In a very similar fashion, we can retrieve a list of contacts in Android with Xamarin.Mobile. All of the APIs in Xamarin.Mobile are identical in Android with the exception of the requirement of Android.Content.Context to be passed in a few places. This is because many native Android APIs require a reference to the current activity (or to the other context such as Application) in order to function properly. To begin, create a standard Android application project by navigating to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

In a parallel iOS, let's create a simple ListView to display a list of contacts as follows:

Let's begin by removing most of the code; we don't need the code that came from the project template. You will also need to add a using statement for Xamarin.Contacts. Next, let's implement a simple BaseAdapter<Contact> class inside the MainActivity class as follows:

class ContactsAdapter : BaseAdapter<Contact>
{
  public Contact[] Contacts { get; set; }

  public override long GetItemId(int position)
  {
    return position;
  }
  public override View GetView(int position, View convertView, ViewGroup parent)
  {
    var contact = this [position];
    var textView = convertView as TextView;
    if (textView == null)
    {
      textView = new TextView(parent.Context);
    }
    textView.Text = contact.LastName + ", " + contact.FirstName;
    return textView;
  }
  public override int Count
  {
    get { return Contacts == null ? 0 : Contacts.Length; }
  }
  public override Contact this[int index]
  {
    get { return Contacts [index]; }
  }
}

This will display each contact in TextView for each row in ListView. One thing that we've done here in order to simplify things is add a property for the array of contacts. This should be pretty straightforward and similar to what we did in the previous chapters.

Now, let's set up the adapter in OnCreate as follows:

protected async override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);

  SetContentView(Resource.Layout.Main);
  var listView = FindViewById<ListView>(Resource.Id.contacts);
  var adapter = new ContactsAdapter();
  listView.Adapter = adapter;
  try
  {
    var book = new AddressBook(this);
    await book.RequestPermission();
    adapter.Contacts = book.ToArray();
    adapter.NotifyDataSetChanged();
  }
  catch
  {
    new AlertDialog.Builder(this).SetTitle("Oops").SetMessage("Something went wrong, try again later.").SetPositiveButton("Ok", delegate { }).Show();
  }
}

This code calls Xamarin.Mobile that is identical to what we did on the code for iOS except that here, this had to be passed for the Android Context in the constructor for AddressBook. Our code changes are complete; however, if you run the application right now, an exception would be thrown. Android requires permission in the manifest file, which will notify the user of its access to the address book when downloaded from Google Play.

We must create an AndroidManifest.xml file and declare a permission as follows:

Now, if you run the application, you will get a list of all the contacts on the device, as shown in the following screenshot:

In a very similar fashion, we can retrieve a list of contacts in Android with Xamarin.Mobile. All of the APIs in Xamarin.Mobile are identical in Android with the exception of the requirement of Android.Content.Context to be passed in a few places. This is because many native Android APIs require a reference to the current activity (or to the other context such as Application) in order to function properly. To begin, create a standard Android application project by navigating to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

In a parallel iOS, let's create a simple ListView to display a list of contacts as follows:

Let's begin by removing most of the code; we don't need the code that came from the project template. You will also need to add a using statement for Xamarin.Contacts. Next, let's implement a simple BaseAdapter<Contact> class inside the MainActivity class as follows:

class ContactsAdapter : BaseAdapter<Contact>
{
  public Contact[] Contacts { get; set; }

  public override long GetItemId(int position)
  {
    return position;
  }
  public override View GetView(int position, View convertView, ViewGroup parent)
  {
    var contact = this [position];
    var textView = convertView as TextView;
    if (textView == null)
    {
      textView = new TextView(parent.Context);
    }
    textView.Text = contact.LastName + ", " + contact.FirstName;
    return textView;
  }
  public override int Count
  {
    get { return Contacts == null ? 0 : Contacts.Length; }
  }
  public override Contact this[int index]
  {
    get { return Contacts [index]; }
  }
}

This will display each contact in TextView for each row in ListView. One thing that we've done here in order to simplify things is add a property for the array of contacts. This should be pretty straightforward and similar to what we did in the previous chapters.

Now, let's set up the adapter in OnCreate as follows:

protected async override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);

  SetContentView(Resource.Layout.Main);
  var listView = FindViewById<ListView>(Resource.Id.contacts);
  var adapter = new ContactsAdapter();
  listView.Adapter = adapter;
  try
  {
    var book = new AddressBook(this);
    await book.RequestPermission();
    adapter.Contacts = book.ToArray();
    adapter.NotifyDataSetChanged();
  }
  catch
  {
    new AlertDialog.Builder(this).SetTitle("Oops").SetMessage("Something went wrong, try again later.").SetPositiveButton("Ok", delegate { }).Show();
  }
}

This code calls Xamarin.Mobile that is identical to what we did on the code for iOS except that here, this had to be passed for the Android Context in the constructor for AddressBook. Our code changes are complete; however, if you run the application right now, an exception would be thrown. Android requires permission in the manifest file, which will notify the user of its access to the address book when downloaded from Google Play.

We must create an AndroidManifest.xml file and declare a permission as follows:

Now, if you run the application, you will get a list of all the contacts on the device, as shown in the following screenshot:

Just as in the previous section, using Xamarin.Mobile for GPS location is almost identical to the APIs we used in iOS. To begin with our Android example, go to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

Let's create ListView to display a list of messages of the GPS location updates as follows:

As usual, remove any extra code that was created by the project template. Next, add a using statement for Xamarin.Geolocation. Then, add a simple BaseAdapter<string> to the MainActivity class as follows:

class Adapter : BaseAdapter<string>
{
  List<string> messages = new List<string>();

  public void Add(string message)
  {
    messages.Add(message);
    NotifyDataSetChanged();
  }
  public override long GetItemId(int position)
  {
    return position;
  }
  public override View GetView(int position, View convertView, ViewGroup parent)
  {
    var textView = convertView as TextView;
    if (textView == null)
      textView = new TextView(parent.Context);
    textView.Text = messages [position];
    return textView;
  }
  public override int Count
  {
    get { return messages.Count; }
  }
  public override string this[int index]
  {
    get { return messages [index]; }
  }
}

This is similar to other Android adapters we have set up in the past. One difference here is that we made a member variable that contains a List<string> of messages and a method to add the new messages to the list.

Now, let's add a few methods to the MainActivity class in order to set up the GPS location updates as follows:

Geolocator locator;
Adapter adapter;

protected override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);
  SetContentView(Resource.Layout.Main);
  var listView = FindViewById<ListView>(Resource.Id.locations);
  listView.Adapter = adapter = new Adapter();
  locator = new Geolocator(this);
  locator.PositionChanged += OnPositionChanged;
  locator.PositionError += OnPositionError;
}
protected override void OnResume()
{
  base.OnResume();
  locator.StartListening(1000, 50);
}
protected override void OnPause()
{
  base.OnPause();
  locator.StopListening();
}
void OnPositionChanged (object sender, PositionEventArgs e)
{
  adapter.Add(string.Format("Long: {0:0.##} Lat: {1:0.##}",e.Position.Longitude, e.Position.Latitude));
}
void OnPositionError (object sender, PositionErrorEventArgs e)
{
  adapter.Add(e.Error.ToString());
}

Again, this looks identical to the code for iOS except for the constructor for Geolocator. If you run the application at this point, it would start with no errors. However, no events will be fired from the Geolocator object. We first need to add a permission to access the location from the Android Manifest file. It is also a good idea to start the locator in OnResume and stop it in OnPause. This will conserve the battery by stopping GPS location when this activity is no longer on the screen.

Let's create an AndroidManifest.xml file and declare two permissions as follows:

Now, if you compile and run the application, you will get the GPS location updates over time, as shown in the following screenshot:

Just as in the previous section, using Xamarin.Mobile for GPS location is almost identical to the APIs we used in iOS. To begin with our Android example, go to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

Let's create ListView to display a list of messages of the GPS location updates as follows:

As usual, remove any extra code that was created by the project template. Next, add a using statement for Xamarin.Geolocation. Then, add a simple BaseAdapter<string> to the MainActivity class as follows:

class Adapter : BaseAdapter<string>
{
  List<string> messages = new List<string>();

  public void Add(string message)
  {
    messages.Add(message);
    NotifyDataSetChanged();
  }
  public override long GetItemId(int position)
  {
    return position;
  }
  public override View GetView(int position, View convertView, ViewGroup parent)
  {
    var textView = convertView as TextView;
    if (textView == null)
      textView = new TextView(parent.Context);
    textView.Text = messages [position];
    return textView;
  }
  public override int Count
  {
    get { return messages.Count; }
  }
  public override string this[int index]
  {
    get { return messages [index]; }
  }
}

This is similar to other Android adapters we have set up in the past. One difference here is that we made a member variable that contains a List<string> of messages and a method to add the new messages to the list.

Now, let's add a few methods to the MainActivity class in order to set up the GPS location updates as follows:

Geolocator locator;
Adapter adapter;

protected override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);
  SetContentView(Resource.Layout.Main);
  var listView = FindViewById<ListView>(Resource.Id.locations);
  listView.Adapter = adapter = new Adapter();
  locator = new Geolocator(this);
  locator.PositionChanged += OnPositionChanged;
  locator.PositionError += OnPositionError;
}
protected override void OnResume()
{
  base.OnResume();
  locator.StartListening(1000, 50);
}
protected override void OnPause()
{
  base.OnPause();
  locator.StopListening();
}
void OnPositionChanged (object sender, PositionEventArgs e)
{
  adapter.Add(string.Format("Long: {0:0.##} Lat: {1:0.##}",e.Position.Longitude, e.Position.Latitude));
}
void OnPositionError (object sender, PositionErrorEventArgs e)
{
  adapter.Add(e.Error.ToString());
}

Again, this looks identical to the code for iOS except for the constructor for Geolocator. If you run the application at this point, it would start with no errors. However, no events will be fired from the Geolocator object. We first need to add a permission to access the location from the Android Manifest file. It is also a good idea to start the locator in OnResume and stop it in OnPause. This will conserve the battery by stopping GPS location when this activity is no longer on the screen.

Let's create an AndroidManifest.xml file and declare two permissions as follows:

Now, if you compile and run the application, you will get the GPS location updates over time, as shown in the following screenshot:

In comparison to iOS, we have to use a slightly different pattern on Android to retrieve photos from the camera or photo library. A common pattern in Android is that it calls StartActivityForResult to launch an activity from another application. When this activity is completed, OnActivityResult will be called from your activity. Because of this, Xamarin.Mobile could not use the same APIs on Android as the other platforms. To start our example, create an Android Application project by going to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

Let's create two Buttons and an ImageView to mimic our UI on iOS as follows:

As usual, remove any extra code that was created by the project template. Next, add a using statement for Xamarin.Media. Then, we can add a new OnCreate method and some member variables for our activity as follows:

MediaPicker picker;
ImageView imageView;

protected override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);
  SetContentView(Resource.Layout.Main);
  var library = FindViewById<Button>(Resource.Id.library);
  var camera = FindViewById<Button>(Resource.Id.camera);
  imageView = FindViewById<ImageView>(Resource.Id.imageView);
  picker = new MediaPicker(this);
  library.Click += OnLibrary;
  camera.Click += OnCamera;
  if (!picker.IsCameraAvailable)
    camera.Enabled = false;
}

We retrieved the instance of our views and created a new MediaPicker by passing our activity as Context to its constructor. We hooked up some Click event handlers, and disabled the camera button since a camera is not available.

Next, let's implement the two Click event handlers as follows:

void OnLibrary (object sender, EventArgs e)
{
  var intent = picker.GetPickPhotoUI();
  StartActivityForResult (intent, 1);
}
void OnCamera (object sender, EventArgs e)
{
  var intent = picker.GetTakePhotoUI(new StoreCameraMediaOptions
  {
    Name = "test.jpg", Directory = "PhotoPicker"
  });
  StartActivityForResult (intent, 1);
}

In each case, we make a call to GetPickPhotoUI or GetTakePhotoUI in order to get an instance of an Android Intent object. This object is used to start the new activities within an application. StartActivityForResult will also start the Intent object, expecting a result to be returned from the new activity. We also set some values with StoreCameraMediaOptions to specify a filename and temporary directory to store the photo.

Next, we need to implement OnActivityResult in order to handle what will happen when the new activity is completed:

protected async override void OnActivityResult(
  int requestCode, Result resultCode, Intent data)
{
  if (resultCode == Result.Canceled)
  return;
  var file = await data.GetMediaFileExtraAsync(this);
  using (var stream = file.GetStream())
  {
    imageView.SetImageBitmap(await
      BitmapFactory.DecodeStreamAsync(stream));
  }
}

If this is successful, we retrieve MediaFile and load a new Bitmap with the returned Stream. Next, all that is needed is to call SetImageBitmap to display the image on the screen.

Let's create an AndroidManifest.xml file and declare two permissions as follows:

You should now be able to run the application and load photos to be displayed on the screen, as shown in the following screenshot:

In comparison to iOS, we have to use a slightly different pattern on Android to retrieve photos from the camera or photo library. A common pattern in Android is that it calls StartActivityForResult to launch an activity from another application. When this activity is completed, OnActivityResult will be called from your activity. Because of this, Xamarin.Mobile could not use the same APIs on Android as the other platforms. To start our example, create an Android Application project by going to Android | Android Application in Xamarin Studio. Make sure you add Xamarin.Mobile to the project from the Component Store.

Let's create two Buttons and an ImageView to mimic our UI on iOS as follows:

As usual, remove any extra code that was created by the project template. Next, add a using statement for Xamarin.Media. Then, we can add a new OnCreate method and some member variables for our activity as follows:

MediaPicker picker;
ImageView imageView;

protected override void OnCreate(Bundle bundle)
{
  base.OnCreate(bundle);
  SetContentView(Resource.Layout.Main);
  var library = FindViewById<Button>(Resource.Id.library);
  var camera = FindViewById<Button>(Resource.Id.camera);
  imageView = FindViewById<ImageView>(Resource.Id.imageView);
  picker = new MediaPicker(this);
  library.Click += OnLibrary;
  camera.Click += OnCamera;
  if (!picker.IsCameraAvailable)
    camera.Enabled = false;
}

We retrieved the instance of our views and created a new MediaPicker by passing our activity as Context to its constructor. We hooked up some Click event handlers, and disabled the camera button since a camera is not available.

Next, let's implement the two Click event handlers as follows:

void OnLibrary (object sender, EventArgs e)
{
  var intent = picker.GetPickPhotoUI();
  StartActivityForResult (intent, 1);
}
void OnCamera (object sender, EventArgs e)
{
  var intent = picker.GetTakePhotoUI(new StoreCameraMediaOptions
  {
    Name = "test.jpg", Directory = "PhotoPicker"
  });
  StartActivityForResult (intent, 1);
}

In each case, we make a call to GetPickPhotoUI or GetTakePhotoUI in order to get an instance of an Android Intent object. This object is used to start the new activities within an application. StartActivityForResult will also start the Intent object, expecting a result to be returned from the new activity. We also set some values with StoreCameraMediaOptions to specify a filename and temporary directory to store the photo.

Next, we need to implement OnActivityResult in order to handle what will happen when the new activity is completed:

protected async override void OnActivityResult(
  int requestCode, Result resultCode, Intent data)
{
  if (resultCode == Result.Canceled)
  return;
  var file = await data.GetMediaFileExtraAsync(this);
  using (var stream = file.GetStream())
  {
    imageView.SetImageBitmap(await
      BitmapFactory.DecodeStreamAsync(stream));
  }
}

If this is successful, we retrieve MediaFile and load a new Bitmap with the returned Stream. Next, all that is needed is to call SetImageBitmap to display the image on the screen.

Let's create an AndroidManifest.xml file and declare two permissions as follows:

You should now be able to run the application and load photos to be displayed on the screen, as shown in the following screenshot: