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<li jsinstance="0">
<a href="#what">The basics</a>
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<a>h3Name</a>
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</li><li jsinstance="1">
<a href="#files">Files</a>
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<li jsinstance="0">
<a href="#H3-2">Referring to files</a>
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<a href="#H3-3">The manifest file</a>
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<a href="#arch">Architecture</a>
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<a href="#contentScripts">Content scripts</a>
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<a href="#H2-7">Communication between pages </a>
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<a href="#methods">Methods</a>
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<a href="#method-anchor">methodName</a>
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<a href="#events">Events</a>
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<a href="#event-anchor">eventName</a>
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<!-- STATIC CONTENT PLACEHOLDER -->
<div id="static"><div id="pageData-title" class="pageData">Overview</div>
<div id="pageData-showTOC" class="pageData">true</div>
<p>
Read this page!
It has vital information about the extension architecture.
Once you've finished this page
and the
<a href="getstarted.html">Getting Started</a> tutorial,
you'll be all set to start writing extensions.
</p>
<h2 id="what">The basics</h2>
<p>
An extension is a zipped bundle of files —
HTML, CSS, JavaScript, images, and anything else you need —
that adds functionality to the Google Chrome browser.
Extensions are essentially web pages,
and they can use all the APIs
that the browser provides
to web pages,
from XMLHttpRequest to JSON to HTML5 local storage.
</p>
<p>
Many extensions add UI to Google Chrome,
in the form of
<a href="toolstrip.html">toolstrips</a> (toolbar additions),
<a href="browserAction.html">browser actions</a>,
or <a href="pageAction.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>.
To interact with web pages or servers,
extensions can use
<a href="content_scripts.html">content scripts</a> or
<a href="xhr.html">cross-origin XMLHttpRequests</a>.
</p>
<p>
See the <a href="devguide.html">Developer's Guide</a>
for a complete list of extension features,
with implementation details
for each one.
</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 —
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 ZIP file
that has a <code>.crx</code> suffix,
as described in <a href="packaging.html">Packaging</a>.
</p>
<a name="H3-2"></a><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 subfolder named <code>images</code>.
</p>
<pre><img <b>src="images/myimage.png"</b>>
</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><extensionID></em><b>/</b><em><pathToFile></em>
</blockquote>
<p>
In that URL, the <em><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><pathToFile></em> is the location of the file
under the extension's top folder;
it's the same as the relative URL.
</p>
<p class="comment">
[PENDING: Should mention/reflect/link to
<a href="http://dev.chromium.org/developers/design-documents/extensions/i18n">internationalization</a>
when it's ready.]
</p>
<a name="H3-3"></a><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>
Remember that extensions are essentially web pages.
The following figure shows what
a simple extension with a toolstrip
might look like
when there's a single browser window.
The toolstrip (T1) is at the bottom of the window.
The HTML and JavaScript code for T1
is in a web page
(<code>toolstrip.html</code>).
</p>
<img src="images/arch-1.gif">
<p>
Each time the user creates a new window,
the browser creates new toolstrips, page actions, and so on.
Each new bit of UI gets its own web page,
which means that toolstrips (for example)
that are in different windows can have different states.
In the following figure,
the two toolstrips (T1 and T2)
are associated with two separate web pages,
both of which contain code from <code>toolstrip.html</code>.
This means that although both toolstrips use the same code,
one can be blue while the other is green.
</p>
<img src="images/arch-2.gif">
<h3 id="background">The background page</h3>
<p>
If your extension's web pages need to communicate
or otherwise share information —
for example, if a browser event
can affect more than just one window's toolstrip —
the extension can use a background page.
The following figure shows a background page
(<code>background.html</code>)
that controls
the toolstrips in an extension.
</p>
<img src="images/arch-3.gif">
<p>
The background page is an invisible page
where you can 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>
For more information, see
<a href="background_pages.html">Background Pages</a>.
</p>
<h3 id="contentScripts">Content scripts</h3>
<p>
If your extension needs to interact with web pages,
then it needs a <em>content script</em>.
Content scripts are JavaScript files that run in the context of web pages.
Using the standard
<a href="http://www.w3.org/TR/DOM-Level-2-HTML/">Document Object Model</a>
(DOM),
content scripts can read details of the web pages the browser visits,
and they can make changes to the pages.
</p>
<p>
In the following figure,
the content script
can read and modify
the DOM for the web page displayed in Window 1.
If the toolstrip's UI needs to change
to reflect the web page's contents,
the content script can request that change
by sending a message to the parent extension.
</p>
<img src="images/arch-cs.gif">
<p class="comment">
[PENDING: Add overview of message passing.]
</p>
<p>
For more information,
see <a href="content_scripts.html">Content Scripts</a>.
</p>
<a name="H2-7"></a><h2>Communication between pages </h2>
<p>
The HTML pages within an extension often need to communicate.
An example is when the background page tells
UI pages (such as toolstrips)
to update their appearance.
Because all of an extension's pages
execute in same process on the same thread,
the pages can make direct function calls to each other.
</p>
<p>
To find pages in the extension, use
<a href="extension.html"><code>chrome.extension</code></a>
methods such as
<code>getViews()</code>,
<code>getBackgroundPage()</code>,
and <code>getToolstrips()</code>.
Once a page has a reference to other pages within the extension,
the first page can invoke functions on the other pages.
It can even manipulate their DOMs.
</p>
<p>
Here's an example of
communication between toolstrips and the background page.
</p>
<pre>//In background.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>
<a name="H2-8"></a><h2> Now what? </h2>
<p>
Now that you've been introduced to extensions,
you should be ready to write your own.
Here are some ideas for where to go next:
</p>
<ul>
<li> <a href="getstarted.html">Tutorial: Getting Started</a> </li>
<li> <a href="tut_debugging.html">Tutorial: Debugging</a> </li>
<li> <a href="devguide.html">Developer's Guide</a> </li>
<li> <a href="http://dev.chromium.org/developers/design-documents/extensions/samples">Samples</a> </li>
</ul>
</div>
<!-- API PAGE -->
<div class="apiPage" style="display: none; ">
<a name="apiReference"></a>
<h2>API reference: chrome.apiname </h2>
<!-- PROPERTIES -->
<div class="apiGroup">
<a name="properties"></a>
<h3 id="properties">Properties</h3>
<div>
<a></a>
<h4>getLastError</h4>
<div class="summary">
<!-- Note: intentionally longer 80 columns -->
<span>chrome.extension</span><span>lastError</span>
</div>
<div>
</div>
</div>
</div> <!-- /apiGroup -->
<!-- METHODS -->
<div class="apiGroup" id="methods">
<a name="methods"></a>
<h3>Methods</h3>
<!-- iterates over all functions -->
<div class="apiItem">
<a></a> <!-- method-anchor -->
<h4>method name</h4>
<div class="summary"><span>void</span>
<!-- Note: intentionally longer 80 columns -->
<span>chrome.module.methodName</span>(<span><span>, </span><span></span>
<var><span></span></var></span>)</div>
<div class="description">
<p class="todo">Undocumented.</p>
<p>
A description from the json schema def of the function goes here.
</p>
<!-- PARAMETERS -->
<h4>Parameters</h4>
<dl>
<div>
<div>
</div>
</div>
</dl>
<!-- RETURNS -->
<h4>Returns</h4>
<dl>
<div>
<div>
</div>
</div>
</dl>
<!-- CALLBACK -->
<div>
<div>
<h4>Callback function</h4>
<p>
If you specify the <em>callback</em> parameter,
it should specify a function that looks like this:
</p>
<!-- Note: intentionally longer 80 columns -->
<pre>function(<span>Type param1, Type param2</span>) <span class="subdued">{...}</span>);</pre>
<dl>
<div>
<div>
</div>
</div>
</dl>
</div>
</div>
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<!-- iterates over all events -->
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<h4>event name</h4>
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<span class="subdued">chrome.bookmarks</span><span>onEvent</span><span class="subdued">.addListener</span>(function(<span>Type param1, Type param2</span>) <span class="subdued">{...}</span>);
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<p class="todo">Undocumented.</p>
<p>
A description from the json schema def of the event goes here.
</p>
<!-- PARAMETERS -->
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<dl>
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</div>
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</div> <!-- /decription -->
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