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.
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.
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:
targetSdkVersion to 22
or higher.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.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.
<application>
<uses-library android:name="com.google.android.wearable"
android:required="false" />
...
</application>
WAKE_LOCK
permission to the handheld and wearable app manifest:
<uses-permission android:name="android.permission.WAKE_LOCK" />
To enable ambient mode in your activity, use the
WearableActivity
class and methods.
WearableActivity.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();
...
}
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 that when your app runs on a device without a hardware button, palming the screen does not switch an app into ambient mode. Rather, it causes the app to exit and the home screen to appear. This behavior is intended to ensure that users can exit apps gracefully. However, these devices still go to ambient mode when the screen times out.
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);
}
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.
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
}
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:
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.
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();
}
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 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.
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();
}
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" />