Chrome Devpanel Extension Communicating with Background Page

Question

I have an extension to the chrome devtools panel. I can send messages to the page using chrome.devtools.inspectedWindow.eval ... but how do I receive messages in the dev panel? Specifically, I need my devpanel to hook into events that happen on the page. I can't get it to listen to events on my content script, nor the background page.

I've tried chrome.extension.sendMessage in the content script, along with chrome.extension.onMessage.addListener in the dev panel script. But sendMessage complains with Port error: Could not establish connection. Receiving end does not exist.

The issue persists with long-lived connections:

In content script or background page:

var port = chrome.extension.connect({name: "test"});
port.postMessage({msg: "testing"});

In dev tools panel javascript:

chrome.extension.onConnect.addListener(function(port) {
    port.onMessage.addListener(function(msg) {
         // never gets here
    });
 });

How can I listen for events that are triggered in my content script-- in my dev tool panel? A diagram like this from Firefox's Add-On SDK would be great: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/static-files/media/content-scripting-overview.png

Answer 1

The goal is to create a channel ("port") for communication. It does not matter how the port is created, as long as the connection is correctly maintained.

The devtools script has to initiate the port, because the background script does not know when a devtools panel is created.

Here's a basic example, which shows a bidirectional communication method:

devtools.js

chrome.devtools.panels.create('Test', '/icon.png', '/panel.html', function(extensionPanel) {
    var _window; // Going to hold the reference to panel.html's `window`

    var data = [];
    var port = chrome.runtime.connect({name: 'devtools'});
    port.onMessage.addListener(function(msg) {
        // Write information to the panel, if exists.
        // If we don't have a panel reference (yet), queue the data.
        if (_window) {
            _window.do_something(msg);
        } else {
            data.push(msg);
        }
    });
    
    extensionPanel.onShown.addListener(function tmp(panelWindow) {
        extensionPanel.onShown.removeListener(tmp); // Run once only
        _window = panelWindow;

        // Release queued data
        var msg;
        while (msg = data.shift()) 
            _window.do_something(msg);
        // Just to show that it's easy to talk to pass a message back:
        _window.respond = function(msg) {
            port.postMessage(msg);
        };
    });
});

Now, the panel is capable of sending/receiving messages over a port. The panel's script (external script file, because of the CSP) may look like:

panel.js

function do_something(msg) {
    document.body.textContent += '\n' + msg; // Stupid example, PoC
}
document.documentElement.onclick = function() {
    // No need to check for the existence of `respond`, because
    // the panel can only be clicked when it's visible...
    respond('Another stupid example!');
};

Now, the background page's script:

background.js

var ports = [];
chrome.runtime.onConnect.addListener(function(port) {
    if (port.name !== "devtools") return;
    ports.push(port);
    // Remove port when destroyed (eg when devtools instance is closed)
    port.onDisconnect.addListener(function() {
        var i = ports.indexOf(port);
        if (i !== -1) ports.splice(i, 1);
    });
    port.onMessage.addListener(function(msg) {
        // Received message from devtools. Do something:
        console.log('Received message from devtools page', msg);
    });
});
// Function to send a message to all devtools.html views:
function notifyDevtools(msg) {
    ports.forEach(function(port) {
        port.postMessage(msg);
    });
}

To test, simply run notifyDevtools('Foo'); on the background page (e.g. via the console). In this demo, the message will be sent to all devtools. Upon receipt, the devtools panel will contain the received message.

Put the extension together using:

manifest.json

{
  "name": "Test",
  "manifest_version": 2,
  "version": "1",
  "devtools_page": "devtools.html",
  "background":{"scripts":["background.js"]}
}

panel.html

<script src="panel.js"></script> <!-- Doctype etc not added for conciseness-->

devtools.html

<script src="devtools.js"></script>

See also

• How to modify content under a devtools panel in a Chrome extension?
• chrome.devtools API
• Message passing: Long-lived connections
• Content Security Policy in Chrome extensions ("Inline JavaScript (...) will not be executed. This restriction bans both inline <script> blocks and inline event handlers.")