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09May

Building Xamarin.Android application in C#

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:

    Creating Solution
  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:

    Create New Solution
  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.

Solution Components

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.

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 DebugDebug 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:

Launch Emulator

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:

Launch Emulator VS

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

Start Emulator

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:

Select Device

Deploying and Launching the Application

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

Deploying

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 UI

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

Walkthrough

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 += (sendere) => { 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;
 
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 += (sendere) => { aLabel.Text = "Hello from the button"; };
 
Instead, we could use a C# 2.0 anonymous method with the delegate syntax:
aButton.Click += delegate(object senderEventArgs 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. 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:

HelloWorld

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:

 

Application icons

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

Android Icons

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

Properties

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

Add File

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:

Resources

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:

Android App

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

Project Options

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>
 
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.

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>
 
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 += (sendere) => { 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. For more samples and documentation on installing Xamarin on Windows and building Xamarin.iOS application, please refer Xamarin website.

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Comments

We we need to call SetContentView everytime in the code? Is there any other example we can deal with code here?
3/26/2021 4:19:34 PM | Reply

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