Sync data items on Wear

A DataItem defines the interface that the system uses to synchronize data between handhelds and wearables. A DataItem generally consists of the following components:

• Payload — A byte array, which you can set with whatever data you wish, allowing you to do your own object serialization and deserialization. The size of the payload is limited to 100KB.
• Path — A unique string that must start with a forward slash (for instance, "/path/to/data")

Note: A Wear app can communicate with a phone app using the Data Layer API, but connecting to a network using this API is discouraged.

You normally don't implement DataItem directly. Instead, you do the following:

1. Create a PutDataRequest object, specifying a string path to uniquely identify the item.
2. Call setData() to set the payload.
3. If a delay in syncing would negatively impact user experience, call setUrgent().
4. Use the putDataItem method of the DataClient class to request that the system create the data item.

When requesting data items, the system returns objects that properly implement the DataItem interface. However, instead of working with raw bytes using setData(), we recommend you use a data map, which exposes a data item with an easy-to-use Bundle-like interface.

Sync data with a data map

When possible, use the DataMap class. This approach lets you work with data items in the form of an Android Bundle, so the system does object serialization and deserialization for you, and you can manipulate data with key-value pairs.

To use a data map:

1. Create a PutDataMapRequest object, setting the path of the data item.

   Note: The path string is a unique identifier for the data item that allows you to access it from either side of the connection. The path must begin with a forward slash. If you're using hierarchical data in your app, you should create a path scheme that matches the structure of the data.

2. Call PutDataMapRequest.getDataMap() to obtain a data map that you can set values on.
3. Set any desired values for the data map using the put...() methods, such as putString().
4. If a delay in syncing would negatively impact user experience, call setUrgent().
5. Call PutDataMapRequest.asPutDataRequest() to obtain a PutDataRequest object.
6. Use the putDataItem method of the DataClient class to request that the system create the data item.

   Note: If the handset and wearable devices are disconnected, the data is buffered and synced when the connection is re-established.

The increaseCounter() method in the following example shows how to create a data map and put data in it:

public class MainActivity extends Activity {
    private static final String COUNT_KEY = "com.example.key.count";
    private DataClient mDataClient;
    private int count = 0;
    ...
    // Create a data map and put data in it
    private void increaseCounter() {
        PutDataMapRequest putDataMapReq = PutDataMapRequest.create("/count");
        putDataMapReq.getDataMap().putInt(COUNT_KEY, count++);
        PutDataRequest putDataReq = putDataMapReq.asPutDataRequest();
        Task<DataItem> putDataTask = mDataClient.putDataItem(putDataReq);
    }
  ...
}

For more information about handling Tasks, see the reference documentation.

Set DataItem priority

The DataClient API allows urgent requests for syncing of DataItems. Normally, the system may delay delivery of DataItems to the Wear network in order to improve battery life for user devices, but if a delay in syncing DataItems would negatively impact user experience, you can mark them as urgent. For example, in a remote control app where the user expects their actions to be reflected immediately, you can have the system sync your DataItems immediately by calling setUrgent().

If you do not call setUrgent(), the system may delay up to 30 minutes before syncing non-urgent DataItems, but you can usually expect the delay to be a few minutes, if at all. The default urgency is now non-urgent, so you must use setUrgent() if you wish to retain the immediate-sync behavior that existed in previous versions of the Wear API.

Listen for data item events

If one side of the data layer connection changes a data item, you probably want to be notified of any changes on the other side of the connection. You can do this by implementing a listener for data item events.

The code snippet in the following example notifies your app when the value of the counter defined in the previous example changes:

public class MainActivity extends Activity implements DataClient.OnDataChangedListener {
    private static final String COUNT_KEY = "com.example.key.count";
    private int count = 0;

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

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

    @Override
    public void onDataChanged(DataEventBuffer dataEvents) {
        for (DataEvent event : dataEvents) {
            if (event.getType() == DataEvent.TYPE_CHANGED) {
                // DataItem changed
                DataItem item = event.getDataItem();
                if (item.getUri().getPath().compareTo("/count") == 0) {
                    DataMap dataMap = DataMapItem.fromDataItem(item).getDataMap();
                    updateCount(dataMap.getInt(COUNT_KEY));
                }
            } else if (event.getType() == DataEvent.TYPE_DELETED) {
                // DataItem deleted
            }
        }
    }

    // Our method to update the count
    private void updateCount(int c) { ... }

    ...
}

This activity implements the DataClient.OnDataChangedListener interface. This activity adds itself as a listener for data item events inside the onResume() method and removes the listener in the onPause() method.

You can also implement the listener as a service. For more information, see Listen for data layer events.