path: root/chrome/common/extensions/docs/static/overview.html

<div id="pageData-title" class="pageData">Overview</div>
<div id="pageData-showTOC" class="pageData">true</div>

<p>
Please read this page;
it has vital information about the extension architecture.
If you get bored or restless,
take a break!
We suggest bouncing between this page
and tutorials
such as <a href="getstarted.html">Getting Started</a>
and <a href="tut_debugging.html">Debugging</a>.
</p>


<h2 id="what">The basics</h2>

<p>
An extension is a zipped bundle of files &mdash;
HTML, CSS, JavaScript, images, and anything else you need &mdash;
that adds functionality to the Google Chrome browser.
Because an extension is just a special kind of web page,
it can use all the APIs
that the browser provides
to web pages and apps,
from XMLHttpRequest to JSON to localStorage.
</p>

<p>
Extensions can add UI to Google Chrome,
in the form of
<a href="toolstrip.html">toolstrips</a> (toolbar additions)
and <a href="pageActions.html">page actions</a>
(clickable badges in the address bar).
Extensions can also interact programmatically
with browser features such as 
<a href="bookmarks.html">bookmarks</a>
and <a href="tabs.html">tabs</a>.
</p>

<p>
To find a complete list of extension features,
with implementation details
for each one,
see the <a href="devguide.html">Developer's Guide</a>.
</p>


<h2 id="files">Files</h2>
<p>
Each extension has the following files:
<!-- PENDING: This could use a picture -->
</p>

<ul>
  <li>A <b>manifest file</b></li>
  <li>One or more <b>HTML files</b> (unless the extension is a theme)</li>
  <li><em>Optional:</em> One or more <b>JavaScript files</b></li>
  <li><em>Optional:</em> Any other files your extension needs &mdash; for example, image files</li>
</ul>

<p>
While you're working on your extension,
you put all these files into a single folder.
When you distribute your extension,
the contents of the folder are packaged into a special zipfile
that has a <code>.crx</code> suffix,
as described in <a href="packaging.html">Packaging</a>.
</p>


<h3>Referring to files</h3>

<p>
You can put any file you like into an extension,
but how do you use it?
Usually,
you can refer to the file using a relative URL,
just as you would in an ordinary HTML page.
Here's an example of referring to
a file named <code>myimage.png</code>
that's in a subdirectory named <code>images</code>.
</p>

<pre>
&lt;img <b>src="images/myimage.png"</b> style="width:auto; height:auto">
</pre>

<p>
As you might notice while you use the Google Chrome debugger,
every file in an extension is also accessible by an absolute URL like this:
</p>

<blockquote>
<b>chrome-extension://</b><em>&lt;extensionID></em><b>/</b><em>&lt;pathToFile></em>
</blockquote>

<p>
In that URL, the <em>&lt;extensionID></em> is a unique identifier
that the extension system generates for each extension.
You can see the IDs for all your loaded extensions
by going to the URL <b>chrome://extensions/</b>.
The <em>&lt;pathToFile></em> is the location of the file
under the extension's top directory;
it's the same as the relative URL.
</p>

<p>
For example, assume your extension has
the ID <b>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</b>
and the files shown in the following figure:
</p>

<pre>
<span class="tbd">[PENDING: someday this will be a real figure]</span>
toolstrip.html
styles.css
images:
  myimage.png
  ...
other:
  more.html
  morestyles.css
  ...
</pre>

<p>
Here's a table that shows the relative and absolute URLs of these files.
Keep in mind that the relative URL is the same no matter where you're using it &mdash;
the relative URL of <code>myimage.png</code> is
<b>images/myimage.png</b>,
no matter whether it's being used by
<code>toolstrip.html</code> or <code>other/more.html</code>.
</p>

<table>
<tr>
  <th> File </th>
  <th> Relative URL </th>
  <th> Absolute URL </th>
</tr>
<tr>
  <td> toolstrip.html </td>
  <td> toolstrip.html </td>
  <td> chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/toolstrip.html </td>
</tr>
<tr>
  <td> styles.css </td>
  <td> styles.css </td>
  <td> chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/styles.css </td>
</tr>
<tr>
  <td> myimage.png </td>
  <td> images/myimage.png </td>
  <td> chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/images/myimage.png </td>
</tr>
<tr>
  <td> more.html </td>
  <td> other/more.html </td>
  <td> chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/other/more.html </td>
</tr>
<tr>
  <td> morestyles.css </td>
  <td> other/morestyles.css </td>
  <td> chrome-extension://aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/other/morestyles.css </td>
</tr>
</table>

<p class="tbd">
[PENDING: Mention/reflect/link to
<a href="http://dev.chromium.org/developers/design-documents/extensions/i18n">internationalization</a>]
</p>


<h3>The manifest file</h3>

<p>
The manifest file, called <code>manifest.json</code>,
gives information about the extension,
such as the most important files
and the capabilities that the extension might use.
Here's a typical manifest file for a toolstrip
that uses information from google.com:
</p>

<pre>
{
  "name": "My Extension",
  "version": "2.1",
  "description": "Gets information from Google.",
  "update_url": "http://example.com/mytestextension/updates.xml",
  "permissions": ["http://*.google.com/", "https://*.google.com/"],
  "toolstrips": ["my_toolstrip.html"]
}</pre>

<p>
For details, see
<a href="manifest.html">Manifest Files</a>.
</p>

<h2 id="arch">Architecture</h2>

<p>
The following pictures shows the web pages associated with a typical extension
(in this case, a simple toolstrip).
The first figure shows what an extension might look like
when there's a single browser window.
</p>

<p class="tbd"> [PENDING: image goes here] </p>

<p>
The next figure shows that all the code in the extension's main HTML file
is duplicated each time you create a new window.
In other words, each window has its own widgets for the extension,
which means each window adds another web page for that extension.
</p>

<p class="tbd"> [PENDING: image goes here] </p>

<p>
To more easily coordinate all these web pages and to avoid duplication,
you should design your extension
so that most of the code &mdash;
especially the state of the extension &mdash;
is in a <em>background page</em>,
as shown in the following figure.
</p>

<p class="tbd"> [PENDING: image goes here] </p>


<h3 id="background">The background page</h3>
<p>
Any non-trivial extension can (and probably should)
have a background page.
The background page is an invisible page
where you put the main logic of the extension.
The extension's other pages should have
only the code that's necessary
to show the state of the extension
and to get input from the user.</p>

<p>
An extension's background page exists
before any of the extension's other pages exist.
It continues to exist as long as the browser is running
and the extension is installed,
even if other pages and windows go away.
</p>

<p>
Your extension's UI &mdash;
its toolstrips, page actions, and so on &mdash;
should be dumb views.
When the view needs some state,
it should request it from the background page.
When the background page notices some state change,
it should update all the views.
</p>

<p>
For more information, see
<a href="background_pages.html">Background Pages</a>.
</p>

<h3 id="contentScripts">Content scripts</h3>

<p>
If you want your extension to interact with web pages,
you need a content script.
Content scripts are JavaScript files that run in the context of web pages.
By using the standard Document Object Model (DOM),
they can read details of the web pages the browser visits,
and they can make changes to the pages.
</p>

<p class="comment">
[PENDING: add an architectural figure here,
showing the extension's pages, a web page, and the content script
interacting with the web page. explain the figure.]
</p>

<p>
For more information,
see <a href="content_scripts.html">Content Scripts</a>.
</p>


<h2>Communication </h2>

<p>
Two kinds of communication happen within an extension:
</p>

<ul>
  <li>
    Communication <b>between pages</b> in the extension.
    <br />
    For example, sometimes the background page
    needs to update all the UI pages
    to reflect a change in the extension's state.
  </li>
  <li>
    Communication <b>between content scripts and the extension</b>.
    <br />
    For example,
    <span class="tbd">[PENDING: typical use case goes here]</span>.
    See <a href="content_scripts.html">Content Scripts</a>
    for information about this type of communication.
  </li>
</ul>

<p>
Here are some keys to communication between an extension's pages:
</p>

<ul>
  <li>
    All the extension's pages execute in same process,
    on the same thread.
  </li>
  <li>
    Extension pages can use
    <a href="extension.html"><code>chrome.extension</code></a>
    methods such as
    <code>getViews()</code>,
    <code>getBackgroundPage()</code>,
    and <code>getToolstrips()</code>
    to get access to the extension's pages.
  </li>
  <li>
    Once you have references to an extension's pages,
    you can manipulate the DOM for each page.
  </li>
  <li>
    You can also make direct function calls to the pages,
    as shown in the following example.
  </li>
</ul>

<p>
Here's an example of 
communication between toolstrips and the background page.
</p>

<pre>
//In background_page.html:
function updateUI(checked) {
  var toolstrips = chrome.extension.getToolstrips();
  for (var i in toolstrips) {
    if (toolstrips[i].enableCheckbox)
      toolstrips[i].enableCheckbox(checked);
  }
}

//In toolstrip.html:
function enableCheckbox(checked) {
  var cb = document.getElementById('checkbox');
  cb.checked = checked;
}
</pre>

<p>
A good summary of communication mechanisms is at
<a href="http://www.chromeplugins.org/google/plugins-development/communication-7883.html">http://www.chromeplugins.org/google/plugins-development/communication-7883.html</a>.
</p>


<h2> Summary </h2>

<p class="tbd">
[
PENDING: summarize, suggest where to go next. Probably:
<a href="getstarted.html">Getting Started</a>,
<a href="tut_debugging.html">Debugging</a>,
<a href="devguide.html">Developer's Guide</a>.
]
</p>