Receiving Location Updates

If your app can continuously track location, it can deliver more relevant information to the user. For example, if your app helps the user find their way while walking or driving, or if your app tracks the location of assets, it needs to get the location of the device at regular intervals. As well as the geographical location (latitude and longitude), you may want to give the user further information such as the bearing (horizontal direction of travel), altitude, or velocity of the device. This information, and more, is available in the Location object that your app can retrieve from the fused location provider.

While you can get a device's location with getLastLocation(), as illustrated in the lesson on Getting the Last Known Location, a more direct approach is to request periodic updates from the fused location provider. In response, the API updates your app periodically with the best available location, based on the currently-available location providers such as WiFi and GPS (Global Positioning System). The accuracy of the location is determined by the providers, the location permissions you've requested, and the options you set in the location request.

This lesson shows you how to request regular updates about a device's location using the requestLocationUpdates() method in the fused location provider.

Connect to Location Services

Location services for apps are provided through Google Play services and the fused location provider. In order to use these services, you connect your app using the Google API Client and then request location updates. For details on connecting with the GoogleApiClient, follow the instructions in Getting the Last Known Location, including requesting the current location.

The last known location of the device provides a handy base from which to start, ensuring that the app has a known location before starting the periodic location updates. The lesson on Getting the Last Known Location shows you how to get the last known location by calling getLastLocation(). The snippets in the following sections assume that your app has already retrieved the last known location and stored it as a Location object in the global variable mCurrentLocation.

Apps that use location services must request location permissions. In this lesson you require fine location detection, so that your app can get as precise a location as possible from the available location providers. Request this permission with the uses-permission element in your app manifest, as shown in the following example:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.google.android.gms.location.sample.locationupdates" >

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

Set Up a Location Request

To store parameters for requests to the fused location provider, create a LocationRequest. The parameters determine the levels of accuracy requested. For details of all the options available in the location request, see the LocationRequest class reference. This lesson sets the update interval, fastest update interval, and priority, as described below:

Update interval

setInterval() - This method sets the rate in milliseconds at which your app prefers to receive location updates. Note that the location updates may be faster than this rate if another app is receiving updates at a faster rate, or slower than this rate, or there may be no updates at all (if the device has no connectivity, for example).

Fastest update interval

setFastestInterval() - This method sets the fastest rate in milliseconds at which your app can handle location updates. You need to set this rate because other apps also affect the rate at which updates are sent. The Google Play services location APIs send out updates at the fastest rate that any app has requested with setInterval(). If this rate is faster than your app can handle, you may encounter problems with UI flicker or data overflow. To prevent this, call setFastestInterval() to set an upper limit to the update rate.

Priority

setPriority() - This method sets the priority of the request, which gives the Google Play services location services a strong hint about which location sources to use. The following values are supported:

• PRIORITY_BALANCED_POWER_ACCURACY - Use this setting to request location precision to within a city block, which is an accuracy of approximately 100 meters. This is considered a coarse level of accuracy, and is likely to consume less power. With this setting, the location services are likely to use WiFi and cell tower positioning. Note, however, that the choice of location provider depends on many other factors, such as which sources are available.
• PRIORITY_HIGH_ACCURACY - Use this setting to request the most precise location possible. With this setting, the location services are more likely to use GPS (Global Positioning System) to determine the location.
• PRIORITY_LOW_POWER - Use this setting to request city-level precision, which is an accuracy of approximately 10 kilometers. This is considered a coarse level of accuracy, and is likely to consume less power.
• PRIORITY_NO_POWER - Use this setting if you need negligible impact on power consumption, but want to receive location updates when available. With this setting, your app does not trigger any location updates, but receives locations triggered by other apps.

Create the location request and set the parameters as shown in this code sample:

protected void createLocationRequest() {
    LocationRequest mLocationRequest = new LocationRequest();
    mLocationRequest.setInterval(10000);
    mLocationRequest.setFastestInterval(5000);
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
}

The priority of PRIORITY_HIGH_ACCURACY, combined with the ACCESS_FINE_LOCATION permission setting that you've defined in the app manifest, and a fast update interval of 5000 milliseconds (5 seconds), causes the fused location provider to return location updates that are accurate to within a few feet. This approach is appropriate for mapping apps that display the location in real time.

Performance hint: If your app accesses the network or does other long-running work after receiving a location update, adjust the fastest interval to a slower value. This adjustment prevents your app from receiving updates it can't use. Once the long-running work is done, set the fastest interval back to a fast value.

Request Location Updates

Now that you've set up a location request containing your app's requirements for the location updates, you can start the regular updates by calling requestLocationUpdates(). Do this in the onConnected() callback provided by Google API Client, which is called when the client is ready.

Depending on the form of the request, the fused location provider either invokes the LocationListener.onLocationChanged() callback method and passes it a Location object, or issues a PendingIntent that contains the location in its extended data. The accuracy and frequency of the updates are affected by the location permissions you've requested and the options you set in the location request object.

This lesson shows you how to get the update using the LocationListener callback approach. Call requestLocationUpdates(), passing it your instance of the GoogleApiClient, the LocationRequest object, and a LocationListener. Define a startLocationUpdates() method, called from the onConnected() callback, as shown in the following code sample:

@Override
public void onConnected(Bundle connectionHint) {
    ...
    if (mRequestingLocationUpdates) {
        startLocationUpdates();
    }
}

protected void startLocationUpdates() {
    LocationServices.FusedLocationApi.requestLocationUpdates(
            mGoogleApiClient, mLocationRequest, this);
}

Notice that the above code snippet refers to a boolean flag, mRequestingLocationUpdates, used to track whether the user has turned location updates on or off. For more about retaining the value of this flag across instances of the activity, see Save the State of the Activity.

Define the Location Update Callback

The fused location provider invokes the LocationListener.onLocationChanged() callback method. The incoming argument is a Location object containing the location's latitude and longitude. The following snippet shows how to implement the LocationListener interface and define the method, then get the timestamp of the location update and display the latitude, longitude and timestamp on your app's user interface:

public class MainActivity extends ActionBarActivity implements
        ConnectionCallbacks, OnConnectionFailedListener, LocationListener {
    ...
    @Override
    public void onLocationChanged(Location location) {
        mCurrentLocation = location;
        mLastUpdateTime = DateFormat.getTimeInstance().format(new Date());
        updateUI();
    }

    private void updateUI() {
        mLatitudeTextView.setText(String.valueOf(mCurrentLocation.getLatitude()));
        mLongitudeTextView.setText(String.valueOf(mCurrentLocation.getLongitude()));
        mLastUpdateTimeTextView.setText(mLastUpdateTime);
    }
}

Stop Location Updates

Consider whether you want to stop the location updates when the activity is no longer in focus, such as when the user switches to another app or to a different activity in the same app. This can be handy to reduce power consumption, provided the app doesn't need to collect information even when it's running in the background. This section shows how you can stop the updates in the activity's onPause() method.

To stop location updates, call removeLocationUpdates(), passing it your instance of the GoogleApiClient object and a LocationListener, as shown in the following code sample:

@Override
protected void onPause() {
    super.onPause();
    stopLocationUpdates();
}

protected void stopLocationUpdates() {
    LocationServices.FusedLocationApi.removeLocationUpdates(
            mGoogleApiClient, this);
}

Use a boolean, mRequestingLocationUpdates, to track whether location updates are currently turned on. In the activity's onResume() method, check whether location updates are currently active, and activate them if not:

@Override
public void onResume() {
    super.onResume();
    if (mGoogleApiClient.isConnected() && !mRequestingLocationUpdates) {
        startLocationUpdates();
    }
}

Save the State of the Activity

A change to the device's configuration, such as a change in screen orientation or language, can cause the current activity to be destroyed. Your app must therefore store any information it needs to recreate the activity. One way to do this is via an instance state stored in a Bundle object.

The following code sample shows how to use the activity's onSaveInstanceState() callback to save the instance state:

public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putBoolean(REQUESTING_LOCATION_UPDATES_KEY,
            mRequestingLocationUpdates);
    savedInstanceState.putParcelable(LOCATION_KEY, mCurrentLocation);
    savedInstanceState.putString(LAST_UPDATED_TIME_STRING_KEY, mLastUpdateTime);
    super.onSaveInstanceState(savedInstanceState);
}

Define an updateValuesFromBundle() method to restore the saved values from the previous instance of the activity, if they're available. Call the method from the activity's onCreate() method, as shown in the following code sample:

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

private void updateValuesFromBundle(Bundle savedInstanceState) {
    if (savedInstanceState != null) {
        // Update the value of mRequestingLocationUpdates from the Bundle, and
        // make sure that the Start Updates and Stop Updates buttons are
        // correctly enabled or disabled.
        if (savedInstanceState.keySet().contains(REQUESTING_LOCATION_UPDATES_KEY)) {
            mRequestingLocationUpdates = savedInstanceState.getBoolean(
                    REQUESTING_LOCATION_UPDATES_KEY);
            setButtonsEnabledState();
        }

        // Update the value of mCurrentLocation from the Bundle and update the
        // UI to show the correct latitude and longitude.
        if (savedInstanceState.keySet().contains(LOCATION_KEY)) {
            // Since LOCATION_KEY was found in the Bundle, we can be sure that
            // mCurrentLocationis not null.
            mCurrentLocation = savedInstanceState.getParcelable(LOCATION_KEY);
        }

        // Update the value of mLastUpdateTime from the Bundle and update the UI.
        if (savedInstanceState.keySet().contains(LAST_UPDATED_TIME_STRING_KEY)) {
            mLastUpdateTime = savedInstanceState.getString(
                    LAST_UPDATED_TIME_STRING_KEY);
        }
        updateUI();
    }
}

For more about saving instance state, see the Android Activity class reference.

Note: For a more persistent storage, you can store the user's preferences in your app's SharedPreferences. Set the shared preference in your activity's onPause() method, and retrieve the preference in onResume(). For more information about saving preferences, read Saving Key-Value Sets.

The next lesson, Displaying a Location Address, shows you how to display the street address for a given location.