Interacting with Web content¶

Interacting with Web content and WebExtensions¶

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

Running extensions in GeckoView
Communicating with Web Content
Connection-based messaging
- /assets/messaging/background.js

Running extensions in GeckoView ¶

Extensions bundled with applications can be provided in a folder in the /assets section of the APK. Like ordinary extensions, every extension bundled with GeckoView requires a manifest.json file.

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.

Note: Every installed extension will need an id and version specified in the manifest.json file.

To install a bundled extension in GeckoView, simply call WebExtensionController.installBuiltIn.

runtime.getWebExtensionController()
  .installBuiltIn("resource://android/assets/messaging/")

Note that the lifetime of the extension is not tied with the lifetime of the GeckoRuntime instance. The extension persists even when your app is restarted. Installing at every start up is fine, but it could be slow. To avoid installing multiple times you can use WebExtensionRuntime.ensureBuiltIn, which will only install if the extension is not installed yet.

runtime.getWebExtensionController()
  .ensureBuiltIn("resource://android/assets/messaging/", "messaging@example.com")
  .accept(
        extension -> Log.i("MessageDelegate", "Extension installed: " + extension),
        e -> Log.e("MessageDelegate", "Error registering WebExtension", e)
  );

Communicating with Web Content ¶

GeckoView allows bidirectional communication with Web pages through extensions.

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

runtime.sendNativeMessage for one-off messages.
runtime.connectNative for connection-based messaging.

Note: these APIs are only available when the geckoViewAddons permission is present in the manifest.json file of the extension.

One-off messages ¶

The easiest way to send messages from a content script or a background script is using runtime.sendNativeMessage.

Note: Ordinarily, native extensions would use a native manifest to define what native app identifier to use. For GeckoView this is not needed, the nativeApp parameter in setMessageDelegate will be use to determine what native app string is used.

Messaging Example ¶

To receive messages from the background script, call setMessageDelegate on the WebExtension object.

extension.setMessageDelegate(messageDelegate, "browser");

SessionController.setMessageDelegate allows the app to receive messages from content scripts.

session.getWebExtensionController()
    .setMessageDelegate(extension, messageDelegate, "browser");

Note: the "browser" parameter in the code above determines what native app id the extension can use to send native messages.

Note: extension can only send messages from content scripts if explicitly authorized by the app by adding nativeMessagingFromContent in the manifest.json file, e.g.

"permissions": [
  "nativeMessaging",
  "nativeMessagingFromContent",
  "geckoViewAddons"
]

Example ¶

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

You can find the full example here: MessagingExample.

Activity.java ¶

WebExtension.MessageDelegate messageDelegate = new WebExtension.MessageDelegate() {
    @Nullable
    public GeckoResult<Object> onMessage(final @NonNull String nativeApp,
                                         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;
    }
};

// Let's make sure the extension is installed
runtime.getWebExtensionController()
        .ensureBuiltIn(EXTENSION_LOCATION, "messaging@example.com").accept(
            // Set delegate that will receive messages coming from this extension.
            extension -> session.getWebExtensionController()
                    .setMessageDelegate(extension, messageDelegate, "browser"),
            // Something bad happened, let's log an error
            e -> Log.e("MessageDelegate", "Error registering extension", e)
        );

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

/assets/messaging/manifest.json ¶

{
  "manifest_version": 2,
  "name": "messaging",
  "version": "1.0",
  "description": "Example messaging web extension.",
  "browser_specific_settings": {
    "gecko": {
      "id": "messaging@example.com"
    }
  },
  "content_scripts": [
    {
      "matches": ["*://*.twitter.com/*"],
      "js": ["messaging.js"]
    }
  ],
  "permissions": [
    "nativeMessaging",
    "nativeMessagingFromContent",
    "geckoViewAddons"
  ]
}

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 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 Activity.java.

/assets/messaging/messaging.js ¶

let manifest = document.querySelector("head > link[rel=manifest]");
if (manifest) {
     fetch(manifest.href)
        .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.

@Nullable
public GeckoResult<Object> onMessage(final @NonNull String nativeApp,
                                     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 extension.

The type of message will be JSONObject when the extension sends a javascript object, but could also be a primitive type if the extension 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 extension, runtime.connectNative is the appropriate API to use.

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

The following example can be found here.

For this example, the extension 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.

/assets/messaging/background.js ¶

// 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;

@Override
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
            // extension 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
    // extension
    WebExtension.MessageDelegate messageDelegate = new WebExtension.MessageDelegate() {
        @Nullable
        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.
            mPort.setDelegate(portDelegate);
        }
    };

    runtime.getWebExtensionController()
        .ensureBuiltIn("resource://android/assets/messaging/", "messaging@example.com")
        .accept(
            // Register message delegate for background script
            extension -> extension.setMessageDelegate(messageDelegate, "browser"),
            e -> Log.e("MessageDelegate", "Error registering WebExtension", e)
        );

    // ... other
}

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

@Override
public boolean onKeyLongPress(int keyCode, KeyEvent event) {
    if (mPort == null) {
        // No extension 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);
    }

    mPort.postMessage(message);
    return true;
}

This allows bidirectional communication between the app and the extension.