templates/articles/messaging.html

<h1>Message Passing</h1>


<p>
Since content scripts run in the context of a web page and not the extension,
they often need some way of communicating with the rest of the extension. For
example, an RSS reader extension might use content scripts to detect the
presence of an RSS feed on a page, then notify the background page in order to
display a page action icon for that page.

<p>
Communication between extensions and their content scripts works by using
message passing. Either side can listen for messages sent from the other end,
and respond on the same channel. A message can contain any valid JSON object
(null, boolean, number, string, array, or object). There is a simple API for
<a href="#simple">one-time requests</a>
and a more complex API that allows you to have
<a href="#connect">long-lived connections</a>
for exchanging multiple messages with a shared context. It is also possible to
send a message to another extension if you know its ID, which is covered in
the
<a href="#external">cross-extension messages</a>
section.


<h2 id="simple">Simple one-time requests</h2>
<p>
If you only need to send a single message to another part of your extension
(and optionally get a response back), you should use the simplified
$ref:runtime.sendMessage
or
$ref:tabs.sendMessage
methods. This lets you send a one-time JSON-serializable message from a
content script to extension, or vice versa, respectively. An optional
callback parameter allows you handle the response from the other side, if
there is one.

<p>
Sending a request from a content script looks like this:
<pre data-filename="contentscript.js">
chrome.runtime.sendMessage({greeting: "hello"}, function(response) {
  console.log(response.farewell);
});
</pre>

<p>
Sending a request from the extension to a content script looks very similar,
except that you need to specify which tab to send it to. This example
demonstrates sending a message to the content script in the selected tab.
<pre data-filename="background.html">
chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
  chrome.tabs.sendMessage(tabs[0].id, {greeting: "hello"}, function(response) {
    console.log(response.farewell);
  });
});
</pre>

<p>
On the receiving end, you need to set up an
$ref:runtime.onMessage
event listener to handle the message. This looks the same from a content
script or extension page.
<pre>
chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    console.log(sender.tab ?
                "from a content script:" + sender.tab.url :
                "from the extension");
    if (request.greeting == "hello")
      sendResponse({farewell: "goodbye"});
  });
</pre>

<p class="note">
<b>Note:</b> If multiple pages are listening for onMessage events, only the
first to call sendResponse() for a particular event will succeed in sending the
response. All other responses to that event will be ignored.
</p>


<h2 id="connect">Long-lived connections</h2>
<p>
Sometimes it's useful to have a conversation that lasts longer than a single
request and response. In this case, you can open a long-lived channel from
your content script to an extension page, or vice versa, using
$ref:runtime.connect
or
$ref:tabs.connect respectively. The
channel can optionally have a name, allowing you to distinguish between
different types of connections.

<p>
One use case might be an automatic form fill extension. The content script
could open a channel to the extension page for a particular login, and send a
message to the extension for each input element on the page to request the
form data to fill in. The shared connection allows the extension to keep
shared state linking the several messages coming from the content script.

<p>
When establishing a connection, each end is given a
$ref:runtime.Port
object which is used for sending and receiving messages through that
connection.

<p>
Here is how you open a channel from a content script, and send and listen for
messages:
<pre data-filename="contentscript.js">
var port = chrome.runtime.connect({name: "knockknock"});
port.postMessage({joke: "Knock knock"});
port.onMessage.addListener(function(msg) {
  if (msg.question == "Who's there?")
    port.postMessage({answer: "Madame"});
  else if (msg.question == "Madame who?")
    port.postMessage({answer: "Madame... Bovary"});
});
</pre>

<p>
Sending a request from the extension to a content script looks very similar,
except that you need to specify which tab to connect to. Simply replace the
call to connect in the above example with
$ref:tabs.connect.

<p>
In order to handle incoming connections, you need to set up a
$ref:runtime.onConnect
event listener. This looks the same from a content script or an extension
page. When another part of your extension calls "connect()", this event is
fired, along with the
$ref:runtime.Port
object you can use to send and receive messages through the connection. Here's
what it looks like to respond to incoming connections:
<pre>
chrome.runtime.onConnect.addListener(function(port) {
  console.assert(port.name == "knockknock");
  port.onMessage.addListener(function(msg) {
    if (msg.joke == "Knock knock")
      port.postMessage({question: "Who's there?"});
    else if (msg.answer == "Madame")
      port.postMessage({question: "Madame who?"});
    else if (msg.answer == "Madame... Bovary")
      port.postMessage({question: "I don't get it."});
  });
});
</pre>

<p>
You may want to find out when a connection is closed, for example if you are
maintaining separate state for each open port. For this you can listen to the
$ref:runtime.Port.onDisconnect
event. This event is fired either when the other side of the channel manually
calls
$ref:runtime.Port.disconnect, or when the page
containing the port is unloaded (for example if the tab is navigated).
onDisconnect is guaranteed to be fired only once for any given port.


<h2 id="external">Cross-extension messaging</h2>
<p>
In addition to sending messages between different components in your
extension, you can use the messaging API to communicate with other extensions.
This lets you expose a public API that other extensions can take advantage of.

<p>
Listening for incoming requests and connections is similar to the internal
case, except you use the
$ref:runtime.onMessageExternal
or
$ref:runtime.onConnectExternal
methods. Here's an example of each:
<pre>
// For simple requests:
chrome.runtime.onMessageExternal.addListener(
  function(request, sender, sendResponse) {
    if (sender.id == blacklistedExtension)
      return;  // don't allow this extension access
    else if (request.getTargetData)
      sendResponse({targetData: targetData});
    else if (request.activateLasers) {
      var success = activateLasers();
      sendResponse({activateLasers: success});
    }
  });

// For long-lived connections:
chrome.runtime.onConnectExternal.addListener(function(port) {
  port.onMessage.addListener(function(msg) {
    // See other examples for sample onMessage handlers.
  });
});
</pre>

<p>
Likewise, sending a message to another extension is similar to sending one
within your extension. The only difference is that you must pass the ID of the
extension you want to communicate with. For example:
<pre>
// The ID of the extension we want to talk to.
var laserExtensionId = "abcdefghijklmnoabcdefhijklmnoabc";

// Make a simple request:
chrome.runtime.sendMessage(laserExtensionId, {getTargetData: true},
  function(response) {
    if (targetInRange(response.targetData))
      chrome.runtime.sendMessage(laserExtensionId, {activateLasers: true});
  });

// Start a long-running conversation:
var port = chrome.runtime.connect(laserExtensionId);
port.postMessage(...);
</pre>


<h2 id="external-webpage">Sending messages from web pages</h2>
<p>
Similar to <a href="#external">cross-extension messaging</a>,
your app or extension can receive and
respond to messages from regular web pages.
To use this feature, you must first
specify in your manifest.json which web sites you want to communicate with. For
example:

<pre data-filename="manifest.json">
"externally_connectable": {
  "matches": ["*://*.example.com/*"]
}
</pre>

<p>
This will expose the messaging API to any page which matches the URL patterns
you specify. The URL pattern must contain at least a
<a href="http://en.wikipedia.org/wiki/Second-level_domain">second-level domain</a>
- that is, hostname
patterns like "*", "*.com", "*.co.uk", and "*.appspot.com" are prohibited.
From the web page, use the
$ref:runtime.sendMessage
or
$ref:runtime.connect
APIs to send a message to a specific app or extension. For example:
<pre>
// The ID of the extension we want to talk to.
var editorExtensionId = "abcdefghijklmnoabcdefhijklmnoabc";

// Make a simple request:
chrome.runtime.sendMessage(editorExtensionId, {openUrlInEditor: url},
  function(response) {
    if (!response.success)
      handleError(url);
  });
</pre>

<p>
From your app or extension, you may listen to messages from web pages via the
$ref:runtime.onMessageExternal
or
$ref:runtime.onConnectExternal
APIs, similar to <a href="#external">cross-extension messaging</a>.
Only the web page can initiate a connection.
Here is an example:

<pre>
chrome.runtime.onMessageExternal.addListener(
  function(request, sender, sendResponse) {
    if (sender.url == blacklistedWebsite)
      return;  // don't allow this web page access
    if (request.openUrlInEditor)
      openUrl(request.openUrlInEditor);
  });
</pre>


<h2 id="native-messaging">Native messaging</h2>
<p>
Extensions can exchange messages with native applications. Native
applications that support this feature must register a <em>native messaging
host</em> that knows how to communicate with the extension. Chrome starts the
host in a separate process and communicates with it using standard input and
standard output streams.

<h3 id="native-messaging-host">Native messaging host</h3>
<p>
In order to register a native messaging host the application must install a
manifest file that defines the native messaging host configuration. Below is an
example of the manifest file:
<pre data-filename="manifest.json">
{
  "name": "com.my_company.my_application",
  "description": "My Application",
  "path": "C:\\Program Files\\My Application\\chrome_native_messaging_host.exe",
  "type": "stdio",
  "allowed_origins": [
    "chrome-extension://knldjmfmopnpolahpmmgbagdohdnhkik/"
  ]
}
</pre>

<p>Native messaging host manifest file contains the following fields:
<table class="simple">
  <tr>
    <th>Name</th>
    <th>Description</th>
  </tr>
  <tr>
    <td><code>name</code></td>
    <td>Name of the native messaging host. Clients pass this string to
    $ref:runtime.connectNative or $ref:runtime.sendNativeMessage.</td>
  </tr>
  <tr>
    <td><code>description</code></td>
    <td>Short application description.</td>
  </tr>
  <tr>
    <td><code>path</code></td>
    <td>Path to the native messaging host binary. On Linux and OSX the path must
    be absolute. On Windows it can be relative to the directory in which the
    manifest file is located.</td>
  </tr>
  <tr>
    <td><code>type</code></td>
    <td>Type of the interface used to communicate with the native messaging
    host. Currently there is only one possible value for this parameter:
    <code>stdio</code>. It indicates that Chrome should use <code>stdin</code>
    and <code>stdout</code> to communicate with the host.</td>
  </tr>
  <tr>
    <td><code>allowed_origins</code></td>
    <td>List of extensions that should have access to the native messaging host.</td>
  </tr>
</table>

<p>Location of the manifest file depends on the platform:

<dl>
  <dt>Windows:</dt>
    <dd>The manifest file can be located anywhere in the file system.
     The application installer must create registry key
     <code>HKEY_LOCAL_MACHINE\SOFTWARE\Google\Chrome\NativeMessagingHosts\<em>com.my_company.my_application</em></code>
     and set default value of that key to the full path to the manifest file.
    </dd>

  <dt>OSX:</dt>
    <dd>The manifest file must be placed at
    <code>/Library/Google/Chrome/NativeMessagingHosts/<em>com.my_company.my_application</em>.json</code>
    </dd>
  <dt>Linux:</dt>
    <dd>The manifest file must be placed at
    <code>/etc/opt/chrome/native-messaging-hosts/<em>com.my_company.my_application</em>.json</code>
    </dd>
</dl>

<p>
Chrome starts each native messaging host in a separate process and communicates
with it using standard input (<code>stdin</code>) and standard output
(<code>stdout</code>). The same format is used to send messages in both
directions: each message is serialized using JSON, UTF-8 encoded
and is preceded with 32-bit message length in native byte order.

<p>
When a messaging port is created using $ref:runtime.connectNative Chrome
starts native messaging host process and keeps it running until the port is
destroyed. On the other hand, when a message is sent using
$ref:runtime.sendNativeMessage, without creating a messaging port, Chrome starts
a new native messaging host process for each message. In that case the first
message generated by the host process is handled as a response to the original
request, i.e. Chrome will pass it to the response callback specified when
$ref:runtime.sendNativeMessage is called. All other messages generated by the
native messaging host in that case are ignored.

<h3 id="native-messaging-client">Connecting to a native application</h3>
<p>
Sending and receiving messages to and from a native application is very similar
to cross-extension messaging. The main difference is that
$ref:runtime.connectNative is used instead of $ref:runtime.connect,
and $ref:runtime.sendNativeMessage is used instead of $ref:runtime.sendMessage.

<p>
The Following example creates a $ref:runtime.Port object that's connected to native
messaging host <code>com.my_company.my_application</code>, starts listening for
messages from that port and sends one outgoing message:
<pre>
var port = chrome.runtime.connectNative('com.my_company.my_application');
port.onMessage.addListener(function(msg) {
  console.log("Received" + msg);
});
port.onDisconnect.addListener(function() {
  console.log("Disconnected");
});
port.postMessage({ text: "Hello, my_application" });
</pre>

<p>
$ref:runtime.sendNativeMessage can be used to send a message to native
application without creating a port, e.g.:
<pre>
chrome.runtime.sendNativeMessage('com.my_company.my_application',
  { text: "Hello" },
  function(response) {
    console.log("Received " + response);
  });
</pre>

<h2 id="security-considerations">Security considerations</h2>

<p>
When receiving a message from a content script or another extension, your
background page should be careful not to fall victim to <a
href="http://en.wikipedia.org/wiki/Cross-site_scripting">cross-site
scripting</a>.  Specifically, avoid using dangerous APIs such as the
below:
</p>
<pre data-filename="background.js">
chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // WARNING! Might be evaluating an evil script!
  var resp = eval("(" + response.farewell + ")");
});
</pre>
<pre data-filename="background.js">
chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // WARNING! Might be injecting a malicious script!
  document.getElementById("resp").innerHTML = response.farewell;
});
</pre>
<p>
Instead, prefer safer APIs that do not run scripts:
</p>
<pre data-filename="background.js">
chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // JSON.parse does not evaluate the attacker's scripts.
  var resp = JSON.parse(response.farewell);
});
</pre>
<pre data-filename="background.js">
chrome.tabs.sendMessage(tab.id, {greeting: "hello"}, function(response) {
  // innerText does not let the attacker inject HTML elements.
  document.getElementById("resp").innerText = response.farewell;
});
</pre>

<h2 id="examples">Examples</h2>

<p>
You can find simple examples of communication via messages in the
<a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/messaging/">examples/api/messaging</a>
directory.
<a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/api/nativeMessaging/">examples/api/nativeMessaging</a>
contains an example application that uses native messaging.
Also see the
<a href="http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/howto/contentscript_xhr">contentscript_xhr</a> example,
in which a content script and its parent extension exchange messages,
so that the parent extension can perform
cross-site requests on behalf of the content script.
For more examples and for help in viewing the source code, see
<a href="samples.html">Samples</a>.
</p>