Developing a Plugin on Android

Writing a plugin requires an understanding of the architecture of Cordova-Android. Cordova-Android consists of an Android WebView with hooks attached to it. These plugins are represented as class mappings in the config.xml file.

A plugin will consist of at least a single Java class that extends the CordovaPlugin class. A plugin must override one of the execute methods from CordovaPlugin. In addition to this, there is a best practice that the plugin should handle pause and resume events, and should handle message passing between plugins. Plugins with long-running requests, background activity (e.g. playing media), listeners or internal state should implement the onReset() method as well. This method is run when the WebView navigates to a new page or refreshes, which reloads the Javascript.

Plugin Class Mapping

The JavaScript portion of a plugin always uses the cordova.exec method as follows:

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

This will marshal a request from the WebView to the Android native side, more or less boiling down to calling the action method on the service class, with the arguments passed in the args Array.

Whether you distribute your plugin as Java file or as a JAR of its own, the plugin must be added to the config.xml file in your Cordova-Android application's res/xml/ folder.

<plugin name="<service_name>" value="<full_name_including_namespace>"/>

The service name should match what you use in the JavaScript exec call, and the value will be the full name of the Java class including the namespace. Without this added, the plugin may compile but will not be reachable by Cordova.

Writing an Android Java Plugin

We have JavaScript to fire off a plugin request to the native side. We have the Android Java plugin mapped properly via the config.xml file. So what does the final Android Java Plugin class look like?

What gets dispatched to the plugin via JavaScript's exec function gets 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.
}

We compare the value of the action parameter, and dispatch the request off to a (private) method in the class, optionally passing some of the parameters to the method.

When catching exceptions and returning errors, it's important that the error we return to JavaScript match the Java exception as much as possible, for clarity.

Threading

JavaScript in the WebView does not run on the UI thread. It runs on the WebCore thread. The execute method also runs on the WebCore thread.

If you need to interact with the UI, you should use the following:

@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;
}

If you do not need to run on the UI thread, but do not want to block the WebCore thread:

@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

We would add the following to our config.xml:

<plugin name="Echo" value="org.apache.cordova.plugin.Echo" />

Then we would add the following file to src/org/apache/cordova/plugin/Echo.java inside our Cordova-Android application:

package org.apache.cordova.plugin;

import org.apache.cordova.api.CordovaPlugin;
import org.apache.cordova.api.PluginResult;
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.");
        }
    }
}

Let's take a look at the code. At the top we have all of the necessary Cordova imports. Our class extends from CordovaPlugin. We override the execute() method in order to recieve messages from exec(). Our method first compares against action: this plugin only supports one action, the echo action. Any other action will return false, which results in an error of type INVALID_ACTION - this will translate into an error callback invocation on the JavaScript side. Next, we grab the echo string using the getString method on our args, telling it we want to get the 0th parameter in the parameter array. We do a bit of parameter checking: make sure it is not null, and make sure it is not a zero-length string. If it is, we call callbackContext.error() (which, by now, you should know will invoke the error callback). If all of those checks pass, then we call callbackContext.success(), and pass in the message string we received as a parameter. This will finally translate into a success callback invocation on the JavaScript side. It will also pass the message parameter as a parameter into the JavaScript success callback function.

Debugging Plugins

Eclipse can be used to debug an Android project, and the plugins can be debugged if the Java source is included in the project. Only the latest version of the Android Dev Tools is known to allow source code attachment to JAR dependencies, this is not fully supported at this time.

Common Pitfalls

• Plugins have access to a CordovaInterface object. This object has access to the Android Activity that is running 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 comes back to the application. This is important, since the Intents system is how Android communicates between processes.
• Plugins do not have direct access to the Context as they have in the past. The legacy ctx member is deprecated, and will be removed six months after 2.0 is released. All the methods that ctx has exist on the Context, so both getContext() and getActivity() are capable of returning the proper object required.

Use the Source

One of the best ways to prepare yourself to write your own plugin is to have a look at other plugins that already exist.

You should also read through the comments in CordovaPlugin.java.