Keeping Your App Visible

Some Wear apps are most useful when they are constantly visible to the user. For example, users out on a run can glance at their wearable to see the distance covered and time elapsed, or after recording a grocery list on their wearable, users can quickly see which items are remaining on the list as they shop at the market. Making an app constantly visible has an impact on battery life, so you should carefully consider that impact when adding this feature to your app.

Android Wear devices running Android version 5.1 or higher allow apps to remain in the foreground while saving battery power. Android Wear apps can control what’s displayed on the wearable device screen while the device is in a low-power ambient mode. Wear apps that run in both ambient and interactive mode are called always-on apps.

This lesson describes how to enable your wearable app to be always-on, update the screen while in ambient mode, and maintain backwards compatibility.

Enable Ambient Mode in a Wearable App

For new and existing projects, you can add ambient mode support to your Wear app by updating your development project configuration. After you complete the project configuration, extend the WearableActivity class, which provides all the methods you need to enable ambient mode in your app. The following sections describe these steps in detail.

Configure your development project

In order to support ambient mode in your Wear app, you must update your Android SDK and configure your development project. Follow these steps to make the necessary changes:

• Update your SDK to include the Android 5.1 (API 22) or higher platform, which provides the APIs to allow activities to go into ambient mode. For more information on how to update your SDK, see Adding SDK Packages.
• Create a project or modify an existing project to target Android 5.1 or higher. This means you must set the manifest targetSdkVersion to 22 or higher.
• Set the manifest minSdkVersion to 20 or higher, if you want to support devices on versions prior to Android 5.1. For more information on backwards compatibility, see Maintain Backward-compatibility.
• Add or update the following dependencies to your build.gradle file:

  dependencies {
      ...
      compile 'com.google.android.support:wearable:1.2.0'
      provided 'com.google.android.wearable:wearable:1.0.0'
  }
  

  Note: The provided dependency ensures that the classes loaded at run-time to support ambient mode are also available at compile-time.

• Add the wearable shared library entry into the wearable app manifest:

  <application>
    <uses-library android:name="com.google.android.wearable"
                  android:required="false" />
    ...
  </application>
  

• Add the WAKE_LOCK permission to the handheld and wearable app manifest:

  <uses-permission android:name="android.permission.WAKE_LOCK" />
  

Create an activity that supports ambient mode

To enable ambient mode in your activity, use the WearableActivity class and methods.

1. Create an activity that extends WearableActivity.
2. In the onCreate() method of your activity, call the setAmbientEnabled() method.

Enable ambient mode in your activity as follows:

public class MainActivity extends WearableActivity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setAmbientEnabled();
    ...
}

Handle transitions between modes

If the user does not interact with your app for a period of time while it is displayed, or if the user covers the screen with their palm, the system switches the activity to ambient mode. After the app switches to ambient mode, update the activity UI to a more basic layout to reduce power consumption. You should use a black background with minimal white graphics and text. To ease a user into the transition from interactive to ambient mode, try to maintain similar placement of items on the screen. For more information on presenting content on an ambient screen, see the Watch Faces for Android Wear design guide.

Note: In ambient mode, disable any interactive elements on the screen, such as buttons. For more information on how to design user interactions for an always-on app, see the App Structure for Android Wear design guide.

When the activity switches to ambient mode, the system calls the onEnterAmbient() method in your wearable activity. The following code snippet shows how to change the text color to white and disable anti-aliasing after the system switches to ambient mode:

@Override
public void onEnterAmbient(Bundle ambientDetails) {
    super.onEnterAmbient(ambientDetails);

    mStateTextView.setTextColor(Color.WHITE);
    mStateTextView.getPaint().setAntiAlias(false);
}

When the user taps the screen or brings up their wrist, the activity switches from ambient mode to interactive mode. The system calls the onExitAmbient() method. Override this method to update the UI layout so that your app displays in a full-color, interactive state.

The following code snippet shows how to change the text color to green and enable anti-aliasing when the system switches to interactive mode:

@Override
public void onExitAmbient() {
    super.onExitAmbient();

    mStateTextView.setTextColor(Color.GREEN);
    mStateTextView.getPaint().setAntiAlias(true);
}

Update Content in Ambient Mode

Ambient mode allows you to update the screen with new information for the user, but you must carefully balance display updates against the battery life. You should strongly consider only overriding the onUpdateAmbient() method to update the screen once a minute in ambient mode. If your app requires more frequent updates, take into consideration that there is a trade-off between battery life and the frequency of updates. To realize battery savings, updates should be no more than once every 10 seconds. In practice, however, you should update your app less frequently than that.

Update once a minute

In order to preserve battery power, most wear apps should not frequently update the screen while in ambient mode. We recommend designing your app to update the screen once per minute while in this mode. The system provides a callback method, onUpdateAmbient(), that allows you to update the screen at this recommended frequency.

To update your app content, override the onUpdateAmbient() method in your wearable activity:

@Override
public void onUpdateAmbient() {
    super.onUpdateAmbient();

    // Update the content
}

Update more frequently

For apps that require more frequent updates, such as a fitness, time-keeping, and travel information apps, use an AlarmManager object to wake the processor and update the screen more frequently.

To implement an alarm that updates content more frequently in ambient mode, follow these steps:

1. Prepare the alarm manager.
2. Set the frequency of the updates.
3. Schedule the next update when the activity switches to ambient mode or is currently in ambient mode.
4. Cancel the alarm when the activity switches to interactive mode or the activity is stopped

Note: The alarm manager may create new instances of your activity as they are triggered. To prevent this situation, ensure that your activity is declared with the android:launchMode="singleInstance" parameter in the manifest.

The following sections describe these steps in detail.

Prepare the alarm manager

The alarm manager launches a pending intent that updates the screen and schedules the next alarm. The following example shows how to declare the alarm manager and the pending intent in the onCreate() method of your activity:

private AlarmManager mAmbientStateAlarmManager;
private PendingIntent mAmbientStatePendingIntent;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setAmbientEnabled();

    mAmbientStateAlarmManager =
        (AlarmManager) getSystemService(Context.ALARM_SERVICE);
    Intent ambientStateIntent =
        new Intent(getApplicationContext(), MainActivity.class);

    mAmbientStatePendingIntent = PendingIntent.getActivity(
        getApplicationContext(),
        0,
        ambientStateIntent,
        PendingIntent.FLAG_UPDATE_CURRENT);
    ...
}

When the alarm triggers and launches the pending intent, update the screen and schedule the next alarm by overriding the onNewIntent() method:

@Override
public void onNewIntent(Intent intent) {
    super.onNewIntent(intent);

    setIntent(intent);

    // Described in the following section
    refreshDisplayAndSetNextUpdate();
}

Update screen and schedule data updates

In this example activity, the alarm manager triggers every 20 seconds in ambient mode. When the timer ticks, the alarm triggers the intent to update the screen and then sets the delay for the next update.

The following example shows how to update information on the screen and set the alarm for the next update:

// Milliseconds between waking processor/screen for updates
private static final long AMBIENT_INTERVAL_MS = TimeUnit.SECONDS.toMillis(20);

private void refreshDisplayAndSetNextUpdate() {

    if (isAmbient()) {
        // Implement data retrieval and update the screen for ambient mode
    } else {
        // Implement data retrieval and update the screen for interactive mode
    }

    long timeMs = System.currentTimeMillis();

    // Schedule a new alarm
    if (isAmbient()) {
        // Calculate the next trigger time
        long delayMs = AMBIENT_INTERVAL_MS - (timeMs % AMBIENT_INTERVAL_MS);
        long triggerTimeMs = timeMs + delayMs;

        mAmbientStateAlarmManager.setExact(
            AlarmManager.RTC_WAKEUP,
            triggerTimeMs,
            mAmbientStatePendingIntent);

    } else {
        // Calculate the next trigger time for interactive mode
    }
}

Schedule the next alarm

Schedule the alarm to update the screen when the activity is entering ambient mode or when the activity is already in ambient mode by overriding the onEnterAmbient() method and the onUpdateAmbient() method:

@Override
public void onEnterAmbient(Bundle ambientDetails) {
    super.onEnterAmbient(ambientDetails);

    refreshDisplayAndSetNextUpdate();
}

@Override
public void onUpdateAmbient() {
    super.onUpdateAmbient();

    refreshDisplayAndSetNextUpdate();
}

Note: In this example, the refreshDisplayAndSetNextUpdate() method is called whenever the screen needs to be updated. For more examples of when to call this method, see the AlwaysOn sample.

Cancel the alarm

When the device switches to interactive mode, cancel the alarm in the onExitAmbient() method:

@Override
public void onExitAmbient() {
    super.onExitAmbient();

    mAmbientStateAlarmManager.cancel(mAmbientStatePendingIntent);
}

When the user exits or stops your activity, cancel the alarm in the onDestroy() method of your activity:

@Override
public void onDestroy() {
    mAmbientStateAlarmManager.cancel(mAmbientStatePendingIntent);
    super.onDestroy();
}

Maintain Backward-compatibility

Activities that support ambient mode automatically fall back to normal activities on Wear devices that are on Android versions prior to 5.1 (API level 22). No special app code is required to support devices on these versions of Android. When the device switches to ambient mode, the device returns to the home screen and exits your activity.

If your app should not be installed or updated on devices with Android versions prior to 5.1, update your manifest with the following:

<uses-library android:name="com.google.android.wearable" android:required="true" />