Hello, Android Multiscreen: Deep Dive

In this two-part guide, the basic Phoneword application (created in the Hello, Android guide) is expanded to handle a second screen. Along the way, the basic Android application building blocks are introduced. A deeper dive into Android architecture is included to help you develop a better understanding of Android application structure and functionality.

In the Hello, Android Multiscreen Quickstart, you built and ran your first multi-screen Xamarin.Android application.

In this guide you will explore more advanced Android architecture. Android navigation with Intents is explained, and Android hardware navigation options are explored. New additions to the Phoneword app are dissected as you develop a more holistic view of the application's relationship with the operating system and other applications.

Android architecture basics

In the Hello, Android Deep Dive, you learned that Android applications are unique programs because they lack a single entry point. Instead, the operating system (or another application) starts any one of the application's registered Activities, which in turn starts the process for the application. This deep dive into Android architecture expands your understanding of how Android applications are constructed by introducing the Android Application Building Blocks and their functions.

Android application building blocks

An Android application consists of a collection of special Android classes called Application Blocks bundled together with any number of app resources - images, themes, helper classes, etc. – these are coordinated by an XML file called the Android Manifest.

Application Blocks form the backbone of Android applications because they allow you to do things you couldn't normally accomplish with a regular class. The two most important ones are Activities and Services:

• Activity – An Activity corresponds to a screen with a user interface, and it is conceptually similar to a web page in a web application. For example, in a newsfeed application, the login screen would be the first Activity, the scrollable list of news items would be another Activity, and the details page for each item would be a third. You can learn more about Activities in the Activity Lifecycle guide.

• Service – Android Services support Activities by taking over long-running tasks and running them in the background. Services don't have a user interface and are used to handle tasks that aren't tied to screens – for example, playing a song in the background or uploading photos to a server. For more information about Services, see the Creating Services and Android Services guides.

An Android application may not use all types of Blocks, and often has several Blocks of one type. For example, the Phoneword application from the Hello, Android Quickstart was composed of just one Activity (screen) and some resource files. A simple music player app might have several Activities and a Service for playing music when the app is in the background.

Intents

Another fundamental concept in Android applications is the Intent. Android is designed around the principle of least privilege – applications have access only to the Blocks they require to work, and they have limited access to the Blocks that make up the operating system or other applications. Similarly, Blocks are loosely-coupled – they are designed to have little knowledge of and limited access to other Blocks (even blocks that are part of the same application).

To communicate, Application Blocks send asynchronous messages called Intents back and forth. Intents contain information about the receiving Block and sometimes some data. An Intent sent from one App component triggers something to happen in another App component, binding the two App components and allowing them to communicate. By sending Intents back and forth, you can get Blocks to coordinate complex actions such as launching the camera app to take and save, gathering location information, or navigating from one screen to the next.

AndroidManifest.XML

When you add a Block to the application, it is registered with a special XML file called the Android Manifest. The Manifest keeps track of all Application Blocks in an application, as well as version requirements, permissions, and linked libraries – everything that the operating system needs to know for your application to run. The Android Manifest also works with Activities and Intents to control what actions are appropriate for a given Activity. These advanced features of the Android Manifest are covered in the Working with the Android Manifest guide.

In the single-screen version of the Phoneword application, only one Activity, one Intent, and the AndroidManifest.xml were used, alongside additional resources like icons. In the multi-screen version of Phoneword, an additional Activity was added; it was launched from the first Activity using an Intent. The next section explores how Intents help to create navigation in Android applications.

Android navigation

Intents were used to navigate between screens. It's time to dive into this code to see how Intents work and understand their role in Android navigation.

Launching a second activity with an intent

In the Phoneword application, an Intent was used to launch a second screen (Activity). Start by creating an Intent, passing in the current Context (this, referring to the current Context) and the type of Application Block that you're looking for (TranslationHistoryActivity):

Intent intent = new Intent(this, typeof(TranslationHistoryActivity));

The Context is an interface to global information about the application environment – it lets newly-created objects know what's going on with the application. If you think of an Intent as a message, you are providing the name of the message recipient (TranslationHistoryActivity) and the receiver's address (Context).

Android provides an option to attach simple data to an Intent (complex data is handled differently). In the Phoneword example, PutStringArrayExtra is used to attach a list of phone numbers to the Intent and StartActivity is called on the recipient of the Intent. The completed code looks like this:

translationHistoryButton.Click += (sender, e) =>
{
    var intent = new Intent(this, typeof(TranslationHistoryActivity));
    intent.PutStringArrayListExtra("phone_numbers", _phoneNumbers);
    StartActivity(intent);
};

Additional concepts introduced in phoneword

The Phoneword application introduced several concepts not covered in this guide. These concepts include:

String Resources – In the Phoneword application, the text of the TranslationHistoryButton was set to "@string/translationHistory". The @string syntax means that the string's value is stored in the string resources file, Strings.xml. The following value for the translationHistory string was added to Strings.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="translationHistory">Call History</string>
</resources>

For more information on string resources and other Android resources, refer to the Android Resources guide.

ListView and ArrayAdapter – A ListView is a UI component that provides a simple way to present a scrolling list of rows. A ListView instance requires an Adapter to feed it with data contained in row views. The following line of code was used to populate the user interface of TranslationHistoryActivity:

this.ListAdapter = new ArrayAdapter<string>(this, Android.Resource.Layout.SimpleListItem1, phoneNumbers);

ListViews and Adapters are beyond the scope of this document, but they are covered in the very comprehensive ListViews and Adapters guide. Populating a ListView With Data deals specifically with using built-in ListActivity and ArrayAdapter classes to create and populate a ListView without defining a custom layout, as was done in the Phoneword example.

Summary

Congratulations, you've completed your first multi-screen Android application! This guide introduced Android Application Building Blocks and Intents and used them to build a multi-screened Android application. You now have the solid foundation you need to start developing your own Xamarin.Android applications.

Next, you'll learn to build cross-platform applications with Xamarin in the Building Cross-Platform Applications guides.