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Sample webextension experiment

This project contains a trivial example of a webextension experiment.

Using an experimental API in 4 simple steps

  1. Run a Firefox 51 build and navigate to about:debugging
  2. Choose "Load Temporary Add-on" and select (a file from) the experiment/ directory in this project. You should see a new entry in the list of extensions titled "Experimental API".
  3. Choose "Load Temporary Add-on" and select (a file from) the extension/ directory in this project. You should see a new entry in the list of extensions titled "hello test".
  4. Click the "Debug" button next to "hello test" (you may need to enable debugging first). When the console appears, you should see the string hello sez: "Hello, world!". That's it! Read on for a more detailed explanation.


The API extension

The extension in the experiment/ directory is a new type of Firefox extension called an api extension. This is specified by the type property in the extension's install.rdf:


The rest of install.rdf is standard extension boilerplate with one twist -- the extension id must be in the something@somethingelse format (i.e., it may not be a UUID) and the portion before the at sign is interpreted as the name of the API being created by this extension. In this case, we are using the name "simple":


To use this API, a webextension will need to declare that is requires the "experiments.simple" permission -- more on that below.

An API extension must contain two files: schema.json and api.js. The file schema.json is a standard JSON schema for the new webextension API implemented in the extension. When the API extension is loaded, the schema is loaded and processed just like the schema files built into the base browser. In this case, we have a simple schema that creates a new namespace called hello, which contains a single function hello().

The file api.js contains the actual implementation of this API. This file is evaluated with Chrome privileges, and after it is evaluated, it should create a new class called API. A new instance of this class will be created every time an extension that uses this API (by declaring the permission described above). This class should include a method getAPI() that returns an object suitable for being cloned into the chrome / browser objects visible to the extension.

Like the built-in webextensions API implementations, functions available to a webextension are wrapped with code generated from the schema that validates permissions, function arguments, etc. In this simple example, we have a single function hello() that just returns a fixed string.

The webextension

The webextension in the extension/ directory is quite simple. One thing to note is that manifest.json includes the permission "experiments.simple". As described above this creates a dependency between this webextension and an API extension with an ID of the form "simple@(something)".

The add-ons manager currently enforces this dependency in that it will not enable a webextension if it depends on an API extension that is not installed and active, but it does not attempt to install or enable the API extension -- that must be done manually by the user. If a webextension depends on an API extension that is not available, the webextension is simply disabled.

From here, the operation of the extension is pretty simple, it includes a background script that calls the hello() function from the API extension and logs the result to the console.


A demonstration of webextension experiments



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