Android Status Bar Notifications

A status bar notification adds an icon to the system's status bar (with an optional ticker-text message) and a notification message in the notifications window. When the user selects the notification, Android fires an Intent that is defined by theNotification (usually to launch an Activity). You can also configure the notification to alert the user with a sound, a vibration, and flashing lights on the device.

A status bar notification should be used for any case in which a background service needs to alert the user about an event that requires a response. A background service should never launch an activity on its own in order to receive user interaction. The service should instead create a status bar notification that will launch the activity when selected by the user.

Figure 1 shows the status bar with a notification icon on the left side.

Figure 1. Status bar with a notification.

Figure 2 shows the notification's message in the notifications window.

Figure 2. The notifications window.

The Basics

An Activity or Service can initiate a status bar notification. Because an activity can perform actions only while it is running in the foreground and its window has focus, you will usually create status bar notifications from a service. This way, the notification can be created from the background, while the user is using another application or while the device is asleep. To create a notification, you must use two classes: Notification and NotificationManager.

Use an instance of the Notification class to define the properties of your status bar notification, such as the status bar icon, the notification message, and extra settings such as a sound to play. The NotificationManager is an Android system service that executes and manages all status bar notifications. You do not instantiate the NotificationManagerdirectly. In order to give it your Notification, you must retrieve a reference to the NotificationManager withgetSystemService() and then, when you want to notify the user, pass it your Notification with notify().

To create a status bar notification:

1. Get a reference to the NotificationManager:

   String ns = Context.NOTIFICATION_SERVICE;
   NotificationManager mNotificationManager = (NotificationManager) getSystemService(ns);

2. Instantiate the Notification:

   int icon = R.drawable.notification_icon;
   CharSequence tickerText = "Hello";
   long when = System.currentTimeMillis();
   Notification notification = new Notification(icon, tickerText, when);

3. Define the notification's message and PendingIntent:

   Context context = getApplicationContext();
   CharSequence contentTitle = "My notification";
   CharSequence contentText = "Hello World!";
   Intent notificationIntent = new Intent(this, MyClass.class);
   PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
   
   notification.setLatestEventInfo(context, contentTitle, contentText, contentIntent);

4. Pass the Notification to the NotificationManager:

   private static final int HELLO_ID = 1;
   
   mNotificationManager.notify(HELLO_ID, notification);

   That's it. Your user has now been notified.

Managing your Notifications

The NotificationManager is a system service that manages all notifications. You must retrieve a reference to it with thegetSystemService() method. For example:

String ns = Context.NOTIFICATION_SERVICE;
NotificationManager mNotificationManager = (NotificationManager) getSystemService(ns);

When you want to deliver your status bar notification, pass the Notification to the NotificationManager withnotify(int, Notification). The first parameter is the unique ID for the notification and the second is theNotification object. The ID uniquely identifies the notification from within your application. The ID is necessary if you need to update the notification or (if your application manages different kinds of notifications) select the appropriate action when the user returns to your application via the intent defined in the notification.

To clear the status bar notification when the user selects it from the notifications window, add the "FLAG_AUTO_CANCEL" flag to your Notification. You can also clear it manually with cancel(int), passing it the notification ID, or clear all your notifications with cancelAll().

Creating a Notification

A Notification object defines the details of the notification message that is displayed in the status bar and notifications window, and any other alert settings, such as sounds and blinking lights.

A status bar notification requires all of the following:

• An icon for the status bar
• A title and message, unless you define a custom notification layout
• A PendingIntent, to be fired when the notification is selected

Optional settings for the status bar notification include:

• A ticker-text message for the status bar
• An alert sound
• A vibrate setting
• A flashing LED setting

The starter-kit for a new notification includes the Notification(int, CharSequence, long) constructor and thesetLatestEventInfo(Context, CharSequence, CharSequence, PendingIntent) method. These define all the required settings for a notification. The following snippet demonstrates a basic notification setup:

int icon = R.drawable.notification_icon;        // icon from resources
CharSequence tickerText = "Hello";              // ticker-text
long when = System.currentTimeMillis();         // notification time
Context context = getApplicationContext();      // application Context
CharSequence contentTitle = "My notification";  // message title
CharSequence contentText = "Hello World!";      // message text
Intent notificationIntent = new Intent(this, MyClass.class);
PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
// the next two lines initialize the Notification, using the configurations above
Notification notification = new Notification(icon, tickerText, when);
notification.setLatestEventInfo(context, contentTitle, contentText, contentIntent);

Updating the notification

You can update the information in your status bar notification as events continue to occur in your application. For example, when a new SMS text message arrives before previous messages have been read, the Messaging application updates the existing notification to display the total number of new messages received. This practice of updating an existing notification is much better than adding new notifications, because it avoids clutter in the notifications window.

Because each notification is uniquely identified by the NotificationManager with an integer ID, you can revise the notification by calling setLatestEventInfo() with new values, change some field values of the notification, and then callnotify() again.

You can revise each property with the object member fields (except for the Context and the notification title and text). You should always revise the text message when you update the notification by calling setLatestEventInfo() with new values for contentTitle and contentText. Then call notify() to update the notification. (Of course, if you've created a custom notification layout, then updating these title and text values has no effect.)

Adding a sound

You can alert the user with the default notification sound (which is defined by the user) or with a sound specified by your application.

To use the user's default sound, add "DEFAULT_SOUND" to the defaults field:

notification.defaults |= Notification.DEFAULT_SOUND;

To use a different sound with your notifications, pass a Uri reference to the sound field. The following example uses a known audio file saved to the device SD card:

notification.sound = Uri.parse("file:///sdcard/notification/ringer.mp3");

In the next example, the audio file is chosen from the internal MediaStore's ContentProvider:

notification.sound = Uri.withAppendedPath(Audio.Media.INTERNAL_CONTENT_URI, "6");

In this case, the exact ID of the media file ("6") is known and appended to the content Uri. If you don't know the exact ID, you must query all the media available in the MediaStore with a ContentResolver. See the Content Providersdocumentation for more information on using a ContentResolver.

If you want the sound to continuously repeat until the user responds to the notification or the notification is cancelled, addFLAG_INSISTENT to the flags field.

Note: If the defaults field includes DEFAULT_SOUND, then the default sound overrides any sound defined by the soundfield.

Adding vibration

You can alert the user with the the default vibration pattern or with a vibration pattern defined by your application.

To use the default pattern, add DEFAULT_VIBRATE to the defaults field:

notification.defaults |= Notification.DEFAULT_VIBRATE;

To define your own vibration pattern, pass an array of long values to the vibrate field:

long[] vibrate = {0,100,200,300};
notification.vibrate = vibrate;

The long array defines the alternating pattern for the length of vibration off and on (in milliseconds). The first value is how long to wait (off) before beginning, the second value is the length of the first vibration, the third is the next length off, and so on. The pattern can be as long as you like, but it can't be set to repeat.

Note: If the defaults field includes DEFAULT_VIBRATE, then the default vibration overrides any vibration defined by thevibrate field.

Adding flashing lights

To alert the user by flashing LED lights, you can implement the default light pattern (if available), or define your own color and pattern for the lights.

To use the default light setting, add DEFAULT_LIGHTS to the defaults field:

notification.defaults |= Notification.DEFAULT_LIGHTS;

To define your own color and pattern, define a value for the ledARGB field (for the color), the ledOffMS field (length of time, in milliseconds, to keep the light off), the ledOnMS (length of time, in milliseconds, to keep the light on), and also addFLAG_SHOW_LIGHTS to the flags field:

notification.ledARGB = 0xff00ff00;
notification.ledOnMS = 300;
notification.ledOffMS = 1000;
notification.flags |= Notification.FLAG_SHOW_LIGHTS;

In this example, the green light repeatedly flashes on for 300 milliseconds and turns off for one second. Not every color in the spectrum is supported by the device LEDs, and not every device supports the same colors, so the hardware estimates to the best of its ability. Green is the most common notification color.

More features

You can add several more features to your notifications using Notification fields and flags. Some useful features include the following:

FLAG_AUTO_CANCEL flag

Add this to the flags field to automatically cancel the notification after it is selected from the notifications window.

FLAG_INSISTENT flag

Add this to the flags field to repeat the audio until the user responds.

FLAG_ONGOING_EVENT flag

Add this to the flags field to group the notification under the "Ongoing" title in the notifications window. This indicates that the application is on-going — its processes are still running in the background, even when the application is not visible (such as with music or a phone call).

FLAG_NO_CLEAR flag

Add this to the flags field to indicate that the notification should not be cleared by the "Clear notifications" button. This is particularly useful if your notification is on-going.

number field

This value indicates the current number of events represented by the notification. The appropriate number is overlaid on top of the status bar icon. If you intend to use this field, then you must start with "1" when the Notification is first created. (If you change the value from zero to anything greater during an update, the number is not shown.)

iconLevel field

This value indicates the current level of a LevelListDrawable that is used for the notification icon. You can animate the icon in the status bar by changing this value to correlate with the drawable's defined in a LevelListDrawable. See theLevelListDrawable reference for more information.

See the Notification class reference for more information about additional features that you can customize for your application.

Creating a Custom Notification Layout

Figure 3. Notification with a custom layout.

By default, the notification that appears in the notifications window includes a title and the message text. These are defined by the contentTitle and contentTextparameters of the setLatestEventInfo() method. However, you can also define a custom layout for the notification using RemoteViews. Figure 3 shows an example of a custom notification layout. It looks similar to the default notification, but is actually created with a custom XML layout.

To define your own layout for the notification, instantiate a RemoteViews object that inflates a custom layout file, then pass the RemoteViews to the contentViewfield of your Notification.

Creating a custom notification layout is best understood with an example:

1. Create the XML layout for the notification. For example, the following layout is called custom_notification.xml:

   <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
       android:id="@+id/layout"
       android:layout_width="fill_parent"
       android:layout_height="fill_parent"
       android:padding="10dp" >
       <ImageView android:id="@+id/image"
           android:layout_width="wrap_content"
           android:layout_height="fill_parent"
           android:layout_alignParentLeft="true"
           android:layout_marginRight="10dp" />
       <TextView android:id="@+id/title"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:layout_toRightOf="@id/image"
           style="@style/NotificationTitle" />
       <TextView android:id="@+id/text"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:layout_toRightOf="@id/image"
           android:layout_below="@id/title"
           style="@style/NotificationText" />
   </RelativeLayout>

   Notice that the two TextView elements include the style attribute. It's important that you use style resources for the text in your custom notifications, because the background color of the notification can vary across different devices and platform versions. Beginning with Android 2.3 (API level 9), the system defines a style for the text it uses for the default notification layouts. Thus, you should apply that style when running on Android 2.3 or higher to ensure that your text is visible against the background.
   For example, to use the standard text colors on versions of Android lower than 2.3, you should use the following styles for res/values/styles.xml:

   <?xml version="1.0" encoding="utf-8"?>
   <resources>
       <style name="NotificationText">
         <item name="android:textColor">?android:attr/textColorPrimary</item>
       </style>
       <style name="NotificationTitle">
         <item name="android:textColor">?android:attr/textColorPrimary</item>
         <item name="android:textStyle">bold</item>
       </style>
       <!-- If you want a slightly different color for some text,
            consider using ?android:attr/textColorSecondary -->
   </resources>

   Then, to apply the system's default colors for notifications on Android 2.3 and higher, use the following styles forres/values-v9/styles.xml:

   <?xml version="1.0" encoding="utf-8"?>
   <resources>
       <style name="NotificationText" parent="android:TextAppearance.StatusBar.EventContent" />
       <style name="NotificationTitle" parent="android:TextAppearance.StatusBar.EventContent.Title" />
   </resources>

   Now, when running on Android 2.3 (API level 9) or higher, the text in your custom view will use the same colors that the system does for default notifications. This is important because later versions of Android actually change the background color of the notifications to be dark. Inheriting the system's styles ensures that your text will be light in such cases, but also if the background is some other unexpected color, your text will also change as appropriate.
2. Now, in the application code, use the RemoveViews methods to define the image and text. Then pass the RemoteViews object to the contentView field of the Notification, as shown in this example:

   RemoteViews contentView = new RemoteViews(getPackageName(), R.layout.custom_notification_layout);
   contentView.setImageViewResource(R.id.image, R.drawable.notification_image);
   contentView.setTextViewText(R.id.title, "Custom notification");
   contentView.setTextViewText(R.id.text, "This is a custom layout");
   notification.contentView = contentView;

   As shown here, pass the application's package name and the layout resource ID to the RemoteViews constructor. Then, define the content for the ImageView and TextView, using the setImageViewResource() andsetTextViewText(). In each case, pass the reference ID of the appropriate View object that you want to set, along with the value for that View. Finally, the RemoteViews object is passed to the Notification in the contentView field.
3. Because you don't need the setLatestEventInfo() method when using a custom view, you must define the Intent for the Notification with the contentIntent field, as in this example:

   Intent notificationIntent = new Intent(this, MyClass.class);
   PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
   notification.contentIntent = contentIntent;

4. The notification can now be sent as usual:

   mNotificationManager.notify(CUSTOM_VIEW_ID, notification);

The RemoteViews class also includes methods that you can use to easily add a Chronometer or ProgressBar in your notification's layout. For more information about creating custom layouts for your notification, refer to the RemoteViews class reference.

Caution: When creating a custom notification layout, you must take special care to ensure that your custom layout functions properly in different device orientations and resolutions. While this advice applies to all View layouts created on Android, it is especially important in this case because your layout real estate is very restricted. So don't make your custom layout too complex and be sure to test it in various configurations.