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+<h1>Overview</h1>
+
+
+<p>
+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 and packaged apps.
+</p>
+
+<p class="caution">
+<strong>Note:</strong>
+<em>Packaged apps</em> are implemented as extensions,
+so unless otherwise stated,
+everything in this page applies to packaged apps.
+</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.
+Extensions are essentially web pages,
+and they can use all the
+<a href="api_other.html">APIs that the browser provides to web pages</a>,
+from XMLHttpRequest to JSON to HTML5.
+</p>
+
+<p>
+Extensions can interact with web pages or servers using
+<a href="content_scripts.html">content scripts</a> or
+<a href="xhr.html">cross-origin XMLHttpRequests</a>.
+Extensions can also interact programmatically
+with browser features such as
+<a href="bookmarks.html">bookmarks</a>
+and <a href="tabs.html">tabs</a>.
+</p>
+
+<h3 id="extension-ui">Extension UIs</h3>
+
+<p>
+Many extensions&mdash;but not packaged apps&mdash;add
+UI to Google Chrome in the form of
+<a href="browserAction.html">browser actions</a>
+or <a href="pageAction.html">page actions</a>.
+Each extension can have at most one browser action or page action.
+Choose a <b>browser action</b> when the extension is relevant to most pages.
+Choose a <b>page action</b> when the extension's icon
+should appear or disappear,
+depending on the page.
+</p>
+
+<table class="columns">
+<tr>
+  <td width="33%">
+    <img src="{{static}}/images/overview/browser-action.png"
+      width="147" height="100"
+      alt="screenshot" />
+  </td>
+  <td width="33%">
+    <img src="{{static}}/images/overview/page-action.png"
+      width="147" height="100"
+      alt="screenshot" />
+  </td>
+  <td>
+    <img src="{{static}}/images/overview/browser-action-with-popup.png"
+      width="147" height="100"
+      alt="screenshot" />
+  </td>
+</tr>
+
+<tr>
+  <td>
+    This <a href="samples.html#gmail">mail extension</a>
+    uses a <em>browser action</em>
+    (icon in the toolbar).
+  </td>
+  <td>
+    This <a href="samples.html#mappy">map extension</a>
+    uses a <em>page action</em>
+    (icon in the address bar)
+    and <em>content script</em>
+    (code injected into a web page).
+  </td>
+  <td>
+    This <a href="samples.html#news">news extension</a>
+    features a browser action that,
+    when clicked,
+    shows a <em>popup</em>.
+  </td>
+</tr>
+</table>
+
+<p>
+Extensions (and packaged apps) can also present a UI in other ways,
+such as adding to the Chrome context menu,
+providing an options page,
+or using a content script that changes how pages look.
+See the <a href="devguide.html">Developer's Guide</a>
+for a complete list of extension features,
+with links to implementation details
+for each one.
+</p>
+
+
+<h3 id="packagedapp-ui">Packaged app UIs</h3>
+
+<p>
+A packaged app usually presents its main functionality using
+an HTML page that's bundled into the app.
+For example, the following packaged app
+displays a Flash file within an HTML page.
+</p>
+
+<img src="{{static}}/images/overview/flash-app.png"
+  width="372" height="300"
+  alt="screenshot" />
+
+<p>
+For more information,
+see <a href="apps.html">Packaged Apps</a>.
+</p>
+
+<h2 id="files">Files</h2>
+<p>
+Each extension has the following files:
+
+</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 ZIP file
+that has a <code>.crx</code> suffix.
+If you upload your extension using the
+<a href="https://chrome.google.com/webstore/developer/dashboard">Chrome Developer Dashboard</a>,
+the <code>.crx</code> file is created for you.
+For details on distributing extensions,
+see <a href="hosting.html">Hosting</a>.
+</p>
+
+
+<h3 id="relative-urls">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>
+&lt;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>&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 folder;
+it's the same as the relative URL.
+</p>
+
+<p>
+While you're working on an extension
+(before it's packaged),
+the extension ID can change.
+Specifically, the ID of an unpacked extension will change
+if you load the extension from a different directory;
+the ID will change again when you package the extension.
+If your extension's code
+needs to specify the full path to a file within the extension,
+you can use the <code>@@extension_id</code>
+<a href="i18n.html#overview-predefined">predefined message</a>
+to avoid hardcoding the ID during development.
+</p>
+
+<p>
+When you package an extension
+(typically, by uploading it with the dashboard),
+the extension gets a permanent ID,
+which remains the same even after you update the extension.
+Once the extension ID is permanent,
+you can change all occurrences of
+<code>@@extension_id</code> to use the real ID.
+</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 browser action
+that uses information from google.com:
+</p>
+
+<pre>
+{
+  "name": "My Extension",
+  "version": "2.1",
+  "description": "Gets information from Google.",
+  "icons": { "128": "icon_128.png" },
+  "background": {
+    "scripts": ["bg.js"]
+  },
+  "permissions": ["http://*.google.com/", "https://*.google.com/"],
+  "browser_action": {
+    "default_title": "",
+    "default_icon": "icon_19.png",
+    "default_popup": "popup.html"
+  }
+}</pre>
+
+<p>
+For details, see
+<a href="manifest.html">Manifest Files</a>.
+</p>
+
+<h2 id="arch">Architecture</h2>
+
+<p>
+Many extensions have a <em>background page</em>,
+an invisible page
+that holds the main logic of the extension.
+An extension can also contain other pages
+that present the extension's UI.
+If an extension needs to interact with web pages that the user loads
+(as opposed to pages that are included in the extension),
+then the extension must use a content script.
+</p>
+
+
+<h3 id="background_page">The background page</h3>
+
+<p>
+The following figure shows a browser
+that has at least two extensions installed:
+a browser action (yellow icon)
+and a page action (blue icon).
+Both the browser action and the page action
+have background pages.
+This figure shows the browser action's background page,
+which is defined by <code>background.html</code>
+and has JavaScript code that controls
+the behavior of the browser action in both windows.
+</p>
+
+<img src="{{static}}/images/overview/arch-1.gif"
+ width="232" height="168"
+ alt="Two windows and a box representing a background page (background.html). One window has a yellow icon; the other has both a yellow icon and a blue icon. The yellow icons are connected to the background page." />
+
+<p>
+Although background pages can be useful,
+don't use one if you don't need it.
+Background pages are always open,
+so when a user installs many extensions that have background pages,
+Chrome's performance can suffer.
+</p>
+
+<!-- PENDING: Perhaps show a picture of many background page processes.
+  This could build on a figure that shows the process architecture,
+  and perhaps the differences between packaged apps and extensions. -->
+
+<p>
+Here are some examples of extensions that usually
+<em>do not need</em> a background page:
+</p>
+
+<ul>
+  <li> An extension with a browser action that
+    presents its UI solely through a popup
+    (and perhaps an options page).
+  </li>
+  <li>
+    An extension that provides an <em>override page</em>&mdash;a
+    page that replaces a standard Chrome page.
+  </li>
+  <li>
+    An extension with a content script
+    that <em>doesn't use</em> localStorage or
+    <a href="#apis">extension APIs</a>.
+  </li>
+  <li>
+    An extension that has no UI except for an options page.
+  </li>
+</ul>
+
+<p>
+See <a href="background_pages.html">Background Pages</a>
+for more details.
+</p>
+
+<h3 id="pages">UI pages</h3>
+
+<p>
+Extensions can contain ordinary HTML pages that display the extension's UI.
+For example, a browser action can have a popup,
+which is implemented by an HTML file.
+Any extension can have an options page,
+which lets users customize how the extension works.
+Another type of special page is the override page.
+And finally, you can
+use <a href="tabs.html#method-create">chrome.tabs.create()</a>
+or <code>window.open()</code>
+to display any other HTML files that are in the extension.
+</p>
+
+<p>
+The HTML pages inside an extension
+have complete access to each other's DOMs,
+and they can invoke functions on each other.
+</p>
+
+<!-- PENDING: Change the following example and figure
+to use something that's not a popup?
+(It might lead people to think that popups need background pages.) -->
+
+<p>
+The following figure shows the architecture
+of a browser action's popup.
+The popup's contents are a web page
+defined by an HTML file
+(<code>popup.html</code>).
+This extension also happens to have a background page
+(<code>background.html</code>).
+The popup doesn't need to duplicate code
+that's in the background page
+because the popup can invoke functions on the background page.
+</p>
+
+<img src="{{static}}/images/overview/arch-2.gif"
+ width="256" height="168"
+ alt="A browser window containing a browser action that's displaying a popup. The popup's HTML file (popup.html) can communicate with the extension's background page (background.html)." />
+
+<p>
+See <a href="browserAction.html">Browser Actions</a>,
+<a href="options.html">Options</a>,
+<a href="override.html">Override Pages</a>,
+and the <a href="#pageComm">Communication between pages</a> section
+for more details.
+</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>.
+A content script is some JavaScript
+that executes in the context of a page
+that's been loaded into the browser.
+Think of a content script as part of that loaded page,
+not as part of the extension it was packaged with
+(its <em>parent extension</em>).
+</p>
+
+<!-- [PENDING: Consider explaining that the reason content scripts are separated from the extension is due to chrome's multiprocess design.  Something like:
+
+Each extension runs in its own process.
+To have rich interaction with a web page, however,
+the extension must be able to
+run some code in the web page's process.
+Extensions accomplish this with content scripts.]
+-->
+
+<p>
+Content scripts can read details of the web pages the browser visits,
+and they can make changes to the pages.
+In the following figure,
+the content script
+can read and modify
+the DOM for the displayed web page.
+It cannot, however, modify the DOM of its parent extension's background page.
+</p>
+
+<img src="{{static}}/images/overview/arch-3.gif"
+ width="238" height="169"
+ alt="A browser window with a browser action (controlled by background.html) and a content script (controlled by contentscript.js)." />
+
+<p>
+Content scripts aren't completely cut off from their parent extensions.
+A content script can exchange messages with its parent extension,
+as the arrows in the following figure show.
+For example, a content script might send a message
+whenever it finds an RSS feed in a browser page.
+Or a background page might send a message
+asking a content script to change the appearance of its browser page.
+</p>
+
+<img src="{{static}}/images/overview/arch-cs.gif"
+ width="238" height="194"
+ alt="Like the previous figure, but showing more of the parent extension's files, as well as a communication path between the content script and the parent extension." />
+
+
+
+<p>
+For more information,
+see <a href="content_scripts.html">Content Scripts</a>.
+</p>
+
+
+<h2 id="apis"> Using the chrome.* APIs </h2>
+
+<p>
+In addition to having access to all the APIs that web pages and apps can use,
+extensions can also use Chrome-only APIs
+(often called <em>chrome.* APIs</em>)
+that allow tight integration with the browser.
+For example, any extension or web app can use the
+standard <code>window.open()</code> method to open a URL.
+But if you want to specify which window that URL should be displayed in,
+your extension can use the Chrome-only 
+<a href="tabs.html#method-create">chrome.tabs.create()</a>
+method instead.
+</p>
+
+<h3 id="sync"> Asynchronous vs. synchronous methods </h3>
+<p>
+Most methods in the chrome.* APIs are <b>asynchronous</b>:
+they return immediately, without waiting for the operation to finish.
+If you need to know the outcome of that operation,
+then you pass a callback function into the method.
+That callback is executed later (potentially <em>much</em> later),
+sometime after the method returns.
+Here's an example of the signature for an asynchronous method:
+</p>
+
+<p>
+<code>
+chrome.tabs.create(object <em>createProperties</em>, function <em>callback</em>)
+</code>
+</p>
+
+<p>
+Other chrome.* methods are <b>synchronous</b>.
+Synchronous methods never have a callback
+because they don't return until they've completed all their work.
+Often, synchronous methods have a return type.
+Consider the
+<a href="extension.html#method-getBackgroundPage">chrome.extensions.getBackgroundPage()</a> method:
+</p>
+
+<p>
+<code>
+Window chrome.extension.getBackgroundPage()
+</code>
+</p>
+
+<p>
+This method has no callback and a return type of <code>Window</code>
+because it synchronously returns the background page
+and performs no other, asynchronous work.
+</p>
+
+
+<h3 id="sync-example"> Example: Using a callback </h3>
+
+<p>
+Say you want to navigate
+the user's currently selected tab to a new URL.
+To do this, you need to get the current tab's ID
+(using <a href="tabs.html#method-getSelected">chrome.tabs.getSelected()</a>)
+and then make that tab go to the new URL
+(using <a href="tabs.html#method-update">chrome.tabs.update()</a>).
+</p>
+
+<p>
+If <code>getSelected()</code> were synchronous,
+you might write code like this:
+</p>
+
+<pre>
+   <b>//THIS CODE DOESN'T WORK</b>
+<span class="linenumber">1: </span>var tab = chrome.tabs.getSelected(null); <b>//WRONG!!!</b>
+<span class="linenumber">2: </span>chrome.tabs.update(tab.id, {url:newUrl});
+<span class="linenumber">3: </span>someOtherFunction();
+</pre>
+
+<p>
+That approach fails
+because <code>getSelected()</code> is asynchronous.
+It returns without waiting for its work to complete,
+and it doesn't even return a value
+(although some asynchronous methods do).
+You can tell that <code>getSelected()</code> is asynchronous
+by the <em>callback</em> parameter in its signature:
+
+<p>
+<code>
+chrome.tabs.getSelected(integer <em>windowId</em>, function <em>callback</em>)
+</code>
+</p>
+
+<p>
+To fix the preceding code,
+you must use that callback parameter.
+The following code shows
+how to define a callback function
+that gets the results from <code>getSelected()</code>
+(as a parameter named <code>tab</code>)
+and calls <code>update()</code>.
+</p>
+
+<pre>
+   <b>//THIS CODE WORKS</b>
+<span class="linenumber">1: </span>chrome.tabs.getSelected(null, <b>function(tab) {</b>
+<span class="linenumber">2: </span>  chrome.tabs.update(tab.id, {url:newUrl});
+<span class="linenumber">3: </span><b>}</b>);
+<span class="linenumber">4: </span>someOtherFunction();
+</pre>
+
+<p>
+In this example, the lines are executed in the following order: 1, 4, 2.
+The callback function specified to <code>getSelected</code> is called
+(and line 2 executed)
+only after information about the currently selected tab is available,
+which is sometime after <code>getSelected()</code> returns.
+Although <code>update()</code> is asynchronous,
+this example doesn't use its callback parameter,
+since we don't do anything about the results of the update.
+</p>
+
+
+<h3 id="chrome-more"> More details </h3>
+
+<p>
+For more information, see the
+<a href="api_index.html">chrome.* API docs</a>
+and watch this video:
+</p>
+
+<p>
+<iframe title="YouTube video player" width="640" height="390" src="http://www.youtube.com/embed/bmxr75CV36A?rel=0" frameborder="0" allowfullscreen></iframe>
+</p>
+
+<h2 id="pageComm">Communication between pages </h2>
+
+<p>
+The HTML pages within an extension often need to communicate.
+
+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> and
+<code>getBackgroundPage()</code>.
+Once a page has a reference to other pages within the extension,
+the first page can invoke functions on the other pages,
+and it can manipulate their DOMs.
+</p>
+
+
+
+
+<h2 id="incognito"> Saving data and incognito mode </h2>
+
+<p>
+Extensions can save data using
+the HTML5 <a href="http://dev.w3.org/html5/webstorage/">web storage API</a>
+(such as <code>localStorage</code>)
+or by making server requests that result in saving data.
+Whenever you want to save something,
+first consider whether it's
+from a window that's in incognito mode.
+By default, extensions don't run in incognito windows,
+and packaged apps <em>do</em>.
+You need to consider what a user expects
+from your extension or packaged app
+when the browser is incognito.
+</p>
+
+<p>
+<em>Incognito mode</em> promises that the window will leave no tracks.
+When dealing with data from incognito windows,
+do your best to honor this promise.
+For example, if your extension normally
+saves browsing history to the cloud,
+don't save history from incognito windows.
+On the other hand, you can store
+your extension's settings from any window,
+incognito or not.
+</p>
+
+<p class="note">
+<b>Rule of thumb:</b>
+If a piece of data might show where a user
+has been on the web or what the user has done,
+don't store it if it's from an incognito window.
+</p>
+
+<p>
+To detect whether a window is in incognito mode,
+check the <code>incognito</code> property of the relevant
+<a href="tabs.html#type-tabs.Tab">Tab</a> or
+<a href="windows.html#type-windows.Window">Window</a> object.
+For example:
+</p>
+
+<pre>
+var bgPage = chrome.extension.getBackgroundPage();
+
+function saveTabData(tab, data) {
+  if (tab.incognito) {
+    bgPage[tab.url] = data;       // Persist data ONLY in memory
+  } else {
+    localStorage[tab.url] = data; // OK to store data
+}
+</pre>
+
+
+<h2 id="now-what"> 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>
+  <li> <a href="http://www.youtube.com/view_play_list?p=CA101D6A85FE9D4B">Videos</a>,
+    such as
+    <a href="http://www.youtube.com/watch?v=B4M_a7xejYI&feature=PlayList&p=CA101D6A85FE9D4B&index=6">Extension Message Passing</a>
+    </li>
+</ul>