Android Plugins

This section provides details for how to implement native plugin code on the Android platform. Before reading this, see Application Plugins for an overview of the plugin's structure and its common JavaScript interface. This section continues to demonstrate the sample echo plugin that communicates from the Cordova webview to the native platform and back. For another sample, see also the comments in CordovaPlugin.java.

Android plugins are based on Cordova-Android, which consists of an Android WebView with hooks attached to it. Plugins are represented as class mappings in the config.xml file. A plugin consists of at least one Java class that extends the CordovaPlugin class, overriding one of its execute methods. As best practice, the plugin should also handle pause and resume events, along with any message passing between plugins. Plugins with long-running requests, background activity such as media playback, listeners, or internal state should implement the onReset() method as well. It executes when the WebView navigates to a new page or refreshes, which reloads the JavaScript.

Plugin Class Mapping

The plugin's JavaScript interface uses the cordova.exec method as follows:

    exec(<successFunction>, <failFunction>, <service>, <action>, [<args>]);

This marshals a request from the WebView to the Android native side, effectively calling the action method on the service class, with additional arguments passed in the args array.

Whether you distribute a plugin as Java file or as a jar file of its own, the plugin must be specified in your Cordova-Android application's res/xml/config.xml file. See Application Plugins for more information on how to use the plugin.xml file to inject this feature element:

    <feature name="<service_name>">
        <param name="android-package" value="<full_name_including_namespace>" />
    </feature>

The service name matches the one used in the JavaScript exec call. The value is the Java class's fully qualified namespace identifier. Otherwise, the plugin may compile but still be unavailable to Cordova.

Writing an Android Java Plugin

A JavaScript call fires off a plugin request to the native side, and the correspoinding Java plugin is mapped properly in the config.xml file, but what does the final Android Java Plugin class look like? Whatever is dispatched to the plugin with JavaScript's exec function is passed into the plugin class's execute method. Most execute implementations look like this:

    @Override
    public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
        if ("beep".equals(action)) {
            this.beep(args.getLong(0));
            callbackContext.success();
            return true;
        }
        return false;  // Returning false results in a "MethodNotFound" error.
    }

The JavaScript exec function's action parameter corresponds to a private class method to dispatch with optional parameters.

When catching exceptions and returning errors, it's important for the sake of clarity that errors returned to JavaScript match Java's exception names as much as possible.

Threading

The plugin's JavaScript does not run in the main thread of the WebView interface; instead, it runs on the WebCore thread, as does the execute method. If you need to interact with the user interface, you should use the following variation:

    @Override
    public boolean execute(String action, JSONArray args, final CallbackContext callbackContext) throws JSONException {
        if ("beep".equals(action)) {
            final long duration = args.getLong(0);
            cordova.getActivity().runOnUiThread(new Runnable() {
                public void run() {
                    ...
                    callbackContext.success(); // Thread-safe.
                }
            });
            return true;
        }
        return false;
    }

Use the following if you do not need to run on the main interface's thread, but do not want to block the WebCore thread either:

    @Override
    public boolean execute(String action, JSONArray args, final CallbackContext callbackContext) throws JSONException {
        if ("beep".equals(action)) {
            final long duration = args.getLong(0);
            cordova.getThreadPool().execute(new Runnable() {
                public void run() {
                    ...
                    callbackContext.success(); // Thread-safe.
                }
            });
            return true;
        }
        return false;
    }

Echo Android Plugin Example

To match the JavaScript interface's echo feature described in Application Plugins, use the plugin.xml to inject a feature specification to the local platform's config.xml file:

    <platform name="android">
        <config-file target="config.xml" parent="/*">
            <feature name="Echo">
                <param name="android-package" value="org.apache.cordova.plugin.Echo"/>
            </feature>
        </config-file>
    </platform>

Then add the following to the src/org/apache/cordova/plugin/Echo.java file:

    package org.apache.cordova.plugin;

    import org.apache.cordova.CordovaPlugin;
    import org.apache.cordova.CallbackContext;

    import org.json.JSONArray;
    import org.json.JSONException;
    import org.json.JSONObject;

    /**
     * This class echoes a string called from JavaScript.
     */
    public class Echo extends CordovaPlugin {

        @Override
        public boolean execute(String action, JSONArray args, CallbackContext callbackContext) throws JSONException {
            if (action.equals("echo")) {
                String message = args.getString(0);
                this.echo(message, callbackContext);
                return true;
            }
            return false;
        }

        private void echo(String message, CallbackContext callbackContext) {
            if (message != null && message.length() > 0) {
                callbackContext.success(message);
            } else {
                callbackContext.error("Expected one non-empty string argument.");
            }
        }
    }

The necessary imports at the top of the file extends the class from CordovaPlugin, whose execute() method it overrides to receive messages from exec(). The execute() method first tests the value of action, for which in this case there is only one valid echo value. Any other action returns false and results in an INVALID_ACTION error, which translates to an error callback invoked on the JavaScript side.

Next, the method retrieves the echo string using the args object's getString method, specifying the first parameter passed to the method. After the value is passed to a private echo method, it is parameter-checked to make sure it is not null or an empty string, in which case callbackContext.error() invokes JavaScript's error callback. If the various checks pass, the callbackContext.success() passes the original message string back to JavaScript's success callback as a parameter.

Android Integration

Android features an Intent system that allows processes to communicate with each other. Plugins have access to a CordovaInterface object, which can access the Android Activity that runs the application. This is the Context required to launch a new Android Intent. The CordovaInterface allows plugins to start an Activity for a result, and to set the callback plugin for when the Intent returns to the application.

As of Cordova 2.0, Plugins can no longer directly access the Context, and the legacy ctx member is deprecated. All ctx methods exist on the Context, so both getContext() and getActivity() can return the required object.

Debugging Android Plugins

Eclipse allows you to debug plugins as Java source included in the project. Only the latest version of the Android Developer Tools allows you to attach source code to JAR dependencies, so this feature is not yet fully supported.