run separately from code running in Javascript modules and the main process. Communication must occur via message passing.

Preferences needed to be available in the content process, as there are many adjustable parameters that affect the user interface.

Localization of HTML pages within the content script should be as easy as possible.

In VDH, the choice was made to handle all of these requirements using the same Client-Server architecture commonly used in regular Web applications: the components that have access to the preferences, localization, and data storage APIs (running in the main process) serve this data to the UI components and the components injected into the page (running in the content process), through the messaging API provided by the SDK.

Limitations

Migrating to the SDK enabled us to become compatible with multiprocess Firefox, but it wasn’t a perfect solution. Low-level SDK APIs, which aren’t guaranteed to work with e10s or stay compatible with future versions of Firefox, were required to implement anything more than simple features. Also, an increased amount of communication between processes is required even for seemingly simple interactions.

Resizing content panels can only occur in the background process, but only the content process knows what the dimensions should be. This gets more complicated when the size dynamically changes or depends on various parameters.

Critical features like monitoring network traffic or launching external programs in VDH requires low-level APIs.

Capturing tab thumbnails from the Add-on SDK API does not work in e10s mode. This feature had to be reimplemented in the add-on using a framescript.

When intercepting network responses, the Add-on SDK does not decode compressed responses.

The SDK provides no easy means to determine if e10s is enabled or not, which would be useful as long as glitches remain where the add-on has to act differently.

Future Direction

Regardless of the limitations posed, making VDH compatible to multiprocess Firefox was a great success. Taking the time to rewrite the add-on also improved the general architecture and prepared it for changes needed for WebExtensions. The first e10s-compatible version of VDH is version 5.0.1 and had been available since March 2015.

Looking forward, the next big challenge is making VDH compatible with WebExtensions. We considered migrating directly to WebExtensions, but the legacy and low-level SDK APIs used in VDH could not be replaced at the time without compromising the add-on’s features.

To fully complete the transition to WebExtensions, additional APIs may need to be created. As an extension developer we’ve found it helpful to work with Mozilla to define those APIs, and design them in a way that is general enough for them to be useful in many other types of add-ons.

A note from the add-ons team: resources for migrating your add-ons to WebExtensions can be found here.