Overview

In this article we’ll look at how to create, deploy, and run a Xamarin.Android application. We’ll create a hello world application, showing how to build the user interface both in code and by using Android XML.

Creating a Xamarin.Android Application

Let’s begin by creating a new Xamarin.Android solution. Xamarin.Android includes several templates for creating projects, including:

• Android Library Project – A reusable .NET library project for Android.
• Android Application – A basic starter project with a single activity.
• Android OpenGL Application – An OpenGL starter project.

We’re going to use the Android Application template for this walkthrough. Let’s create an application by doing the following:

New Solution – Xamarin Studio

1. From the File menu select New > Solution, bringing up the dialog shown below:
2. Expand the C# item in the tree on the left.
3. Choose Android, and select the Android Application template from the list on the right.
4. Enter Hello_World for the project name, and then click OK.

New Solution – Visual Studio

1. From the File menu select New>Project, bringing up the New Project dialog shown below:
2. Expand Visual C# under Installed Templates.
3. Choose Android under Installed Templates, and then select the Android Application template.
4. Enter Hello_World for the Name, enter a Location, and then click OK.

Solution Components

In the previous section, we created a simple Xamarin.Android project. In doing so, Xamarin.Android generated a variety of things for us. Let’s take a look at what it created.

The Hello_World project includes three folders named Assets, Properties, and Resources. These items are summarized in the table below:

The project template also created a class called Activity1 in the file MainActivity.cs. An Activity is a class that models a destination where a user can perform some action while using an app, typically via a user interface.

Conceptually, an activity can be thought of as being mapped to an application screen. An activity is similar in some ways to a Page in ASP.NET, in that every activity has a lifecycle associated with it. An Activity contains methods to be called at certain points in the lifecycle. These methods can also be overridden. For example, the Activity subclass created by the project template overrides the OnCreate method, which is called after an Activity first starts.

Before we begin to implement our own simple Hello, World app, let’s run the application as created from the template. This will let you see a good example of the running app and it will help you become familiar with the process of deploying and launching in the emulator.

Launching the Emulator – Xamarin Studio

Under the Xamarin Studio Run menu, you have the options to either Run or Debug. Debug will attach the application to the debugger after it launches. For now, let’s just select Run With... and thenMono Soft Debugger for Android. This will launch the Select Device dialog as shown below:

Select the emulator in the list and chooseStart Emulator.

Launching the Emulator – Visual Studio

We can run the app as usual in Visual Studio by choosing Debug > Start Without Debugging (orStart Debugging to run with the debugger attached). Visual Studio will launch the Select Device dialog as shown below:

Selecting Start emulator image in this dialog will open a list of available emulators:

When we choose the emulator we want to run, which by default should be an emulator with an API level >= 8, and then click OK, Xamarin.Android launches the emulator. After it starts up, the emulator will appear in the Running devices list as shown below:

Deploying and Launching the Application

After the emulator starts up, slide the lock button to the right to show the Android home screen:

Back in the Select Device dialog, we can now build and deploy our app by selecting the emulator in the list, and then clicking OK. The first time a Xamarin.Android application is installed, the Xamarin.Android shared runtime will be installed, followed by the application.

Xamarin.Android deploys the app to the emulator, and then launches it. The default app created from the template consists of a Button. Clicking the button increments a counter, as shown below:

Hello, World Walkthrough

Now we’re going to create a simple application consisting of a TextView and a Button. Clicking the button will change the text of the TextView. Here is a screenshot that shows the application running in the emulator

Creating the User Interface with Code

Now let’s create the user interface in code. After an Activity starts, its OnCreate method is called. This is the appropriate place to perform initialization of the application, such as loading the user interface for the Activity.

For our app, we need to create a Button and a TextView. When the user clicks the button, we want the TextView to display a message. To accomplish this, open the MainActivity.cs file and replace the OnCreate method with the following code:

protected override void OnCreate (Bundle bundle)
{ 
 base.OnCreate (bundle); 
 //Create the user interface in code 
 var layout = new LinearLayout (this); 
 layout.Orientation = Orientation.Vertical; 
 var aLabel = new TextView (this); 
 aLabel.Text = "Hello, Xamarin.Android"; 
 var aButton = new Button (this); 
 aButton.Text = "Say Hello"; 
 aButton.Click += (sender, e) => { aLabel.Text = "Hello from the button"; }; 
 layout.AddView (aLabel); 
 layout.AddView (aButton); 
 SetContentView (layout);
}

var layout = new LinearLayout (this); 
layout.Orientation = Orientation.Vertical;

Next, we created a Button and TextView, setting the Text property of each like this:

var aLabel = new TextView (this); 
aLabel.Text = "Hello, Xamarin.Android"; 
var aButton = new Button (this);
aButton.Text = "Say Hello";

aButton.Click += (sender, e) => { aLabel.Text = "Hello from the button"; };

aButton.Click += delegate(object sender, EventArgs e) { aLabel.Text = "Hello from the button"; };

layout.AddView (aLabel); 
layout.AddView (aButton);

SetContentView (layout);

In our simple example, we hard-coded all the strings. Let’s change this to use the Android resource system and show you how to make your programming process even simpler and more foolproof.

3.2. Setting Application Icons

To set custom application icons, first download the Xamarin Android Icons zip.

Icons live in the Resources folder, which houses the images and layout files associated with the Android project. First, let’s change the existing icon. In the project, navigate to the Resources folder. Begin by deleting the existing Icon.png from the drawable folder. Then, right click on the drawable folder to select Add, and thenAdd Files:

Navigate to the unzipped xamarin_android_icons directory, and select the drawable folder:

In the popover, select the icon.png file we want to add:

In the resulting popover, select the option to Copy the file into the directory:

Android devices come in different screen sizes and resolutions. To account for this, it is a good practice to add icons of different resolutions to the Resources folder. Repeat the steps above, selecting the Add Existing Folderoption, to add the remaining directories in xamarin_android_icons to the project:

Finally, we need to set the icon in the Android Manifest. To create the Manifest, double-click on the project to open the Project Options; then, select the Android Application panel. If a Manifest file does not exist, click on the button to automatically generate one:

In the Manifest, set the Application Icon to @drawable/icon:

Creating String Resources

Let’s look at an example where we replace our hard-coded strings with string resources. Under the Resources > values folder, open the file named Strings.xml. This file includes the application’s string resources. Replace the contents of the file with the following code:

<?xml version="1.0" encoding="utf-8"?> 
<resources> 
<string name="helloButtonText">Say Hellostring> 
<string name="helloLabelText">Hello Xamarin.Androidstring> 
resources>

The Resource class is in the file Resource.designer.cs. As it is auto-generated, it should never be changed by hand. For the strings we just added, a nested class named Strings will be created in the Resource class. The integer fields in this class identify each string, as shown in the following code:

public partial class String 
{ 
 // aapt resource value: 0x7f040000 
 public const int helloButtonText = 2130968576; 

 // aapt resource value: 0x7f040001 
 public const int helloLabelText = 2130968577; 
 private String()
 {
 }
}

aLabel.SetText(Resource.String.helloLabelText);

aButton.SetText(Resource.String.helloButtonText);

Creating the User Interface with XML

In addition to creating the user interface in code, Android supports a declarative, XML-based user interface system, similar to technologies such as XAML and HTML. Let’s modify our example to create the UI with XML.

Under the Resources > layout folder is a file called Main.axml. Let’s change this file to contain a LinearLayout with a TextView and a Button, just as we’ve done earlier when we used code to create this application. We can accomplish this by changing Main.axml to include the following XML:

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout
 xmlns:android="http://schemas.android.com/apk/res/android" 
 android:orientation="vertical"
 android:layout_width="fill_parent" 
 android:layout_height="fill_parent"> 
<TextView
 android:id="@+id/helloLabel" 
 android:layout_width="fill_parent" 
 android:layout_height="wrap_content"
 android:text="@string/helloLabelText" /> 
<Button 
 android:id="@+id/aButton"
 android:layout_width="fill_parent" 
 android:layout_height="wrap_content"
 android:text="@string/helloButtonText" /> 
LinearLayout>

Resource IDs

The syntax @+id/name tells the Android parser to generate a resource id for the given element with the supplied name. For example, when we give the TextView class an id of @+id/helloLabel, the Resource class will have a nested class Id with the integer field helloLabel. Similarly, the Id class will also contain a field named aButton, as shown in the code below:

public partial class Id 
{ 
 // aapt resource value: 0x7f050001 
 public const int aButton = 2131034113; 
 // aapt resource value: 0x7f050000 
 public const int helloLabel = 2131034112; 
 private Id()
 {
 }
}

Just like the string resources we saw earlier, we can access these controls by using the generated Resource class, only this time through the Id subclass. Let’s switch back over to the Activity1.cs file to show how.

In the OnCreate method, we now only need code to set the content view and create the event handler, since the LinearLayout, TextView, and Button classes are created from the XML we just defined. Change the OnCreate method with the following code:

protected override void OnCreate (Bundle bundle)
{ 
 base.OnCreate (bundle);
 SetContentView(Resource.Layout.Main); 
 var aButton = FindViewById<Button> (Resource.Id.aButton); 
 var aLabel = FindViewById<TextView> (Resource.Id.helloLabel); 
 aButton.Click += (sender, e) => { aLabel.Text = "Hello from the button"; };
}

Next, we simply looked up the Button and TextView instances, respectively, by using the FindViewById method and passing the appropriate resource ids. Finally, we set the event handler, as we did earlier in the Creating the User Interface with Code section.

When you run the app now, you’ll see that it works exactly as it did in the earlier implementation where everything was accomplished in code.

Summary

In this article, we examined how to use Xamarin.Android to create and deploy an Android application. We looked at the various parts of an Android application that was created by using the default Xamarin.Android application template. We then walked through how to use only code to create a simple application. Finally, we used Android XML and a declarative user interface definition to create the same application we’d just created in code. For more samples and documentation on installing Xamarin on Windows and building Xamarin.iOS application, please refer Xamarin website.