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Android Plugin Development Guide

This section provides details for how to implement native plugin code on the Android platform. Before reading this, see the Plugin Development Guide 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 is built from an Android WebView with a native bridge. The native portion of an Android plugin consists of at least one Java class that extends the CordovaPlugin class and overrides one of its execute methods.

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.

Plugin Initialization and Lifetime

One instance of a plugin object is created for the life of each WebView. Plugins are not instantiated until they are first referenced by a call from JavaScript, unless <param> with an onload name attribute is set to "true" in config.xml. For example,

<feature name="Echo">
    <param name="android-package" value="<full_name_including_namespace>" />
    <param name="onload" value="true" />
</feature>

Plugins should use the initialize method for their start-up logic.

@Override
public void initialize(CordovaInterface cordova, CordovaWebView webView) {
    super.initialize(cordova, webView);
    // your init code here
}

Plugins also have access to Android lifecycle events and can handle them by extending one of the provided methods (onResume, onDestroy, etc). Plugins with long-running requests, background activity such as media playback, listeners, or internal state should implement the onReset() method. It executes when the WebView navigates to a new page or refreshes, which reloads the JavaScript.

Writing an Android Java Plugin

A JavaScript call fires off a plugin request to the native side, and the corresponding 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 Activity's runOnUiThread method like so:

@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 wish to block the WebCore thread either, you should execute your code using the Cordova ExecutorService obtained with cordova.getThreadPool() like so:

@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;
}

Adding Dependency Libraries

If your Android plugin has extra dependencies, they must be listed in the plugin.xml in one of two ways.

The preferred way is to use the <framework /> tag (see the Plugin Specification for more details). Specifying libraries in this manner allows them to be resolved via Gradle's Dependency Management logic. This allows commonly used libraries such as gson, android-support-v4, and google-play-services to be used by multiple plugins without conflict.

The second option is to use the <lib-file /> tag to specify the location of a jar file (see the Plugin Specification for more details). This approach should only be used if you are sure that no other plugin will be depending on the library you are referencing (e.g. if the library is specific to your plugin). Otherwise, you risk causing build errors for users of your plugin if another plugin adds the same library. It is worth noting that Cordova app developers are not necessarily native developers, so native platform build errors can be especially frustrating.

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>

    <source-file src="src/android/Echo.java" target-dir="src/org/apache/cordova/plugin" />
</platform>

Then add the following to the src/android/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.

Android Permissions

Android permissions until recently have been handled at install-time instead of runtime. These permissions are required to be declared on an application that uses the permissions, and these permissions need to be added to the Android Manifest. This can be accomplished by using the config.xml to inject these permissions in the AndroidManifest.xml file. The example below uses the Contacts permission.

<config-file target="AndroidManifest.xml" parent="/*">
    <uses-permission android:name="android.permission.READ_CONTACTS" />
</config-file>

Runtime Permissions (Cordova-Android 5.0.0+)

Android 6.0 "Marshmallow" introduced a new permissions model where the user can turn on and off permissions as necessary. This means that applications must handle these permission changes to be future-proof, which was the focus of the Cordova-Android 5.0.0 release.

The permissions that need to be handled at runtime can be found in the Android Developer documentation here.

As far as a plugin is concerned, the permission can be requested by calling the permission method, which signature is as follows:

cordova.requestPermission(CordovaPlugin plugin, int requestCode, String permission);

To cut down on verbosity, it's standard practice to assign this to a local static variable:

public static final String READ = Manifest.permission.READ_CONTACTS;

It is also standard practice to define the requestCode as follows:

public static final int SEARCH_REQ_CODE = 0;

Then, in the exec method, the permission should be checked:

if(cordova.hasPermission(READ))
{
    search(executeArgs);
}
else
{
    getReadPermission(SEARCH_REQ_CODE);
}

In this case, we just call requestPermission:

protected void getReadPermission(int requestCode)
{
    cordova.requestPermission(this, requestCode, READ);
}

This will call the activity and cause a prompt to appear asking for the permission. Once the user has the permission, the result must be handled with the onRequestPermissionResult method, which every plugin should override. An example of this can be found below:

public void onRequestPermissionResult(int requestCode, String[] permissions,
                                         int[] grantResults) throws JSONException
{
    for(int r:grantResults)
    {
        if(r == PackageManager.PERMISSION_DENIED)
        {
            this.callbackContext.sendPluginResult(new PluginResult(PluginResult.Status.ERROR, PERMISSION_DENIED_ERROR));
            return;
        }
    }
    switch(requestCode)
    {
        case SEARCH_REQ_CODE:
            search(executeArgs);
            break;
        case SAVE_REQ_CODE:
            save(executeArgs);
            break;
        case REMOVE_REQ_CODE:
            remove(executeArgs);
            break;
    }
}

The switch statement above would return from the prompt and depending on the requestCode that was passed in, it would call the method. It should be noted that permission prompts may stack if the execution is not handled correctly, and that this should be avoided.

In addition to asking for permission for a single permission, it is also possible to request permissions for an entire group by defining the permissions array, as what is done with the Geolocation plugin:

String [] permissions = { Manifest.permission.ACCESS_COARSE_LOCATION, Manifest.permission.ACCESS_FINE_LOCATION };

Then when requesting the permission, all that needs to be done is the following:

cordova.requestPermissions(this, 0, permissions);

This requests the permissions specified in the array. It's a good idea to provide a publicly accessible permissions array since this can be used by plugins that use your plugin as a dependency, although this is not required.

Debugging Android Plugins

Android debugging can be done with either Eclipse or Android Studio, although Android studio is recommended. Since Cordova-Android is currently used as a library project, and plugins are supported as source code, it is possible to debug the Java code inside a Cordova application just like a native Android application.

Launching Other Activities

There are special considerations to be made if your plugin launches an Activity that pushes the Cordova Activity to the background. The Android OS will destroy Activities in the background if the device is running low on memory. In that case, the CordovaPlugin instance will be destroyed as well. If your plugin is waiting on a result from the Activity it launched, a new instance of your plugin will be created when the Cordova Activity is brought back to the foreground and the result is obtained. However, state for the plugin will not be automatically saved or restored and the CallbackContext for the plugin will be lost. There are two methods that your CordovaPlugin may implement to handle this situation:

/**
 * Called when the Activity is being destroyed (e.g. if a plugin calls out to an
 * external Activity and the OS kills the CordovaActivity in the background).
 * The plugin should save its state in this method only if it is awaiting the
 * result of an external Activity and needs to preserve some information so as
 * to handle that result; onRestoreStateForActivityResult() will only be called
 * if the plugin is the recipient of an Activity result
 *
 * @return  Bundle containing the state of the plugin or null if state does not
 *          need to be saved
 */
public Bundle onSaveInstanceState() {}

/**
 * Called when a plugin is the recipient of an Activity result after the
 * CordovaActivity has been destroyed. The Bundle will be the same as the one
 * the plugin returned in onSaveInstanceState()
 *
 * @param state             Bundle containing the state of the plugin
 * @param callbackContext   Replacement Context to return the plugin result to
 */
public void onRestoreStateForActivityResult(Bundle state, CallbackContext callbackContext) {}

It is important to note that the above methods should only be used if your plugin launches an Activity for a result and should only restore the state necessary to handle that Activity result. The state of the plugin will NOT be restored except in the case where an Activity result is obtained that your plugin requested using the CordovaInterface's startActivityForResult() method and the Cordova Activity was destroyed by the OS while in the background.

As part of onRestoreStateForActivityResult(), your plugin will be passed a replacement CallbackContext. It is important to realize that this CallbackContext IS NOT the same one that was destroyed with the Activity. The original callback is lost, and will not be fired in the javascript application. Instead, this replacement CallbackContext will return the result as part of the resume event that is fired when the application resumes. The payload of the resume event follows this structure:

{
    action: "resume",
    pendingResult: {
        pluginServiceName: string,
        pluginStatus: string,
        result: any
    }
}
  • pluginServiceName will match the name element from your plugin.xml.
  • pluginStatus will be a String describing the status of the PluginResult passed to the CallbackContext. See PluginResult.java for the String values that correspond to plugin statuses
  • result will be whatever result the plugin passes to the CallbackContext (e.g. a String, a number, a JSON object, etc.)

This resume payload will be passed to any callbacks that the javascript application has registered for the resume event. This means that the result is going directly to the Cordova application; your plugin will not have a chance to process the result with javascript before the application receives it. Consequently, you should strive to make the result returned by the native code as complete as possible and not rely on any javascript callbacks when launching activities.

Be sure to communicate how the Cordova application should interpret the result they receive in the resume event. It is up to the Cordova application to maintain their own state and remember what requests they made and what arguments they provided if necessary. However, you should still clearly communicate the meaning of pluginStatus values and what sort of data is being returned in the resume field as part of your plugin's API.

The complete sequence of events for launching an Activity is as follows

  1. The Cordova application makes a call to your plugin
  2. Your plugin launches an Activity for a result
  3. The Android OS destroys the Cordova Activity and your plugin instance
    • onSaveInstanceState() is called
  4. The user interacts with your Activity and the Activity finishes
  5. The Cordova Activity is recreated and the Activity result is received
    • onRestoreStateForActivityResult() is called
  6. onActivityResult() is called and your plugin passes a result to the new CallbackContext
  7. The resume event is fired and received by the Cordova application

Android provides a developer setting for debugging Activity destruction on low memory. Enable the "Don't keep activities" setting in the Developer Options menu on your device or emulator to simulate low memory scenarios. If your plugin launches external activities, you should always do some testing with this setting enabled to ensure that you are properly handling low memory scenarios.

© 2012–2017 The Apache Software Foundation
Licensed under the Apache License 2.0.
https://cordova.apache.org/docs/en/7.x/guide/platforms/android/plugin.html