You will notice that the second parameter to RequestSingleUpdate() must be an object that implements ILocationListener, which defines the following methods:

For the most part, we will create blank stubs for all of the methods except OnLocationChanged(). While writing more sophisticated applications, it will be useful to provide implementations for some of the other methods. For example, you might call RequestLocationUpdate() to begin receiving updates and then receive a notification via OnProviderEnabled() that a preferred provider is now available, in which case you would want to stop updates and start them again using the preferred provider.

Now that we have some idea of how to add location services to an app, let's add location services to POIListActivity as follows:

Now, run POIApp and view the results in POIListView.

The steps for adding location services to POIDetailActivity will be very similar to the previous section, but will be slightly simpler.

Updating the user interface

Prior to adding the logic, we need to add a few buttons to our app; one for getting our location and one for navigating to the map, which we will cover later in this chapter. We can add these as a row of buttons at the bottom of POIDetail.axml, as depicted in the following screenshot:

Using ImageButton allows a drawable class to be specified.

To add the ImageButton widgets, perform the following steps:

1. Add a LinearLayout instance to the bottom of POIDetail.axml just below the TableLayout used to arrange the latitude and longitude content. The orientation should be "horizontal".
2. On the LinearLayout instance, the orientation should be horizontal, the content should be wrapped (both height and width), and it should be centered horizontally in the parent. The layout gravity can be used to center content horizontally within its parent. The top and bottom padding of 10dp will provide a good spacing for the buttons.
3. Add the two ImageButton widgets, locationImageButton and mapImageButton, within the LinearLayout instance. Images for these buttons can be found in the drawable folder in the assets location.
4. The following XML code shows the result:

   . . .
   </TableLayout>
   <LinearLayout
     p1:orientation="horizontal"
     p1:layout_width="wrap_content"
     p1:layout_height="wrap_content"
     p1:layout_gravity="center_horizontal"
     p1:minWidth="25px"
     p1:minHeight="25px"
     p1:layout_marginTop="10dp"
     p1:layout_marginBottom="10dp">
     <ImageButton
       p1:src="@drawable/ic_locate"
       p1:layout_width="wrap_content"
       p1:layout_height="wrap_content"
       p1:id="@+id/locationImageButton" />
     <ImageButton
       p1:src="@drawable/ic_map"
       p1:layout_width="wrap_content"
       p1:layout_height="wrap_content"
       p1:id="@+id/mapImageButton" />
   </LinearLayout>

Another useful feature provided by the Android platform is called Geocoding. This is the process of obtaining the location in terms of latitude and longitude from a known address. Android also supports Reverse Geocoding, which is what you would expect from the name, obtaining an address from a known location.

The Android.Locations.Geocoder class is the class used to perform geocoding and reverse geocoding. It's a very straightforward process to use, as shown in the following steps:

You can see that we chose to call a method to format the address information. The FeatureName property may contain a title such as Golden Gate Bridge or Empire State Building. More times than not, FeatureName will simply contain the street number. The address contains a list of address lines, which we combine and place in _addrEditText, as follows:

protected void UpdateAddressFields(Address addr)
{
  if (String.IsNullOrEmpty(_nameEditText.Text))
    _nameEditText.Text = addr.FeatureName;

  If (String.IsNullOrEmpty(_addrEditText.Text)) {
    for (int i = 0; i < addr.MaxAddressLineIndex; i++) {
      if (!String.IsNullOrEmpty(_addrEditText.Text))

        _addrEditText.Text += System.Environment.NewLine;
        _addrEditText.Text += addr.GetAddressLine (i);
    }

  }
}

Now, run POIApp and test adding a new POI and getting the address for a location.

After using the get location button, you will notice that requests for location information take some amount of time to process; generally a few seconds or more. It would be best to keep users informed that processing is taking place so that they don't continually click on the button. The ProgressDialog class provides just the solution needed, which is a simple means of displaying a dialog with a spinning progress widget and text description of what process is taking place.

To add a progress dialog, perform the following steps:

Now, run POIApp and test the new progress dialog.

In the previous section, we solved the problem of keeping the user informed while they wait for location updates, but we unknowingly created another problem. As you may recall from Chapter 1, The Anatomy of an Android App, we mentioned that by default Android destroys and recreates an activity when a configuration change such as a device orientation change occurs.

This is done to take advantage of a feature that Android provides, which allows you to specify different layouts to use based on device orientation. In our app, we have a single folder named layout, which holds all our layouts. If we wanted POIDetail.axml to have a different layout when the device is in landscape, we would simply create a new folder in Resources named layout-land and create our alternate layout in it using the same name, POIDetail.axml. When Android destroys and recreates our activity because the device was rotated to landscape, our call to SetContentView() would cause Android to first look in layout-land to see if a layout exists.

This is neat, but a fallout of this approach arises when an activity initiates any type of asynchronous processing such as requesting location updates. The problem is when a configuration change takes place and Android destroys the original activity; you no longer want asynchronous callbacks to come in for the original activity.

To observe the issue, run POIApp, press the get location button, and rotate the screen. In the emulator, press Ctrl + F11 or Ctrl + F12 to rotate the device. If you are in the debug mode, you should see an exception thrown when you try and cancel the progress dialog; this is due to the fact that Android has already removed the dialog from the view hierarchy when it destroyed the activity.

Android provides the following ways of dealing with this issue:

<activity
  android:name="poiapp.POIListActivity" android:configChanges="orientation|screenSize" android:label="POIs" >
. . .
</activity>

[Activity (Label = "POIs", MainLauncher = true, ConfigurationChanges = (Android.Content.PM.ConfigChanges.Orientation | Android.Content.PM.ConfigChanges.ScreenSize))]
public class POIListActivity : Activity, ILocationListener
{
  . . .