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Developing Android Widgets. This article describes how to create home screen widgets in Android.

1. Prerequisites

The following description assume that you already have experience in building standard Android application. Please see https://www.vogella.com/tutorials/Android/article.html - Android Tutorial. It also partly uses Android services. You find an introduction into Android Services in https://www.vogella.com/tutorials/AndroidServices/article.html - Android Service Tutorial.

2. Android Widgets

2.1. Overview about AppWidgets

2.2. Home screen widgets

Home screen widgets are broadcast receivers which provide interactive components. They are primarily used on the Android home screen. They typically display some kind of data and allow the user to perform actions with them. For example, a widget can display a short summary of new emails and if the user selects an email, it could start the email application with the selected email.

To avoid confusion with views (which are also called widgets), this text uses the term home screen widgets, if it speaks about widgets.

A widget runs as part of the process of its host. This requires that the widget preserves the permissions of their application.

Widget use RemoteViews to create their user interface. A RemoteView can be executed by another process with the same permissions as the original application. This way the widget runs with the permissions of its defining application.

The user interface for a Widget is defined by a broadcast receiver. This receiver inflates its layout into an object of type RemoteViews. This object is delivered to Android, which hands it over the home screen application.

2.3. Steps to create a Widget

To create a widget, you:

  • Define a layout file

  • Create an XML file (AppWidgetProviderInfo) which describes the properties of the widget, e.g. size or the fixed update frequency.

  • Create a BroadcastReceiver which is used to build the user interface of the widget.

  • Enter the Widget configuration in the AndroidManifest.xml file.

  • Optional you can specify a configuration activity which is called once a new instance of the widget is added to the widget host.

2.4. Widget size

Before Android 3.1 a widget always took a fixed amount of cells on the home screen. A cell is usually used to display the icon of one application. As a calculation rule you should define the size of the widget with the formula: ((Number of columns / rows) * 74) - 2. These are device independent pixels and the -2 is used to avoid rounding errors.

As of Android 3.1 a widget can be flexible in size, e.g., the user can make it larger or smaller. To enable this for widget, you can use the android:resizeMode="horizontal|vertical" attribute in the XML configuration file for the widget.

3. Creating the Broadcast receiver for the widget

3.1. Create and configure widget

To register a widget, you create a broadcast receiver with an intent filter for the android.appwidget.action.APPWIDGET_UPDATE action.

 <receiver
       android:icon="@drawable/icon"
       android:label="Example Widget"
       android:name="MyWidgetProvider" >
       <intent-filter >
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
       </intent-filter>

       <meta-data
          android:name="android.appwidget.provider"
          android:resource="@xml/widget_info" />
</receiver>

The receiver can get a label and icon assigned. These are used in the list of available widgets in the Android launcher.

You also specify the meta-data for the widget via the android:name="android.appwidget.provider attribute. The configuration file referred by this metadata contains the configuration settings for the widget. It contains, for example, the update interface, the size and the initial layout of the widget.

<?xml version="1.0" encoding="utf-8"?>
<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
    android:initialLayout="@layout/widget_layout"
    android:minHeight="72dp"
    android:minWidth="146dp"
    android:updatePeriodMillis="1800000" >

</appwidget-provider>

3.2. Available views and layouts

A widget is restricted in the View classes it can use. As layouts you can use the FrameLayout, LinearLayout and RelativeLayout classes. As views you can use AnalogClock, Button, Chromometer, ImageButton, ImageView, ProgressBar and TextView.

As of Android 3.0 more views are available: GridView, ListView, StackView, ViewFlipper and AdapterViewFlipper. These adapter views require that you define a collection view widget which is described later in this tutorial.

The only interaction that is possible with the views of a widget is via an OnClickListener event. This OnClickListener can be registered on a widget and is triggered by the user.

3.3. AppWidgetProvider

Your BroadcastReceiver implementation typically extends the AppWidgetProvider class.

The AppWidgetProvider class implements the onReceive() method, extracts the required information and calls the following widget life cycle methods.

As you can add several instances of a widget to the home screen, you have life cycle methods which are called only for the first instance added / removed to the home screen and others which are called for every instance of your widget.

Table 1. Life cycle method
Method Description

onEnabled()

Called the first time an instance of your widget is added to the home screen.

onDisabled()

Called once the last instance of your widget is removed from the home screen.

onUpdate()

Called for every update of the widget. Contains the ids of appWidgetIds for which an update is needed. Note that this may be all of the AppWidget instances for this provider, or just a subset of them, as stated in the method’s JavaDoc. For example, if more than one widget is added to the home screen, only the last one changes (until reinstall).

onDeleted()

Widget instance is removed from the home screen.

3.4. Receiver and asynchronous processing

A widget has the same runtime restrictions as a normal broadcast receiver, i.e., it has only 5 seconds to finish its processing.

A receive (widget) should therefore perform time consuming operations in a service and perform the update of the widgets from the service.

4. Widget updates

A widget gets its data on a periodic timetable. There are two methods to update a widget, one is based on an XML configuration file and the other is based on the Android AlarmManager service.

In the widget configuration file you can specify a fixed update interval. The system will wake up after this time interval and call your broadcast receiver to update the widget. The smallest update interval is 1800000 milliseconds (30 minutes).

The AlarmManager allows you to be more resource efficient and to have a higher frequency of updates. To use this approach, you define a service and schedule this service via the AlarmManager regularly. This service updates the widget.

Please note that a higher update frequency will wake up the phone from the energy safe mode. As a result your widget consumes more energy.

5. Exercise: widget with fixed update interval

5.1. Target

In the following tutorial you create a widget which displays a random number. This random number is updated every 30 minutes. You also register an OnClickListener so that the widgets updates once the user clicks on it.

The resulting widget will look like the following.

Screenshot of the final Widget of this tutorial

5.2. Create project and widget implementation

Create a new Android project called de.vogella.android.widget.example with an activity in the package de.vogella.android.widget.example.

Create a new file myshape.xml in the <filename class="directory">/res/drawable_ folder. This file defines the drawable used as background in the widget.

<?xml version="1.0" encoding="UTF-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <stroke
        android:width="2dp"
        android:color="#FFFFFFFF" />

    <gradient
        android:angle="225"
        android:endColor="#DD2ECCFA"
        android:startColor="#DD000000" />

    <corners
        android:bottomLeftRadius="7dp"
        android:bottomRightRadius="7dp"
        android:topLeftRadius="7dp"
        android:topRightRadius="7dp" />

</shape>

Define the following widget_layout.xml file under the <filename class="directory">res/layout_ folder.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="8dip"
    android:background="@drawable/myshape" >

    <TextView
        android:id="@+id/update"
        style="@android:style/TextAppearance.Medium"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="center"
        android:gravity="center_horizontal|center_vertical"
        android:layout_margin="4dip"
        android:text="Static Text" >
    </TextView>

</LinearLayout>

Create a new resource file called widget_info.xml via right click on the res folder and by selecting New  Android resource file.

Creating the widget provider 1
<?xml version="1.0" encoding="utf-8"?>
<appwidget-provider 
        xmlns:android="http://schemas.android.com/apk/res/android"
    android:initialLayout="@layout/widget_layout"
    android:minHeight="72dp"
    android:minWidth="300dp"
    android:updatePeriodMillis="300000" >

</appwidget-provider>

Create the following receiver class which is called during updates.

package de.vogella.android.widget.example;

import java.util.Random;

import android.app.PendingIntent;
import android.appwidget.AppWidgetManager;
import android.appwidget.AppWidgetProvider;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.util.Log;
import android.widget.RemoteViews;

public class MyWidgetProvider extends AppWidgetProvider {

    private static final String ACTION_CLICK = "ACTION_CLICK";

    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager,
            int[] appWidgetIds) {

        // Get all ids
        ComponentName thisWidget = new ComponentName(context,
                MyWidgetProvider.class);
        int[] allWidgetIds = appWidgetManager.getAppWidgetIds(thisWidget);
        for (int widgetId : allWidgetIds) {
            // create some random data
            int number = (new Random().nextInt(100));

            RemoteViews remoteViews = new RemoteViews(context.getPackageName(),
                    R.layout.widget_layout);
            Log.w("WidgetExample", String.valueOf(number));
            // Set the text
            remoteViews.setTextViewText(R.id.update, String.valueOf(number));

            // Register an onClickListener
            Intent intent = new Intent(context, MyWidgetProvider.class);

            intent.setAction(AppWidgetManager.ACTION_APPWIDGET_UPDATE);
            intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, appWidgetIds);

            PendingIntent pendingIntent = PendingIntent.getBroadcast(context,
                    0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
            remoteViews.setOnClickPendingIntent(R.id.update, pendingIntent);
            appWidgetManager.updateAppWidget(widgetId, remoteViews);
        }
    }
}

Open the AndroidManifest.xml and register your widget similar to the following listing.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.vogella.android.widget.example"
    android:versionCode="1"
    android:versionName="1.0" >

    <application
        android:icon="@drawable/icon"
        android:label="@string/app_name" >
        <receiver android:name="MyWidgetProvider" >
            <intent-filter >
                <action 
                    android:name="android.appwidget.action.APPWIDGET_UPDATE" />
            </intent-filter>

            <meta-data
                android:name="android.appwidget.provider"
                android:resource="@xml/widget_info" />
        </receiver>
    </application>

    <uses-sdk android:minSdkVersion="8" />

</manifest>

This attribute specifies that the AppWidgetProvider accepts the ACTION_APPWIDGET_UPDATE broadcast and specifies the metadata for the widget.

5.3. Validate

Deploy your application on your Android device. Once your application has been deployed use the Android launcher to install your new widget on the home screen and test it.

Installing the widget part 1
Installing the widget part 2

6. Collection View Widgets

Collection view widgets add support for the usage of the ListView, StackView and GridView classes in widgets.

For collection view widgets you need two layout files, one for the widget and one for each item in the widget collection.

The widget items are filled by an instance of the RemoteViewsFactory factory class.

This factory class is provided by an Android service which extends the RemoteViewsService class. This service requires the android.permission.BIND_REMOTEVIEWS permission.

To connect your views with the service, you use your onUpdate() method in your widget implementation.

You define an intent pointing to the service and use the setRemoteAdapter method on the RemoteViews class.

Intent intent = new Intent(context, YourRemoteViewsService.class);
intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId);
views.setRemoteAdapter(
    apppWidgetId,
    R.id.widget_your_id_to_collectionview,
    intent)

7. Enabling a app widget for the lock Screen

Since Android 4.2, it is possible to add home screen app widgets to the lock screen of an Android device. To enable your widget for the look screen you need to add keyguard category in the android:widgetCategory attribute in the AppWidgetProviderInfo XML file. The following code shows an example.

<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
android:widgetCategory="keyguard|home_screen"
    ...
>
...
</appwidget-provider>

In this example you declare a widget to support both - the home and the lock screens. If you recompile and launch your application now, you will be able to add the widget to the lock screen already.

You can also detect a widget category at runtime. For this, in the AppWidgetProvider.onUpdate() method, you can check for the category option of a widget with the following code.

Bundle options = appWidgetManager.getAppWidgetOptions(widgetId);

int category = options.getInt(AppWidgetManager.OPTION_APPWIDGET_HOST_CATEGORY, -1);
boolean isLockScreen = category == AppWidgetProviderInfo.WIDGET_CATEGORY_KEYGUARD;

Using this technique you can decide at runtime whether the widgets your application provides, will look differently, when they are hosted on the lock screen.

Similarly to how you used the android:initialLayout attribute for defining an initial layout for home screen widgets, you can use a new android:initialKeyguardLayout attribute for the lock screen in the AppWidgetProviderInfo XML file. This layout will appear immediately after a widget is added and will be replaced by the real layout once the widget is initialized.

8. Exercise: Update widget via a service

The following will demonstrate the usage of a service to update the widget.

Create the following UpdateWidgetService class in your project.

package de.vogella.android.widget.example;

import java.util.Random;

import android.app.PendingIntent;
import android.app.Service;
import android.appwidget.AppWidgetManager;
import android.content.ComponentName;
import android.content.Intent;
import android.os.IBinder;
import android.util.Log;
import android.widget.RemoteViews;

public class UpdateWidgetService extends Service {
    private static final String LOG = "de.vogella.android.widget.example";

    @Override
    public void onStart(Intent intent, int startId) {
        AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(this
                .getApplicationContext());

        int[] allWidgetIds = intent
                .getIntArrayExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS);

//      ComponentName thisWidget = new ComponentName(getApplicationContext(),
//              MyWidgetProvider.class);
//      int[] allWidgetIds2 = appWidgetManager.getAppWidgetIds(thisWidget);

        for (int widgetId : allWidgetIds) {
            // create some random data
            int number = (new Random().nextInt(100));

            RemoteViews remoteViews = new RemoteViews(this
                    .getApplicationContext().getPackageName(),
                    R.layout.widget_layout);
            Log.w("WidgetExample", String.valueOf(number));
            // Set the text
            remoteViews.setTextViewText(R.id.update,
                    "Random: " + String.valueOf(number));

            // Register an onClickListener
            Intent clickIntent = new Intent(this.getApplicationContext(),
                    MyWidgetProvider.class);

            clickIntent.setAction(AppWidgetManager.ACTION_APPWIDGET_UPDATE);
            clickIntent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS,
                    allWidgetIds);

            PendingIntent pendingIntent = PendingIntent.getBroadcast(
                    getApplicationContext(), 0, clickIntent,
                    PendingIntent.FLAG_UPDATE_CURRENT);
            remoteViews.setOnClickPendingIntent(R.id.update, pendingIntent);
            appWidgetManager.updateAppWidget(widgetId, remoteViews);
        }
        stopSelf();

        super.onStart(intent, startId);
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

Add this class as a Service to your AndroidManifest.xml file.

<service android:name=".UpdateWidgetService"></service>

Change MyWidgetProvider to the following. It will now only construct the service and start it.

package de.vogella.android.widget.example;

import android.appwidget.AppWidgetManager;
import android.appwidget.AppWidgetProvider;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.util.Log;

public class MyWidgetProvider extends AppWidgetProvider {

    private static final String LOG = "de.vogella.android.widget.example";

    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager,
            int[] appWidgetIds) {

        Log.w(LOG, "onUpdate method called");
        // Get all ids
        ComponentName thisWidget = new ComponentName(context,
                MyWidgetProvider.class);
        int[] allWidgetIds = appWidgetManager.getAppWidgetIds(thisWidget);

        // Build the intent to call the service
        Intent intent = new Intent(context.getApplicationContext(),
                UpdateWidgetService.class);
        intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, allWidgetIds);

        // Update the widgets via the service
        context.startService(intent);
    }
}

Once called, this service will update all widgets. You can click on one of the widgets to update all widgets.

9.1. Android Widget Resources

Example Widget from Android API demos

Wiktionary Example Widget

9.2. Android Resources

Introduction to Android Development

Background processing in Android

Android Service and BroadcastReceiver Tutorial

Android Location API and Google Maps

9.3. vogella Java example code

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