.. -*- Mode: rst; fill-column: 80; -*-

============================
Interacting with Web content
============================

Interacting with Web content and WebExtensions
==============================================

GeckoView allows embedder applications to register and run
`WebExtensions <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions>`_
in a GeckoView instance. Extensions are the preferred way to interact
with Web content.

.. contents:: :local:

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 <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/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 <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings>`_
and
`version <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/version>`_
specified in the ``manifest.json`` file.

To install a bundled extension in GeckoView, simply call
`WebExtensionController.installBuiltIn <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtensionController.html#installBuiltIn(java.lang.String)>`_.

.. code:: java

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

Note that the lifetime of the extension is not tied with the lifetime of
the
`GeckoRuntime <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/GeckoRuntime.html>`_
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.

.. code:: java

   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 <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging#Exchanging_messages>`_
can be used for communicating to and from the browser.

- `runtime.sendNativeMessage <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendNativeMessage>`_
  for one-off messages.
- `runtime.connectNative <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/connectNative>`_
  for connection-based messaging.

Note: these APIs are only available when the ``geckoViewAddons``
`permission <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions>`_
is present in the ``manifest.json`` file of the extension.

One-off messages
~~~~~~~~~~~~~~~~

The easiest way to send messages from a `content
script <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts>`_
or a `background
script <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Anatomy_of_a_WebExtension#Background_scripts>`_
is using
`runtime.sendNativeMessage <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendNativeMessage>`_.

Note: Ordinarily, native extensions would use a `native
manifest <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging#App_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 <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html#setMessageDelegate(org.mozilla.geckoview.WebExtension.MessageDelegate,java.lang.String)>`_
on the
`WebExtension <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html>`_
object.

.. code:: java

   extension.setMessageDelegate(messageDelegate, "browser");

`SessionController.setMessageDelegate <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.SessionController.html#setMessageDelegate(org.mozilla.geckoview.WebExtension,org.mozilla.geckoview.WebExtension.MessageDelegate,java.lang.String)>`_
allows the app to receive messages from content scripts.

.. code:: java

   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.

.. code::

     "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 <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.MessageDelegate.html>`_
that will be used to communicate with Web Content.

You can find the full example here:
`MessagingExample <https://searchfox.org/mozilla-central/source/mobile/android/examples/messaging_example>`_.

Activity.java
^^^^^^^^^^^^^

.. code:: 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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code:: 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 <https://developer.mozilla.org/en-US/docs/Web/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 <#activityjava>`_.

/assets/messaging/messaging.js
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code:: javascript

   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 <#activityjava>`_.

.. code:: java

   @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

.. code:: javascript

   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 <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/connectNative>`_
is the appropriate API to use.

``connectNative`` returns a
`runtime.Port <https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/Port>`_
that can be used to send messages to the app. On the app side,
implementing
`MessageDelegate#onConnect <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.MessageDelegate.html#onConnect(org.mozilla.geckoview.WebExtension.Port)>`_
will allow you to receive a
`Port <https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.Port.html>`_
object that can be used to receive and send messages to the extension.

The following example can be found
`here <https://searchfox.org/mozilla-central/source/mobile/android/examples/port_messaging_example>`_.

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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. code:: javascript

   // 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 <#activityjava>`_ example,
``onConnect`` will be storing the ``Port`` object in a member variable
and then using it when needed.

.. code:: java

   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.

.. code:: java

   @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.

.. _GeckoRuntime: https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/GeckoRuntime.html
.. _runtime.sendNativeMessage: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/sendNativeMessage
.. _WebExtension: https://mozilla.github.io/geckoview/javadoc/mozilla-central/org/mozilla/geckoview/WebExtension.html
