1Background
2==========
3
4WebExtensions run in a sandboxed environment much like regular web content.
5The purpose of extensions is to enhance the browser in a way that
6regular content cannot -- WebExtensions APIs bridge this gap by exposing
7browser features to extensions in a way preserves safety, reliability,
8and performance.
9The implementation of a WebExtension API runs with
10`chrome privileges <https://developer.mozilla.org/en-US/docs/Security/Firefox_Security_Basics_For_Developers>`_.
11Browser internals are accessed using
12`XPCOM <https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM>`_
13or `ChromeOnly WebIDL features <https://developer.mozilla.org/en-US/docs/Mozilla/WebIDL_bindings#ChromeOnly>`_.
14
15The rest of this documentation covers how API implementations interact
16with the implementation of WebExtensions.
17To expose some browser feature to WebExtensions, the first step is
18to design the API.  Some high-level principles for API design
19are documented elsewhere (*add links here*).
20
21Javascript APIs
22---------------
23Many WebExtension APIs are accessed directly from extensions through
24Javascript.  Functions are the most common type of object to expose,
25though some extensions expose properties of primitive Javascript types
26(e.g., constants).
27Regardless of the exact method by which something is exposed,
28there are a few important considerations when designing part of an API
29that is accessible from Javascript:
30
31- **Namespace**:
32  Everything provided to extensions is exposed as part of a global object
33  called ``browser``.  For compatibility with Google Chrome, many of these
34  features are also exposed on a global object called ``chrome``.
35  Functions and other objects are not exposed directly as properties on
36  ``browser``, they are organized into *namespaces*, which appear as
37  properties on ``browser``.  For example, the
38  `tabs API <https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs>`_
39  uses a namespace called ``tabs``, so all its functions and other
40  properties appear on the object ``browser.tabs``.
41  For a new API that provides features via Javascript, the usual practice
42  is to create a new namespace with a concise but descriptive name.
43
44- **Environments**:
45  There are several different types of Javascript environments in which
46  extension code can execute: extension pages, content scripts, proxy
47  scripts, and devtools pages.
48  Extension pages include the background page, popups, and content pages
49  accessed via |getURL|_.
50  When creating a new Javascript feature the designer must choose
51  in which of these environments the feature will be available.
52  Most Javascript features are available in extension pages,
53  other environments have limited sets of API features available.
54.. |getURL| replace:: ``browser.runtime.getURL()``
55.. _getURL: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/getURL
56
57- **Permissions**:
58  Many Javascript features are only present for extensions that
59  include an appropriate permission in the manifest.
60  The guidelines for when an API feature requires a permission are
61  described in (*citation needed*).
62
63The specific types of features that can be exposed via Javascript are:
64
65- **Functions**:
66  A function callable from Javascript is perhaps the most commonly
67  used feature in WebExtension APIs.
68  New API functions are asynchronous, returning a
69  `Promise <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise>`_.  Even functions that do not return a result
70  use Promises so that errors can be indicated asynchronously
71  via a rejected Promise as opposed to a synchronously thrown Error.
72  This is due to the fact that extensions run in a child process and
73  many API functions require communication with the main process.
74  If an API function that needs to communicate in this way returned a
75  synchronous result, then all Javascript execution in the child
76  process would need to be paused until a response from the main process
77  was received.  Even if a function could be implemented synchronously
78  within a child process, the standard practice is to make it
79  asynchronous so as not to constrain the implementation of the underlying
80  browser feature and make it impossible to move functionality out of the
81  child process.
82  Another consequence of functions using inter-process communication is
83  that the parameters to a function and its return value must all be
84  simple data types that can be sent between processes using the
85  `structured clone algorithm <https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm>`_.
86
87- **Events**:
88  Events complement functions (which allow an extension to call into
89  an API) by allowing an event within the browser to invoke a callback
90  in the extension.
91  Any time an API requires an extension to pass a callback function that
92  gets invoked some arbitrary number of times, that API method should be
93  defined as an event.
94
95Manifest Keys
96-------------
97In addition to providing functionality via Javascript, WebExtension APIs
98can also take actions based on the contents of particular properties
99in an extension's manifest (or even just the presence of a particular
100property).
101Manifest entries are used for features in which an extension specifies
102some static information that is used when an extension is installed or
103when it starts up (i.e., before it has the chance to run any code to use
104a Javascript API).
105An API may handle a manifest key and implement Javscript functionality,
106see the
107`browser action <https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/browserAction>`_
108API for an example.
109
110Other Considerations
111--------------------
112In addition to the guidelines outlined above,
113there are some other considerations when designing and implementing
114a WebExtension API:
115
116- **Cleanup**: A badly written WebExtension should not be able to permanently
117  leak any resources.  In particular, any action from an extension that
118  causes a resource to be allocated within the browser should be
119  automatically cleaned up when the extension is disabled or uninstalled.
120  This is described in more detail in the section on :ref:`lifecycle`.
121
122- **Performance**: A new WebExtension API should not add any new overhead
123  to the browser when the API is not used.  That is, the implementation
124  of the API should not be loaded at all unless it is actively used by
125  an extension.  In addition, initialization should be delayed when
126  possible -- extensions ared started relatively early in the browser
127  startup process so any unnecessary work done during extension startup
128  contributes directly to sluggish browser startup.
129
130