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Hello, Android

This is the second article in the Xamarin.Android getting started series, the first being Installation. This article demonstrates how to use Xamarin.Android to create and deploy an Android application. This article also examines the default application template, as well as how to create a simple hello world application.

Overview

In this article we’ll look at how to create, deploy, and run a Xamarin.Android application. First, we’ll demonstrate how to use the default application template in the deployment process. Next, we’ll examine some of the basic parts of the android application that are created with the template. We’ll then 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

To get started, we are going to walk through the steps you need to take to create a Xamarin.Android application. Xamarin.Android works with Xamarin Studio on both OS X and Windows; it also works on Windows with Visual Studio 2010 Professional (or greater). The process for creating Xamarin.Android applications is nearly the same on each of these platforms. This walkthrough assumes you already have Xamarin.Android installed. If that’s not the case, refer to the Installation document before continuing.

Creating a New 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:

Folder Purpose
Assets Contains any type of file the application needs included in its package. Files included here are accessible at runtime via the Assets class.
Properties Contains normal .NET assembly metadata.
Resources Contains application resources such as strings and images, as well as declarative XML user interface definitions. These resources are accessible through the generated Resource class.

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. If you want to learn more about activities after you finish the Getting Started series, we recommend reading the Activity Lifecycle article.

Before we begin to implement our own simple Hello, World app, let’s run the application as created from the template. To deploy your application to a device, refer to Set Up Device for Development guide. To run the app in an emulator, refer to Configure the Emulator guide.

Hello, World Walkthrough

The default application template we looked at earlier provides a good starting point for creating a simple hello world application. In order to fully explore how to put an application together ourselves, we’re first 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. Earlier, we talked briefly about Activities and mentioned that activities have a lifecycle. 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);
}

Let’s break this code down line-by-line. First we created a LinearLayout and set its Orientation to Vertical with these lines:

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

Android uses layout classes to group and position controls on the screen. The controls we add, such as the Button and the TextView, will be children of the layout. The LinearLayout class is used to align controls one after another, either horizontally, or vertically as we have done here. This is similar to a StackPanel in Silverlight.

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";

When the user clicks the button, we want to change the text of the TextView. With Xamarin.Android, we accomplish this by using a typical .NET event. To handle such an event, we can use an event handler, an anonymous method, or even a lambda expression as in the following example:

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

Instead, we could use a C# 2.0 anonymous method with the delegate syntax:

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

With the controls created and the event handler wired up, we need to add them to the LinearLayout instance. LinearLayout is a subclass of ViewGroup. A ViewGroup is basically a view that contains other views and determines how to display them. The ViewGroup class contains an AddView method that we can call to add our controls, as we did in this code:

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

The final step in building our app is to add the layout to the screen. We accomplished this by calling the Activity's SetContentView method, passing it the layout like this:

SetContentView (layout);

If we save our work and run the application, we can click the button and see the text change as shown here:

Creating this code shows how easy and familiar programming applications in Xamarin.Android can be for a .NET developer. However, something unique to Android is how it manages resources. 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.

Setting Application Icons

To set custom application icons, first download the Xamarin Android Icons zip included with this tutorial.

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 then Add 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 Folder option, 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:

To learn more about the Android Manifest, refer to the Xamarin Working with the Android Manifest guide

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

Creating String Resources

As we just mentioned, Android accesses resources in an unusual way. Android resources are managed under the Resources folder in the Solution Explorer. To make them accessible from code, generated Resource classes are updated for all resources that are included in the various Resources subfolders.

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 Hello</string>
    <string name="helloLabelText">Hello Xamarin.Android</string>
</resources>

We defined two string resources in the XML above, one with the name helloButtonText and another named helloLabelText. Each of these contains a string value. When we include values like these in the Strings.xml file, the generated Resource class will update when we rebuild, giving us a mechanism to access the resources from code.

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()
    {
    }
}

To access the strings from code, we call the SetText method of the TextView and Button controls respectively, passing the appropriate resource id. For example, to set the TextView’s text, replace the line where we set the aLabel.Text property with a call to the SetText method like this:

aLabel.SetText(Resource.String.helloLabelText);

Likewise, to set the Button’s text, we call its SetText method:

aButton.SetText(Resource.String.helloButtonText);

When we run the application, the behavior is the same using either technique, only now we can easily manage the string values without needing to change them in code.

Resources also come into play when we use XML to declare our user interface, as we’ll see in the next section.

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. This file was automatically generated by Xamarin.Android when we created the project. Just like strings, Android will auto-generate a unique ID that allows us to reference the layout file in source code. The ID will have the name of the layout file and can be found in the nested class Resource.Layout. The following code snippet shows an example of the Layout class:

public partial class Resource
{
    static Resource()
    {
        global::Android.Runtime.ResourceIdManager.UpdateIdValues();
    }

    public partial class Layout
    {

        // aapt resource value: 0x7f030000
        public const int Main = 2130903040;

        static Layout()
        {
            global::Android.Runtime.ResourceIdManager.UpdateIdValues();
        }

        private Layout()
        {
        }
    }
}

When we want to reference the Main.axml layout file in our code, we will use the syntax Resource.Layout.Main.

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>

If we include XML elements like these for the LinearLayout, TextView, and Button classes, it will cause instances of these to be created at runtime. To access these instances from code, we use their resource ids.

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()
       {
       }
}

Also, the syntax @string/name allows us to access the string resources that we created earlier in Strings.xml.

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";
    };
}

We still needed to call SetContentView as we did in our earlier example, only this time we passed the resource id for the layout, Resource.Layout.Main, which we defined in Main.axml.

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.

Next: Hello, Multiscreen Applications