Interacting with Web content

How to interact with Web content and register WebExtensions in GeckoView.

Interacting with Web content and WebExtensions

GeckoView allows embedder applications to register and run [WebExtensions][8] in a GeckoView instance. WebExtensions are the preferred way to interact with Web content.

Running WebExtensions in GeckoView

WebExtensions bundled with applications can be provided either in compressed .xpi files or in regular folders. Like ordinary WebExtensions, every WebExtension requires a [manifest.json][23] file.

To run a WebExtension in GeckoView, simply create a [WebExtension][3] object and register it in your [GeckoRuntime][24] instance.

WebExtension extension = new WebExtension(
    // The location where the web extension is installed, if the location is a
    // folder make sure the path ends with a "/" character.
    // This is the id and must be unique for every web extension
    // Extra flags can be specified here

// Run the WebExtension

Note that the lifetime of the WebExtension is tied with the lifetime of the [GeckoRuntime][24] instance. The WebExtension will need to be registered every time the runtime is created and will not persist once the runtime is closed.

To locate files bundled with the APK, GeckoView provides a shorthand resource://android/ that points to the root of the APK.

E.g. resource://android/assets/messaging/ will point to the /assets/messaging/ folder present in the APK.

Communicating with Web Content

GeckoView allows bidirectional communication with Web pages through WebExtensions.

When using GeckoView, [native messaging][11] can be used for communicating to and from the browser.

  • [runtime.sendNativeMessage][12] for one-off messages.

  • [runtime.connectNative][13] for connection-based messaging.

Note: these APIs are only available when the geckoViewAddons [permission][17] is present in the manifest.json file of the WebExtension.

One-off messages

The easiest way to send messages from a [content script][9] or a [background script][10] is using [runtime.sendNativeMessage][12]. The app will set up a message delegate on the same nativeApp that the WebExtension is using to send messages. In our example we will use the "browser" native app identifier.

To receive messages from the background script, call [setMessageDelegate][14] on the [WebExtension][3] object.

[GeckoSession.setMessageDelegate][16] allows the app to receive messages from content scripts.

Note: WebExtensions can only send messages from content scripts if explicitly authorized by the app setting [WebExtension.Flags.ALLOW_CONTENT_MESSAGING][22] in the [constructor][15].


Let’s set up an activity that registers a WebExtension located in the /assets/messaging/ folder of the APK. This activity will set up a [MessageDelegate][18] that will be used to communicate with Web Content.

You can find the full example here: [MessagingExample][20].

WebExtension.MessageDelegate messageDelegate = new WebExtension.MessageDelegate() {
    public GeckoResult<Object> onMessage(final @NonNull Object message,
                                         final @NonNull WebExtension.MessageSender sender) {
        // The sender object contains information about the session that
        // originated this message and can be used to validate that the message
        // has been sent from the expected location.

        // Be careful when handling the type of message as it depends on what
        // type of object was sent from the WebExtension script.
        if (message instanceof JSONObject) {
            // Do something with message
        return null;

WebExtension extension = new WebExtension(

// Run the WebExtension

// Set the delegate that will receive messages coming from this WebExtension.
session.setMessageDelegate(messageDelegate, "browser");

Now add the geckoViewAddons and nativeMessaging permissions to your manifest.json file.


  "manifest_version": 2,
  "name": "messaging",
  "version": "1.0",
  "description": "Example messaging web extension.",
  "content_scripts": [
      "matches": ["*://**"],
      "js": ["messaging.js"]
  "permissions": [

And finally, write a content script that will send a message to the app when a certain event occurs. For example, you could send a message whenever a [WPA manifest][19] is found on the page. Note that our nativeApp identifier used for sendNativeMessage is the same as the one used in the setMessageDelegate call in


let manifest = document.querySelector("head > link[rel=manifest]");
if (manifest) {
        .then(response => response.json())
        .then(json => {
             let message = {type: "WPAManifest", manifest: json};
             browser.runtime.sendNativeMessage("browser", message);

You can handle this message in the onMessage method in the messageDelegate above.

public GeckoResult<Object> onMessage(final @NonNull Object message,
                                     final @NonNull WebExtension.MessageSender sender) {
    if (message instanceof JSONObject) {
        JSONObject json = (JSONObject) message;
        try {
            if (json.has("type") && "WPAManifest".equals(json.getString("type"))) {
                JSONObject manifest = json.getJSONObject("manifest");
                Log.d("MessageDelegate", "Found WPA manifest: " + manifest);
        } catch (JSONException ex) {
            Log.e("MessageDelegate", "Invalid manifest", ex);
    return null;

Note that, in the case of content scripts, sender.session will be a reference to the GeckoSession instance from which the message originated. For background scripts, sender.session will always be null.

Also note that the type of message will depend on what was sent from the WebExtension.

The type of message will be JSONObject when the WebExtension sends a javascript object, but could also be a primitive type if the WebExtension sends one, e.g. for

runtime.browser.sendNativeMessage("browser", "Hello World!");

the type of message will be java.util.String.

Connection-based messaging

For more complex scenarios or for when you want to send messages from the app to the WebExtension, [runtime.connectNative][13] is the appropriate API to use.

connectNative returns a [runtime.Port][6] that can be used to send messages to the app. On the app side, implementing [MessageDelegate#onConnect][1] will allow you to receive a [Port][7] object that can be used to receive and send messages to the WebExtension.

The following example can be found [here][21].

For this example, the WebExtension side will do the following:

  • open a port on the background script using connectNative

  • listen on the port and log to console every message received

  • send a message immediately after opening the port.


// Establish connection with app
let port = browser.runtime.connectNative("browser");
port.onMessage.addListener(response => {
    // Let's just echo the message back
    port.postMessage(`Received: ${JSON.stringify(response)}`);
port.postMessage("Hello from WebExtension!");

On the app side, following the above example, onConnect will be storing the Port object in a member variable and then using it when needed.

private WebExtension.Port mPort;

protected void onCreate(Bundle savedInstanceState) {
    // ... initialize GeckoView

    // This delegate will handle all communications from and to a specific Port
    // object
    WebExtension.PortDelegate portDelegate = new WebExtension.PortDelegate() {
        public WebExtension.Port port = null;

        public void onPortMessage(final @NonNull Object message,
                                  final @NonNull WebExtension.Port port) {
            // This method will be called every time a message is sent from the
            // WebExtension through this port. For now, let's just log a
            // message.
            Log.d("PortDelegate", "Received message from WebExtension: "
                    + message);

        public void onDisconnect(final @NonNull WebExtension.Port port) {
            // After this method is called, this port is not usable anymore.
            if (port == mPort) {
                mPort = null;

    // This delegate will handle requests to open a port coming from the
    // WebExtension
    WebExtension.MessageDelegate messageDelegate = new WebExtension.MessageDelegate() {
        public void onConnect(final @NonNull WebExtension.Port port) {
            // Let's store the Port object in a member variable so it can be
            // used later to exchange messages with the WebExtension.
            mPort = port;

            // Registering the delegate will allow us to receive messages sent
            // through this port.

    WebExtension extension = new WebExtension(

    // Register message delegate for the background script
    extension.setMessageDelegate(messageDelegate, "browser");

    // ... other

For example, let’s send a message to the WebExtension every time the user long presses on a key on the virtual keyboard, e.g. on the back button.

public boolean onKeyLongPress(int keyCode, KeyEvent event) {
    if (mPort == null) {
        // No WebExtension registered yet, let's ignore this message
        return false;

    JSONObject message = new JSONObject();
    try {
        message.put("keyCode", keyCode);
        message.put("event", KeyEvent.keyCodeToString(event.getKeyCode()));
    } catch (JSONException ex) {
        throw new RuntimeException(ex);

    return true;

This allows bidirectional communication between the app and the WebExtension.

[1]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.MessageDelegate.html#onConnect-org.mozilla.geckoview.WebExtension.Port- [3]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html [6]: [7]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.Port.html [8]: [9]: [10]: [11]: [12]: [13]: [14]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html#setMessageDelegate-org.mozilla.geckoview.WebExtension.MessageDelegate-java.lang.String- [15]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html#WebExtension-java.lang.String-java.lang.String-long- [16]: ../javadoc/mozilla-central/org/mozilla/geckoview/GeckoSession.html#setMessageDelegate-org.mozilla.geckoview.WebExtension.MessageDelegate-java.lang.String- [17]: [18]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.MessageDelegate.html [19]: [20]: [21]: [22]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.Flags.html#ALLOW_CONTENT_MESSAGING [23]: [24]: {{ site.url }}{{ site.baseurl }}/javadoc/mozilla-central/org/mozilla/geckoview/GeckoRuntime.html