Developing a Plugin on BlackBerry

How to make the Echo plugin on BlackBerry

In this article, we will explore how to develop the Echo plugin on BlackBerry. If you haven't read the top level article about the JavaScript part of the plugin, it would be best if you read that first and then this article. In addition, please download the Cordova BlackBerry repo.

To note, the Cordova-BlackBerry project allows you to deploy to BlackBerry devices like the Torch, Bold, etc and as well as the Playbook. There exists a distinction between deploying to normal BlackBerry hand held devices (ie, Torch and Bold) and the Playbook. The code base between the two are separate so when you develop for one, you have to duplicate your efforts for the other! Therefore in this article, the focus will be on the hand held devices and not the tablet. In the future, this guide should cover both platforms.

Continuing on from the previous article, the Echo plugin is essentially returning whatever message a user provides to the window.echo function.

The Echo function:

window.echo = function(str, callback) {
        cordova.exec(callback, function(err) {
            callback('Nothing to echo.');
        }, "Echo", "echo", [str]);
    };

Modifying plugins.xml

This file resides in your project's www folder and contains all of the references to the plugins that your Cordova project uses. We are going to add an additional reference so that when cordova.exec is called, Cordova will know how to map the "Echo" argument of cordova.exec to the Echo class that we want to write natively.

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

Adding Echo.java

If you notice the structure of the value attribute, you'll see a defined path that leads to the Echo plugin. In the root folder of the Cordova BlackBerry WebWorks repo, look for a folder called framework. This folder contains all of the source code that runs natively on the BlackBerry. cd into the folder structure until you reach the path: framework/ext/src/org/apache/cordova. At this point, you'll see all of the plugin folders and inside each folder is the plugins' source code. So, we will add the folder echo to framework/ext/src/org/apache/cordova/echo and create a file called Echo.java at framework/ext/src/org/apache/cordova/echo/Echo.java.

Writing Echo.java

The basic idea of writing a plugin is to create a class that extends the Plugin class and have a method called execute to return a PluginResult class. Any call to cordova.exec will pass in the action that we want to execute within the class as well as the arguments. In this case, "echo" is the action we want to execute within the class "Echo" and [str] are the arguments we are passing in.

package org.apache.cordova.echo;

import org.apache.cordova.api.Plugin;
import org.apache.cordova.api.PluginResult;
import org.apache.cordova.json4j.JSONArray;
import org.apache.cordova.json4j.JSONException;
import org.apache.cordova.json4j.JSONObject;
import org.apache.cordova.util.Logger;
/**
 * A simple plugin to demonstrate how to build a plugin for BlackBerry
 * Basically echos back the msg that a user calls to this plugin 
 */
public final class Echo extends Plugin {

    public static final String echo = "echo";

    public PluginResult execute(String action, JSONArray args, String callbackId) {
        PluginResult result = new PluginResult(PluginResult.Status.INVALID_ACTION, "Echo: Invalid action:" + action);
        if(action.equals(echo)){
            try {
                JSONObject echoObj = new JSONObject();
                String theMsg = args.getString(0);
                if(theMsg.length()>0){   
                    result = new PluginResult(PluginResult.Status.OK, theMsg);
                }else{
                    result = new PluginResult(PluginResult.Status.ERROR, theMsg);
                }
            } catch (JSONException e) {
                result = new PluginResult(PluginResult.Status.JSON_EXCEPTION, e.getMessage());
            }
        }

        return result;
    }

}

So if we look at the code above, we can see that within the execute method, we are first looking for what actions are coming in. The Echo plugin has only one action, "echo" so we will be only checking for that. If our plugin had more actions, it's simply a matter of adding more if-conditionals to check for those actions.

We are then going to grab the message coming in from the arguments which is supplied by the args parameter. We can grab the first argument by simply doing String theMsg = args.getString(0);.

We will do some error checking and if the message looks okay, we will instantiate a new PluginResult with an ok status: PluginResult.Status.OK and return the message: theMsg. After this, we will then return the result which will then pass back to JavaScript to be fired in the success callback. If something should fail, we can return various status exceptions like PluginResult.Status.ERROR, PluginResult.Status.JSON_EXCEPTION, or PluginResult.Status.INVALID_ACTION. When these types of results are passed back, they will fire the fail callback in JavaScript.

Updating the .jar in your project's www folder

The addition of the Echo.java needs to be updated in your project so to build the .jar file, cd to the root directory of the BlackBerry WebWorks repo. Use the ant command:

ant update -Dproject.path="~/path_to_my_project"

This will build a new .jar file in the build/ext folder. Copy the build/ext/cordova.jar file into your project/www/ext folder.

If all goes well, that should allow you to use the Echo plugin in BlackBerry.