When you make a call to the Data Layer API, you can receive the status of the call when it completes. You also can listen for data events resulting from data changes that your app makes anywhere on the Wear OS by Google network.
| **Note:**The Data Layer API can only send messages and synchronize data with Android phones or Wear OS watches. If a Wear OS device is paired with an iOS device, the Data Layer API doesn't work.
|
For this reason, don't use the Data Layer API as the primary way to communicate with a network. Instead, follow the same pattern in your Wear OS app as in a mobile app, with some minor differences as described in[Network access and sync on Wear OS](https://developer.android.com/training/wearables/data-layer/network-access).  
For an example of effectively working with the Data Layer API, check out the[Android DataLayer Sample](https://github.com/android/wear-os-samples/tree/main/DataLayer)app.

## Wait for the status of Data Layer calls

Calls to the Data Layer API---such as a call using the`putDataItem`method of the[DataClient](https://developers.google.com/android/reference/com/google/android/gms/wearable/DataClient)class---sometimes return a[Task<ResultType>](https://developers.google.com/android/guides/tasks)object. As soon as the`Task`object is created, the operation is queued in the background. If you do nothing more after this, the operation eventually completes silently.

However, you usually want to do something with the result after the operation completes, so the`Task`object lets you wait for the result status, either asynchronously or synchronously.

### Asynchronous calls

If your code is running on the main UI thread, don't make blocking calls to the Data Layer API. Run the calls asynchronously by adding a callback method to the`Task`object, which fires when the operation is completed:  

### Kotlin

```kotlin
// Using Kotlin function references
task.addOnSuccessListener(::handleDataItem)
task.addOnFailureListener(::handleDataItemError)
task.addOnCompleteListener(::handleTaskComplete)
...
fun handleDataItem(dataItem: DataItem) { ... }
fun handleDataItemError(exception: Exception) { ... }
fun handleTaskComplete(task: Task<DataItem>) { ... }
```

### Java

```java
// Using Java 8 Lambdas.
task.addOnSuccessListener(dataItem -> handleDataItem(dataItem));
task.addOnFailureListener(exception -> handleDataItemError(exception));
task.addOnCompleteListener(task -> handleTaskComplete(task));
```

See the[Task API](https://developers.google.com/android/reference/com/google/android/gms/tasks/Task)for other possibilities, including chaining the execution of different tasks.

### Synchronous calls

If your code is running on a separate handler thread in a background service, such as in a[WearableListenerService](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html), it's fine for the calls to block. In this case, you can call[`Tasks.await()`](https://developers.google.com/android/reference/com/google/android/gms/tasks/Tasks.html#await(com.google.android.gms.tasks.Task<TResult>, long, java.util.concurrent.TimeUnit))on the`Task`object, which blocks until the request completes and returns a`Result`object. This is shown in the following example.

**Note:**Make sure not to call this while on the main thread.  

### Kotlin

```kotlin
try {
    Tasks.await(dataItemTask).apply {
        Log.d(TAG, "Data item set: $uri")
    }
}
catch (e: ExecutionException) { ... }
catch (e: InterruptedException) { ... }
```

### Java

```java
try {
    DataItem item = Tasks.await(dataItemTask);
    Log.d(TAG, "Data item set: " + item.getUri());
} catch (ExecutionException | InterruptedException e) {
  ...
}
```

## Listen for Data Layer events

Because the data layer synchronizes and sends data across the handheld and wearable devices, you usually need to listen for important events like data items being created and messages being received.

To listen for data layer events, you have two options:

- Create a service that extends[WearableListenerService](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html).
- Create an activity or class that implements the[DataClient.OnDataChangedListener](https://developers.google.com/android/reference/com/google/android/gms/wearable/DataClient.OnDataChangedListener)interface.

With both of these options, you override the data event callback methods for the events you are interested in handling.

**Note:** Consider your app's battery usage when choosing a listener implementation. A`WearableListenerService`is registered in the app's manifest and can launch the app if it is not already running. If you only need to listen for events when your app is already running, which is often the case with interactive applications, then don't use a`WearableListenerService`. Instead, register a live listener. For example, use the`addListener`method of the[DataClient](https://developers.google.com/android/reference/com/google/android/gms/wearable/DataClient)class. This can reduce the load on the system and reduce battery usage.

### Use a WearableListenerService

You typically create instances of[`WearableListenerService`](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService.html)in both your wearable and handheld apps. However, if you are not interested in data events in one of the apps, then you don't need to implement the service in that app.

For example, you can have a handheld app that sets and gets data item objects and a wearable app that listens for these updates to update its UI. The wearable app never updates any of the data items, so the handheld app doesn't listen for any data events from the wearable app.

Some of the events you can listen for using`WearableListenerService`are the following:

- [`onDataChanged()`](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService#onDataChanged(com.google.android.gms.wearable.DataEventBuffer)): whenever a data item object is created, deleted, or changed, the system triggers this callback on all connected nodes.
- [`onMessageReceived()`](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService#onMessageReceived(com.google.android.gms.wearable.MessageEvent)): a message sent from a node triggers this callback on the target node.
- [`onCapabilityChanged()`](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService#onCapabilityChanged(com.google.android.gms.wearable.CapabilityInfo)): when a capability that an instance of your app advertises becomes available on the network, that event triggers this callback. If you're looking for a nearby node, you can query the[`isNearby()`](https://developers.google.com/android/reference/com/google/android/gms/wearable/Node.html#isNearby())method of the nodes provided in the callback.

You can also listen for events from[ChannelClient.ChannelCallback](https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelClient.ChannelCallback), such as`onChannelOpened()`.

All the preceding events are executed in a background thread, not on the main thread.

To create a`WearableListenerService`, follow these steps:

1. Create a class that extends`WearableListenerService`.
2. Listen for the events that you're interested in, such as`onDataChanged()`.
3. Declare an intent filter in your Android manifest to notify the system about your`WearableListenerService`. This declaration lets the system bind your service as needed.

The following example shows how to implement a simple`WearableListenerService`:  

### Kotlin

```kotlin
private const val TAG = "DataLayerSample"
private const val START_ACTIVITY_PATH = "/start-activity"
private const val DATA_ITEM_RECEIVED_PATH = "/data-item-received"

class DataLayerListenerService : WearableListenerService() {

    override fun onDataChanged(dataEvents: DataEventBuffer) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "onDataChanged: $dataEvents")
        }

        // Loop through the events and send a message
        // to the node that created the data item.
        dataEvents.map { it.dataItem.uri }
                .forEach { uri ->
                    // Get the node ID from the host value of the URI.
                    val nodeId: String = uri.host
                    // Set the data of the message to be the bytes of the URI.
                    val payload: ByteArray = uri.toString().toByteArray()

                    // Send the RPC.
                    Wearable.getMessageClient(this)
                            .sendMessage(nodeId, DATA_ITEM_RECEIVED_PATH, payload)
                }
    }
}
```

### Java

```java
public class DataLayerListenerService extends WearableListenerService {
    private static final String TAG = "DataLayerSample";
    private static final String START_ACTIVITY_PATH = "/start-activity";
    private static final String DATA_ITEM_RECEIVED_PATH = "/data-item-received";

    @Override
    public void onDataChanged(DataEventBuffer dataEvents) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "onDataChanged: " + dataEvents);
        }

        // Loop through the events and send a message
        // to the node that created the data item.
        for (DataEvent event : dataEvents) {
            Uri uri = event.getDataItem().getUri();

            // Get the node ID from the host value of the URI.
            String nodeId = uri.getHost();
            // Set the data of the message to be the bytes of the URI.
            byte[] payload = uri.toString().getBytes();

            // Send the RPC.
            Wearable.getMessageClient(this).sendMessage(
                  nodeId,  DATA_ITEM_RECEIVED_PATH, payload);
        }
    }
}
```

The following section explains how to use an intent filter with this listener.

#### Use filters with WearableListenerService

An intent filter for the`WearableListenerService`example shown in the previous section might look like this:  

```xml
<service android:name=".DataLayerListenerService" android:exported="true" tools:ignore="ExportedService" >
  <intent-filter>
      <action android:name="com.google.android.gms.wearable.DATA_CHANGED" />
      <data android:scheme="wear" android:host="*"
               android:path="/start-activity" />
  </intent-filter>
</service>
```

In this filter, the`DATA_CHANGED`action replaces the previously recommended`BIND_LISTENER`action so that only specific events wake or launch your app. This change improves system efficiency and reduces battery consumption and other overhead associated with your app. In this example, the watch listens for the`/start-activity`data item, and the phone listens for the`/data-item-received`message response.

Standard Android filter matching rules apply. You can specify multiple services per manifest, multiple intent filters per service, multiple actions per filter, and multiple data stanzas per filter. Filters can match on a wildcard host or on a specific one. To match on a wildcard host, use`host="*"`. To match on a specific host, specify`host=<node_id>`.

You can also match a literal path or path prefix. To do this, you must specify a wildcard or specific host. Otherwise, the system ignores the path you specify.

For more information about the filter types that Wear OS supports, see the API reference documentation for[`WearableListenerService`](https://developers.google.com/android/reference/com/google/android/gms/wearable/WearableListenerService).

For more information on data filters and matching rules, see the API reference documentation for the[`<data>`](https://developer.android.com/guide/topics/manifest/data-element)manifest element.

When matching intent filters, remember two important rules:

- If no scheme is specified for the intent filter, the system ignores all the other URI attributes.
- If no host is specified for the filter, the system ignores all the path attributes.

### Use a live listener

If your app only cares about data-layer events when the user is interacting with the app, it may not need a long-running service to handle every data change. In such a case, you can listen for events in an activity by implementing one or more of the following interfaces:

- [DataClient.OnDataChangedListener](https://developers.google.com/android/reference/com/google/android/gms/wearable/DataClient.OnDataChangedListener)
- [MessageClient.OnMessageReceivedListener](https://developers.google.com/android/reference/com/google/android/gms/wearable/MessageClient.OnMessageReceivedListener)
- [CapabilityClient.OnCapabilityChangedListener](https://developers.google.com/android/reference/com/google/android/gms/wearable/CapabilityClient.OnCapabilityChangedListener)
- [ChannelClient.ChannelCallback](https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelClient.ChannelCallback)

To create an activity that listens for data events, do the following:

1. Implement the desired interfaces.
2. In the`onCreate()`or`onResume()`method, call`Wearable.getDataClient(this).addListener()`,`MessageClient.addListener()`,`CapabilityClient.addListener()`, or`ChannelClient.registerChannelCallback()`to notify Google Play services that your activity is interested in listening for data layer events.
3. In[onStop()](https://developer.android.com/reference/android/app/Activity.html#onStop())or[onPause()](https://developer.android.com/reference/android/app/Activity.html#onPause()), unregister any listeners with`DataClient.removeListener()`,`MessageClient.removeListener()`,`CapabilityClient.removeListener()`, or`ChannelClient.unregisterChannelCallback()`.
4. If an activity is only interested in events with a specific path prefix, you can add a listener with a suitable prefix filter to only receive data that is relevant to the current application state.
5. Implement`onDataChanged()`,`onMessageReceived()`,`onCapabilityChanged()`, or methods from[ChannelClient.ChannelCallback](https://developers.google.com/android/reference/com/google/android/gms/wearable/ChannelClient.ChannelCallback), depending on the interfaces that you implemented. These methods are called on the main thread, or you can specify a custom`Looper`using[WearableOptions](https://developers.google.com/android/reference/com/google/android/gms/wearable/Wearable.WearableOptions).

Here's an example that implements[DataClient.OnDataChangedListener](https://developers.google.com/android/reference/com/google/android/gms/wearable/DataClient.OnDataChangedListener):  

### Kotlin

```kotlin
class MainActivity : Activity(), DataClient.OnDataChangedListener {

    public override fun onResume() {
        Wearable.getDataClient(this).addListener(this)
    }

    override fun onPause() {
        Wearable.getDataClient(this).removeListener(this)
    }

    override fun onDataChanged(dataEvents: DataEventBuffer) {
        dataEvents.forEach { event ->
            if (event.type == DataEvent.TYPE_DELETED) {
                Log.d(TAG, "DataItem deleted: " + event.dataItem.uri)
            } else if (event.type == DataEvent.TYPE_CHANGED) {
                Log.d(TAG, "DataItem changed: " + event.dataItem.uri)
            }
        }
    }
}
```

### Java

```java
public class MainActivity extends Activity implements DataClient.OnDataChangedListener {

    @Override
    public void onResume() {
        Wearable.getDataClient(this).addListener(this);
    }

    @Override
    protected void onPause() {
        Wearable.getDataClient(this).removeListener(this);
    }

    @Override
    public void onDataChanged(DataEventBuffer dataEvents) {
        for (DataEvent event : dataEvents) {
            if (event.getType() == DataEvent.TYPE_DELETED) {
                Log.d(TAG, "DataItem deleted: " + event.getDataItem().getUri());
            } else if (event.getType() == DataEvent.TYPE_CHANGED) {
                Log.d(TAG, "DataItem changed: " + event.getDataItem().getUri());
            }
        }
    }
}
```

**Caution:** Before using the Wearable Data Layer API, check that it's available on a device; otherwise, an exception occurs. Use the[`GoogleApiAvailability`](https://developers.google.com/android/reference/com/google/android/gms/common/GoogleApiAvailability)class, as implemented in[Horologist](https://github.com/google/horologist/blob/release-0.5.x/datalayer/core/src/main/java/com/google/android/horologist/data/WearableApiAvailability.kt#L29).

### Use filters with live listeners

As previously mentioned, just as you can specify intent filters for manifest-based`WearableListenerService`objects, you can use intent filters when registering a live listener through the[Wearable API](https://developers.google.com/android/reference/com/google/android/gms/wearable/Wearable). The same rules apply to both API-based live listeners and manifest-based listeners.

A common pattern is to register a listener with a specific path or path prefix in an activity's[onResume()](https://developer.android.com/reference/android/app/Activity#onResume())method, and then to remove the listener in the activity's[onPause()](https://developer.android.com/reference/android/app/Activity#onPause())method. Implementing listeners in this fashion lets your app more selectively receive events, improving its design and efficiency.